This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

First draft prepared by R.G. Liteplo and M.E. Meek, Health Canada, Ottawa, Canada, and W. Windle, Environment Canada, Ottawa, Canada

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization

Geneva, 2002

The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals.

The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

N-Nitrosodimethylamine.

(Concise international chemical assessment document ; 38)

1.Dimethylnitrosamine - toxicity 2.Risk assessment 3.Environmental exposure

4.Occupational exposure I.International Programme on Chemical Safety II.Series

ISBN 92 4 153038 3           (NLM Classification: QZ 202)

ISSN 1020-616

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Concise International Chemical Assessment Documents (CICADs) are the latest in a family of publications from the International Programme on Chemical Safety (IPCS) — a cooperative programme of the World Health Organization (WHO), the International Labour Organization (ILO), and the United Nations Environment Programme (UNEP). CICADs join the Environmental Health Criteria documents (EHCs) as authoritative documents on the risk assessment of chemicals.

International Chemical Safety Cards on the relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information on the protection of human health and on emergency action. They are produced in a separate peer-reviewed procedure at IPCS. They may be complemented by information from IPCS Poison Information Monographs (PIM), similarly produced separately from the CICAD process.

CICADs are concise documents that provide summaries of the relevant scientific information concerning the potential effects of chemicals upon human health and/or the environment. They are based on selected national or regional evaluation documents or on existing EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review by internationally selected experts to ensure their complete ness, accuracy in the way in which the original data are represented, and the validity of the conclusions drawn.

The primary objective of CICADs is characterization of hazard and dose–response from exposure to a chemical. CICADs are not a summary of all available data on a particular chemical; rather, they include only that information considered critical for characterization of the risk posed by the chemical. The critical studies are, however, presented in sufficient detail to support the conclusions drawn. For additional information, the reader should consult the identified source documents upon which the CICAD has been based.

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This CICAD on N-nitrosodimethylamine (NDMA) was prepared jointly by the Environmental Health Directorate of Health Canada and the Commercial Chemicals Evaluation Branch of Environment Canada based on documentation prepared concurrently as part of the Priority Substances Program under the Canadian Environmental Protection Act (CEPA). The objective of assessments on Priority Substances under CEPA is to assess potential effects of indirect exposure in the general environment on human health as well as environmental effects. Although occupational exposure was not addressed in the source document (Environment Canada & Health Canada, 2001), information on this aspect has been included in this CICAD. Data identified as of the end of August 1998 (environmental effects) and August 1999² (human health effects) were considered in this review. Other reviews that were also consulted include IARC (1978), ATSDR (1989), OME (1991, 1998), and BIBRA Toxicology International (1997, 1998). Information on the nature of the peer review and availability of the source document is presented in Appendix 1. Information on the peer review of this CICAD is presented in Appendix 2. This CICAD was approved as an international assessment at a meeting of the Final Review Board, held in Geneva, Switzerland, on 8–12 January 2001. Participants at the Final Review Board meeting are listed in Appendix 3. The International Chemical Safety Card for NDMA (ICSC 0525), produced by the International Programme on Chemical Safety (IPCS, 1993), has also been reproduced in this document.

N-Nitrosodimethylamine (NDMA) is the simplest dialkylnitrosamine. It is no longer used industrially or commercially in Canada or the USA but continues to be released as a by-product and contaminant from various industries and from municipal wastewater treatment plants. Major releases of NDMA have been from the manufacture of pesticides, rubber tires, alkylamines, and dyes. NDMA may also form under natural conditions in air, water, and soil as a result of chemical, photochemical, and biological processes and has been detected in drinking-water and in automobile exhaust.

Photolysis is the major pathway for the removal of NDMA from surface water, air, and land. However, in surface waters with high concentrations of organic substances and suspended matter, photodegradation is much slower. In subsurface water and in soil, biodegradation is the removal pathway of importance. NDMA is unlikely to be transported over long distances in air or to partition to soil and sediments. Because of its solubility and low partition coefficient, NDMA has the potential to leach into and persist in groundwater. It is metabolized and does not bioaccumulate. NDMA is generally not detectable in surface waters, except for localized contamination from industrial sites, where end-of-pipe effluent concentrations as high as 0.266 µg/litre have been measured.

In limited surveys in the country on which the sample risk characterization is based (i.e., Canada), NDMA has not been detected in ambient air, except in the vicinity of industrial sites. Low concentrations of NDMA — formed in water treatment plants or from groundwater contaminated by industrial effluents, for example — have been measured in drinking-water. The presence of NDMA has been demonstrated in some foods, most frequently in beer, cured meat, fish products, and some cheeses, although levels of NDMA have decreased in these products in recent years owing to changes in food processing. Exposure can also result from the use of consumer products that contain NDMA, such as cosmetics and personal care products, products containing rubber, and tobacco products.

Based upon laboratory studies in which tumours have been induced in all species examined at relatively low doses, NDMA is clearly carcinogenic. There is overwhelming evidence that NDMA is mutagenic and clastogenic. While the mechanism by which NDMA induces tumours is not fully elucidated, DNA adducts (in particular, O⁶-methylguanine) formed by the methyldiazonium ion generated during metabolism likely play a critical role. Qualitatively, the metabolism of NDMA appears to be similar in humans and animals; as a result, it is considered highly likely that NDMA is carcinogenic to humans, potentially at relatively low levels of exposure.

Data on non-neoplastic effects in laboratory animals associated with exposure to NDMA are limited, attributable primarily to the focus on its carcinogenicity. Effects on the liver and kidney in repeated-dose toxicity studies, embryo toxicity and embryo lethality in single- dose developmental studies, and a range of immunological effects (suppression of humoral- and cell-mediated immunity) reversible at lowest concentrations have been reported.

Cancer is clearly the critical end-point for quantitation of exposure–response for risk characterization of NDMA. In addition to it being best characterized, in general, tumours occur at lowest concentration, compared with those typically reported to induce non-cancer effects. The lowest tumorigenic dose₀₅ for the development of hepatic tumours in male and female rats exposed to NDMA in the critical study was 34 µg/kg body weight per day for the development of biliary cystadenomas in female animals. This equates to a unit risk of 1.5 × 10^–3 per µg/kg body weight. Based on estimated intakes of NDMA in ambient air and contaminated drinking-water (groundwater) in the sample risk characterization, risks in the vicinity of industrial point sources are >10^–5. Those for ambient drinking-water are between 10^–7 and 10^–5. NDMA is a genotoxic carcinogen, and exposure should be reduced to the extent possible.

Acute and chronic toxicity data are available for aquatic organisms. The toxic effect that occurred at lowest concentration was a reduction in the growth of algae at 4000 µg/litre. In the sample risk characterization, concentrations of NDMA in surface waters in the source country are less than the threshold for adverse effects estimated for aquatic organisms. Data on concentrations of NDMA in sediments or in soil in the sample country were not identified.

N-Nitrosodimethylamine, or NDMA, is the simplest dialkylnitrosamine, with a molecular formula of C₂H₆N₂O and a relative molecular mass of 74.08 (ATSDR, 1989) (Figure 1). NDMA belongs to a class of chemicals known as N-nitroso compounds, characterized by the N-nitroso functional group (–N–N=O), and to the family of nitrosamines, which, in addition, possess an amine function (–NR₂, where R is H or an alkyl group). NDMA is also known as dimethylnitrosamine, dimethylnitrosoamine, N,N-dimethylnitrosamine, N-methyl-N-nitrosomethanamine, N-nitroso-N,N-dimethylamine, DMN, and DMNA. NDMA has the Chemical Abstracts Service (CAS) registry number 62-75-9.

NDMA is a volatile, combustible, yellow, oily liquid. It is susceptible to photolytic breakdown due to its absorption of ultraviolet light (Sax & Lewis, 1987). The physical/chemical properties relevant to the environmental fate of NDMA and utilized in the modelling of environmental partitioning (section 5.6) are presented in Table 1. Additional properties are presented in the International Chemical Safety Card reproduced in this document.

The conversion factor for NDMA in air is 1 ppm = 3.08 mg/m³.

Analytical methods for NDMA consist of concentration followed by chromatographic separation of the components in the extract and detection of the N-nitrosamine. Concentration steps include liquid–liquid extraction and solid-phase extraction. Chromatographic separations have been achieved almost exclusively by gas chromatography. Detection of NDMA has been accomplished by flame ionization detectors (Nikaido et al., 1977), nitrogen–phosphorus detectors (US EPA, 1984), the Hall electrolytic conductivity detector operated in the reductive mode (von Rappard et al., 1976; US EPA, 1984), the thermal energy analyser or chemiluminescent nitrogen detector (Fine et al., 1975; Fine & Rounbehler, 1976; Webb et al., 1979; Kimoto et al., 1981; Parees & Prescott, 1981; Sen & Seaman, 1981a; Sen et al., 1994; Tomkins et al., 1995; Tomkins & Griest, 1996), and mass spectrometry. NDMA is also analysed by electron ionization low-resolution mass spectrometry (Sen et al., 1994), high-resolution mass spectrometry (Taguchi et al., 1994; Jenkins et al., 1995), chemical ionization tandem mass spectrometry on an ion trap mass spectrometer (Plomley et al., 1994), and laser ionization time-of-flight mass spectrometry (Opsal & Reilly, 1986). Liquid chromatography has also been used in conjunction with a photolysis reactor and (electrospray ionization) mass spectrometry (Volmer et al., 1996). Detection limits range from 0.150 µg/litre using nitrogen–phosphorus detectors (US EPA, 1984) to 0.002 µg/litre using a gas chromatograph–thermal energy analyser (Kimoto et al., 1981; Tomkins et al., 1995; Tomkins & Griest, 1996) to 0.001 µg/litre using gas chromatography–high-resolution mass spectrometry (Taguchi et al., 1994; Jenkins et al., 1995). Comparable detection limits are possible with chemical ionization tandem mass spectrometry on an ion trap mass spectrometer (Plomley et al., 1994).

Data on sources and emissions from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented here as an example. Sources and patterns of emissions in other countries are expected to be similar, although quantitative values may vary.

NDMA can be formed as a result of biological, chemical, or photochemical processes (Ayanaba & Alexander, 1974). It may be present in water, air, and soil due to chemical reaction between ubiquitous, naturally occurring precursors classified as nitrosatable substrates (secondary amines) or nitrosating agents (nitrites) (OME, 1998). For example, NDMA may form in air during nighttime as a result of the atmospheric reaction of dimethylamine (DMA) with nitrogen oxides (Cohen & Bachman, 1978). Soil bacteria may also synthesize NDMA from various precursor substances, such as nitrate, nitrite, and amine compounds (ATSDR, 1989). NDMA precursors are widespread throughout the environment, occurring in plants, fish, algae, urine, and faeces (Ayanaba & Alexander, 1974).

NDMA is produced as a by-product of industrial processes that use nitrate and/or nitrites and amines under a range of pH conditions. This is due to inadvertent formation when alkylamines, mainly DMA and trimethylamine, come into contact and react with nitrogen oxides, nitrous acid, or nitrite salts or when trans-nitrosation via nitro or nitroso compounds occurs (ATSDR, 1989). Therefore, NDMA may be present in discharges of such industries as rubber manufacturing, leather tanning, pesticide manufacturing, food processing, foundries, and dye manufacturing and, as a result, in sewage treatment plant effluent. Almost all of the releases in the source country (i.e., Canada) are to water.

NDMA has also been detected in emissions from diesel vehicle exhaust (Goff et al., 1980).

NDMA may form directly in sewage as a result of the biological and chemical transformation of alkylamines in the presence of nitrate or nitrite (Ayanaba & Alexander, 1974; ATSDR, 1989). It may also be released into the environment as the result of application of sewage sludge to soils rich in nitrate or nitrite.

NDMA may also be formed during the treatment of drinking-water (OME, 1994). NDMA’s precursor, DMA, together with nitrite, may enter surface water streams from agricultural runoff (V.Y. Taguchi, personal communication, 1998). Water treatment plants incorporating a chlorination process (e.g., sodium hypochlorite) will produce NDMA from these precursors (Jobb et al., 1993; Graham et al., 1996). Ultraviolet treatment can decompose NDMA to DMA (Jobb et al., 1994). How ever, it is also possible to generate/regenerate NDMA from the DMA within distribution systems that have post-chlorination (V.Y. Taguchi, personal communication, 1998).

NDMA may be released into the environment as a result of use of certain pesticides contaminated with this compound (Pancholy, 1978). NDMA is present in various technical and commercial pesticides used in agriculture, hospitals, and homes as the result of its formation during the manufacturing process and during storage. The following DMA formulation pesticides may contain NDMA as a microcontaminant: bromacil, benazolin, 2,4-D, dicamba, MCPA, and mecoprop (J. Ballantine, personal communication, 1997; J. Smith, personal communication, 1999).

Since 1990, in testing in Canada of over 100 samples of formulated pesticidal products (DMA salt of phenoxy acid herbicides) potentially contaminated by NDMA, the compound was detected in 49% of the samples, with an average concentration of 0.44 µg/g. Only six samples contained concentrations above1.0 µg/g, with a range from 1.02 to 2.32 µg/g. Concentrations in pesticides have decreased over time. In 1994, approximately 1 million kilograms of DMA-formulated phenoxy acid herbicides for commercial use were applied to the terrestrial environment in Canada (G. Moore, personal communication, 1999). Based on the average concentration of NDMA mentioned above and per cent estimate of detection, it was calculated that approximately 200 g of NDMA may have been released into the environment through the use of these herbicides.

There are no industrial or commercial uses of NDMA in Canada or the USA. NDMA was used in Canada in the past and may still be used in other countries in rubber formulations as a fire retardant and in the organic chemical industry as an intermediate, catalyst, antioxidant, additive for lubricants, and softener of copolymers (ATSDR, 1989; Budavari et al., 1989).

NDMA has a low vapour pressure (1080 Pa at 25 °C), and, if emitted to or formed in air, it is not likely to adsorb to airborne particulate matter and is expected to exist almost entirely in the vapour phase. In daylight, it degrades rapidly by direct photolysis to form dimethylnitramine. The photolytic half-life of NDMA vapour exposed to sunlight ranges between 0.5 and 1.0 h (Hanst et al., 1977). Half-lives for the reaction with hydroxyl radicals range from 25.4 to 254 h in air (Atkinson, 1985). Modelling of environmental partitioning (section 5.6) is based on a half-life for NDMA in air of 5 h (DMER & AEL, 1996). The short half-lives for NDMA in air suggest that it is not persistent in this compartment.

Since NDMA is miscible in water and has a low vapour pressure and a low octanol/water partition coefficient (log K_ow of 0.57), it is not likely to bioaccumulate, adsorb to particulates, or volatilize to any significant extent (Thomas, 1982; ATSDR, 1989; OME, 1991). Oxidation, hydrolysis, biotransformation, and biodegradation are not significant factors affecting the fate of NDMA in lake water (Tate & Alexander, 1975). Photodegradation is the main process for removing NDMA from the aquatic environment. The efficiency of removal of NDMA depends on the characteristics of the particular water environment. Typically, photodegradation of NDMA is much slower in waters with high concentrations of organic substances and suspended solids than in clear water bodies. The rate of degradation through photolysis may be significantly decreased in the presence of interferences with light transmission, such as ice cover on receiving water bodies (Conestoga-Rovers & Associates, 1994; E. McBean, personal communication, 1999). This observation is supported in the groundwater compartment, where, in the absence of light, NDMA has the potential to persist (OME, 1991).

Modelling of environmental partitioning (section 5.6) is based on a mean half-life of 17 h for NDMA in surface water at 25 °C (DMER & AEL, 1996). Howard et al. (1991) reported a half-life range for NDMA in groundwater of 1008–8640 h, based on estimated unacclimated aqueous aerobic biodegradation.

Modelling of environmental partitioning (section 5.6) is based on a mean half-life of 5500 h for NDMA in sediment at 25 °C (DMER & AEL, 1996). Factors that slow degradation include anoxic conditions and lack of illumination, the former by preventing the generation of oxidants and the latter by preventing photolysis and the generation of oxidants by photolytic processes.

On soil surfaces, photolysis and volatilization rapidly remove NDMA. Oliver (1979) reported that 30–80% of an unreported concentration of NDMA volatilized from the soil within the first few hours of application to the soil surface. Once incorporated into subsurface soil, however, NDMA will be highly mobile, with the potential to migrate into groundwater supplies. Subsurface biodegradation is slightly slower under anaerobic than under aerobic conditions (ATSDR, 1989). Soil type only slightly affects biodegradation of NDMA. Aeration of soil improved biodegradation compared with waterlogged soil. Pre-exposure of bacteria to NDMA increased biodegradation in soil (Mallik & Tesfai, 1981). Modelling of environmental partitioning (section 5.6) is based on a mean half-life of 1700 h for NDMA in soil at 25 °C (DMER & AEL, 1996).

Although NDMA is not present in plants under natural conditions, it can be taken up from the growth medium. Lettuce and spinach plants absorb NDMA from sand, soil, and water after exposure for 2 days to concentrations ranging from 10 to 100 mg NDMA/kg wet weight, with 3.25% and 0.38% being taken up from the growth medium by lettuce and spinach plants, respectively (Dean-Raymond & Alexander, 1976).

A bioconcentration factor of 0.2 has been estimated for NDMA (Bysshe, 1982). However, conventional estimates of bioconcentration factors (correlation with K_ow) are precluded, since, generally, biota can biotransform NDMA (OME, 1998).

Fugacity modelling provides an overview of key reaction, intercompartment, and advection (movement out of a system) pathways for NDMA and its overall distribution in the environment. A steady-state, non-equilibrium model (Level III fugacity model) was run using the methods developed by Mackay (1991) and Mackay & Paterson (1991). Values for physical/chemical properties utilized in the modelling are presented in Table 1; those for half-lives in various media are presented in sections 5.1–5.4 above. Modelling was based on an assumed default emission rate of 1000 kg/h into a region of 100 000 km², which includes a surface water area (20 m deep) of 10 000 km². The height of the atmosphere was assumed to be 1000 m. Sediments and soils were assumed to have an organic carbon content of 4% and 2% and a depth of 1 cm and 10 cm, respectively. The estimated per cent distribution predicted by this model is not affected by the assumed emission rate.

Modelling predicts that when NDMA is continuously released into a medium, most of it will be present in that medium at steady state. For example, if NDMA is discharged into water, almost all of it will be present in the aqueous phase, with very small amounts in air and soil. Almost all of the NDMA is removed by reaction in water. Similarly, most NDMA released to air will exist in the atmosphere, with very small amounts in soil and water. Finally, when NDMA is discharged continuously to soil, almost all of the substance is transported to surface water, and about a third goes into the atmosphere. However, since NDMA is much more persistent in soil than in water or air at steady state, almost all of the NDMA is present in soil, with very little in surface water, and even less in the atmosphere (DMER & AEL, 1996).

In summary, the Level III fugacity model predicts that if NDMA is emitted into water or air, it will be found in, and react in, the medium of discharge. Emission of NDMA into water or air will tend to result in localized contamination of short duration. If emitted to soil, NDMA moves to the water or air compartments, where it undergoes reaction, or it reacts slowly in the soil. Because rates of volatilization, adsorption, runoff, and reaction in soil are relatively slow compared with reaction in air and water, the persistence of NDMA emitted to soil is longer, and there is potential for NDMA to move into the groundwater compartment (DMER & AEL, 1996).

Data primarily on concentrations in the environment from the source country of the national assessment on which the CICAD is based (i.e., Canada) are presented here as a basis for the sample risk characterization. Patterns of exposure in other countries are expected to be similar, although quantitative values may vary.

6.1.1 Ambient air

There is little information on the presence or concentrations of NDMA in ambient (i.e., outdoor) air in Canada or elsewhere. Limited Canadian data are restricted to the province of Ontario, where short-term measurements have been taken in the immediate vicinity of potential point sources of discharge to the atmosphere, for comparison with background measurements from other urban locations. No data on airborne concentrations at rural locations were identified.

At industrial and urban locations in Ontario in 1990, based on seven samples taken in five cities, concentrations of NDMA were all below the detection limit (detection limits ranged from 0.0034 to 0.0046 µg/m³).³

In surveys during 1990 of ambient air in the vicinity of a chemical production facility in Elmira, Ontario, concentrations of NDMA in 41 samples ranged from not detected (detection limits ranged from 0.0029 to 0.0048 µg/m³) to 0.230 µg/m³; concentrations in 20 of the 41 samples were at or above the detection limit.³ The highest concentrations were measured within the perimeter of the production facility, while the maxi mum concentration measured beyond this perimeter was 0.079 µg/m³. Concentrations of NDMA in samples taken in the vicinity of an industrial site in Kitchener, Ontario, were similar.⁴

6.1.2 Indoor air

Available data indicate that levels of NDMA were elevated in indoor air contaminated with environmental tobacco smoke (ETS) in the USA (Brunnemann & Hoffmann, 1978) and Austria (Stehlik et al., 1982; Klus et al., 1992). The maximum concentration of NDMA in ETS-contaminated indoor air was 0.24 µg/m³, whereas NDMA was not detected (i.e., <0.003 µg/m³) when the indoor air of a residence of a non-smoker was sampled in the same manner (Brunnemann & Hoffmann, 1978). Concentrations of NDMA in ETS-contaminated indoor air in these countries were generally between 0.01 and 0.1 µg/m³ (Health Canada, 1999).

6.1.3 Water

Releases of NDMA to water in Canada have been measured primarily in Ontario and vary considerably. As an example, in 1996, a chemical plant released wastewater containing NDMA into the St. Clair River at a concentration of 0.266 µg/litre (Environment Canada, 1997). In April 1997, concentrations of NDMA at the point of release to surface water ranged from 0.096 to 0.224 µg/litre for this company. These concentrations are expected to decrease, as the company installed a wastewater treatment plant in1998.

In a survey of sewage treatment plant effluent in Ontario in 1990, NDMA was detected in 27 of 39 samples, with the maximum concentration being 0.22 µg/litre (OME, 1991).

In 390 samples of raw surface water from 101 water treatment plants sampled for NDMA in Ontario from 1990 to July 1998, concentrations were detectable (>0.001 µg/litre) in the raw water at 37 plants. The average concentration in raw water was 1.27 × 10^–3 µg/litre. The highest concentration of NDMA in raw water was 0.008 µg/litre from two water treatment plants in 1996 (Ontario Ministry of Environment and Energy, unpublished data, 1996; P. Lachmaniuk, Ontario Ministry of the Environment, unpublished data, 1998).

In 1990, concentrations of NDMA in 24 groundwater samples taken from various locations in Ontario were below detection limits (detection limits ranged from 0.001 to 0.010 µg/litre). Concentrations of NDMA in the municipal aquifer in Elmira ranged from 1.3 to 2.9 µg/litre, attributed to contamination from a nearby chemical facility (Kornelsen et al., 1989). The municipal wells using this aquifer were closed in 1989 (Ireland, 1989). In 1994 and 1995, concentrations of up to 0.005 µg NDMA/litre (detection limit 0.001 µg/litre) in raw surface water and groundwater supplies in rural areas in southern Ontario were reported (OME, 1991).

In 313 samples of treated water analysed from 100 locations within Ontario between 1994 and 1996, NDMA was detected (i.e., at greater than 0.001 µg/litre) in at least one sample at 40 of these 100 sites. The censored mean concentration was 0.0027 µg/litre. The highest concentrations were measured in samples from drinking-water plants using a specific pre-blended polyamine/alum water treatment coagulant (Ontario Ministry of Environment and Energy, unpublished data, 1996). These included a concentration of 0.04 µg/litre at the water treatment plant in Huntsville, Ontario. NDMA was detected in all (i.e., at greater than 0.001 µg/litre) 20 samples collected from four water treatment plants using the specific coagulant. The mean concentration of NDMA in these 20 samples was 0.012 µg/litre, whereas the (censored) mean concentration in the remaining 293 samples for the locations where the specific coagulant was not used was 0.002 µg/litre.

Treatment studies on groundwater at a chemical plant in southern Ontario indicated that activated sludge can accumulate NDMA, particularly when nitrification and denitrification are applied to increase the age of the sludge. Concentrations of NDMA sampled in activated sludge ranged from 5 to 10 mg/litre (J. Kochany, personal communication, 1999; E. McBean, personal communication, 1999). In the USA, NDMA has been reported to be a common constituent of sewage sludge. Concentrations ranged from 0.6 to 45 µg/g in the dried sludge from 14 of 15 cities (Mumma et al., 1984).

6.1.4 Sediment and soil

No data on concentrations of NDMA in sediments or soils in Canada were identified. Levels of NDMA up to 15.1 ng/g have been measured in soils collected in the vicinity of industrial facilities in the USA (IARC, 1978).

6.1.5 Human tissues

NDMA has been quantified in a variety of tissues and biological fluids. In a study conducted in Quebec, Canada, Cooper et al. (1987) detected NDMA in the liver, kidneys, brain, and pancreas from four (non-occupationally exposed) individuals at postmortem; concentrations ranged from approximately 0.12 to 0.9 ng/g tissue. In studies conducted outside of Canada, reported levels of NDMA in the blood or plasma of non- occupationally exposed individuals have ranged from approximately 0.03 to 1.5 ng/ml (Fine et al., 1977; Lakritz et al., 1980; Yamamoto et al., 1980; Garland et al., 1982; Gough et al., 1983; Dunn et al., 1986). In other studies, concentrations of NDMA in breast milk ranged from 0.1 to 1.8 ng/g (Lakritz & Pensabene, 1984; Mizuishi et al., 1987; Uibu et al., 1996). NDMA has been detected in the urine of individuals having no clearly defined exposure to this nitrosamine; reported concentrations from studies conducted in Canada (Kakizoe et al., 1979) and elsewhere (Lakritz et al., 1982; Webb et al., 1983) have ranged from 0.02 to 0.2 ng/ml.

6.1.6 Food

NDMA can be formed during food processing, preservation, and/or preparation from precursor compounds already present in, or added to, the specific food items. The foodstuffs that have been most commonly contaminated with NDMA can be classified into several broad groups:

• foods preserved by the addition of nitrate and/or nitrite, such as cured meat products (in particular, bacon) and cheeses (since these methods of preservation introduce nitrosating species into the food);
• foods preserved by smoking, such as fish and meat products (since oxides of nitrogen in the smoke act as nitrosating agents);
• foods dried by combustion gases, such as malt, low-fat dried milk products, and spices (since combustion gases can contain oxides of nitrogen);
• pickled and salt-preserved foods, particularly pickled vegetables (since microbial reduction of nitrate to nitrite occurs); and
• foods grown or stored under humid conditions, leading to nitrosamine formation by contaminating bacteria.

It should be noted, however, that most data on levels of NDMA in foodstuffs have been derived from studies conducted in the 1970s and 1980s and may not be reliable with respect to estimating current exposure to this substance, owing to the analytical methodology available at the time. Moreover, efforts have been made to reduce the potential for exposure to NDMA in foodstuffs in Canada and other countries through continued reduction of allowable nitrite levels during preservation, suspension of the use of nitrate for certain food groups, or increased use of nitrosation inhibitors, such as ascorbate or erythorbate (Cassens, 1997; Sen & Baddoo, 1997). For example, in Canada, in regulations amended in 1975, permissible levels of nitrite in cured meat products were lowered and the use of nitrate was eliminated, except for a few classes of products (including "slow-cured" meats) (G. Lawrence, personal communication, 1999). The use of nitrate in seafood preservation was suspended in 1965.⁵

Data concerning the concentrations of NDMA in food items in Canada from each of the groups in which there is potential for exposure are limited and largely predate the introduction of controls outlined above. Concentrations of NDMA in 121 samples of various meat products in Canada ranged from less than 0.1 µg/kg (the limit of detection) to a maximum of 17.2 µg/kg in a sample of bacon (Sen et al., 1979, 1980b). Concentrations of NDMA in 63 samples of various fish and sea food products in Canada ranged from less than 0.1 µg/kg (the limit of detection) to a maximum of 4.2 µg/kg in a sample of salted/dried fish (Sen et al., 1985). Concentrations of NDMA in 62 samples of cheese (31 of Canadian origin and 31 imported) purchased in Canada ranged from less than 1 µg/kg (the limit of detection) to a maxi mum of 68 µg/kg in a sample of wine cheese (Sen et al., 1978).

NDMA was generally not detected in samples of milk products, except for skim milk powder, where it was present in all 11 samples, at a maximum concentration of 0.7 µg/kg (Sen & Seaman, 1981b). In other countries, the presence of NDMA in non-fat dried milk powders has been attributed to the use of natural gas for direct fired heating (Kelly et al., 1989; Scanlan et al., 1994). In Canada, in other foods dried directly, NDMA was detected in 1 of 10 samples of instant coffee at a concentration of 0.3 µg/kg and in 2 of 20 samples of dried soup with a maximum concentration of 0.25 µg/kg (Sen & Seaman, 1981b).

NDMA was not detected (at limits of detection ranging from 0.1 to 0.5 µg/kg) in 25 samples of baby food, including formula, cereal, and mixed food containing meat, analysed from 1979 to 1981 (Sen et al., 1979, 1980b; Sen & Seaman, 1981b). In a survey of other food products in 1979, NDMA was not detected in apple juice or drink, ketchup and other sauces, Ovaltine, margarine, butter, lard, or (fresh and canned) mushrooms (Sen et al., 1980b). The limit of detection was 0.1 µg/litre or 0.1 µg/kg. NDMA was detected at a trace level (<0.2 µg/kg) in 1 of 11 samples of pizza and pizza toppings (Sen et al., 1980b).

Among the cured meat products analysed, bacon was unique, in that it was generally free of nitrosamines in the raw stage. Nitrosamines were formed in bacon only during high-heat frying (Sen et al., 1979). Various factors control the formation of NDMA in fried bacon, including the initial and residual levels of nitrite, processing conditions, the diet of the pigs, the lean to adipose tissue ratio, the presence of inhibitors, frying temperatures, and cooking methods (Sen, 1986). The cooked-out fat contains higher (approximately twice as high) levels of nitrosamines than the cooked lean bacon, and steam-volatile nitrosamines such as NDMA are volatilized in the fumes produced during frying (Sen, 1986).

Concentrations of NDMA in bacon currently consumed in Canada are unlikely to be as high as the maximum of 17.2 µg/kg reported previously (Sen et al., 1979, 1980b), as a result of the introduction of controls on the use of nitrate and nitrite in cured meat products in 1975. However, quantitative data are not available to support this conclusion.

There is consensus among the literature surveyed that concentrations of NDMA in foods from developed countries were an order of magnitude lower in the late 1980s and1990s than in the 1970s (Tricker et al., 1991a; Cornée et al., 1992; Sen et al., 1996). The reduction in the concentrations of preformed NDMA in foods is attributed to improvements in food cooking and preservation techniques. However, no data are available with which to determine whether the concentrations of preformed NDMA in foods in Canada or elsewhere have continued to decline throughout the 1990s or remain at the levels measured in the late 1980s and 1990s.

Most malt beverages, including beer and most brands of whiskey, regardless of origin, contain NDMA (ATSDR, 1989). The presence of NDMA in beer was first reported in 1977 (Sen et al., 1980a; OME, 1991). Malt was found to be the main source of NDMA contamination in beer, and NDMA was shown to be formed during direct drying of malt using hot flue gases — a practice that was common prior to 1980 (Spiegelhalder et al., 1980). Improved malt drying techniques (direct to indirect in 1981) have now significantly reduced the levels of NDMA in malt and beer (OME, 1991; Sen et al., 1996). It is currently believed that NDMA is only a minor component of the total N-nitroso compounds in beer and that the major contribution is made by as yet unidentified non-volatile N-nitroso compounds (Massey et al., 1990; UK MAFF, 1992). Among samples of beer produced in Canada, a maximum concentration of 4.9 µg/litre was reported in a beer from Ontario in 1978, while in more recent samples (i.e., 1988–1989), the maximum concentration was 0.59 µg/litre. Among imported beers purchased in Canada, a maximum concentration of 9.2 µg/litre was reported in a beer sampled in 1991–1992, while in more recent samples (i.e., October–December 1994), the maximum concentration was 3.2 µg/litre.

NDMA may also be endogenously produced in vivo from precursor compounds contained in the food ingested (e.g., DMA in meats and fish and nitrate/nitrite in vegetables) and/or already present in the human body (e.g., nitrate, nitrite) (Vermeer et al., 1998). However, available data are inadequate to serve as a basis for determining the quantities of endogeneous NDMA formed or their relative contribution to exposure via ingestion compared with that from the exogenous presence of NDMA in food (Cornée et al., 1992).

6.1.7 Consumer products

Exposure can result from the use of consumer products that contain NDMA, such as cosmetics and personal care products, products containing rubber, and tobacco products.

NDMA has been detected in a variety of personal care and cosmetic products (e.g., shampoos, hair conditioners and toners, bath and shower gels, creams and oils, face tonics, cleansers), likely due to the reaction of nitrosating agents such as nitrite and/or nitrogen oxides, which occur frequently therein (Spiegelhalder & Preussmann, 1984), with amine-containing compounds, which are used extensively in ingredients of personal care products. Examples include surfactants, detergents, foam boosters, protein additives, and colouring agents (ECETOC, 1990). Nitrosation of precursor compounds in cosmetic matrices, which likely include quaternary ammonium compounds, betaines, and amine oxides (ECETOC, 1991), is often slow, but cosmetic products may remain on store shelves and in consumers’ cabinets for extended periods of time, during which nitrosamines can continue to form in the products (Havery & Chou, 1994).

Fifty (or 34.5%) of 145 products surveyed in Germany in 1984 contained NDMA, at a maximum concentration of 24 µg/kg in one shampoo (Spiegelhalder & Preussmann, 1984). In some countries, controls have been introduced to limit levels of nitrosamines in cosmetics. For example, in Canada, manufacturers who submit cosmetic notifications for formulations that include combinations of such precursor substances are requested to provide evidence that the level of nitrosamines present in the product or formed over a period equivalent to the shelf life of the product does not exceed 10 µg/kg. Failing this, manufacturers are required to reformulate the products to remove either the amines/ amides or the nitrosating agents (R. Green, personal communication, 1995).

Rubber-containing products that come into contact with human skin are another potential source of exposure to NDMA, since dialkylamines used in rubber vulcanization as accelerators and stabilizers can react with nitrosating agents to form nitrosamines (Biaudet et al., 1997). NDMA has been detected in a diverse selection of workplace, consumer, and medical products containing rubber (Health Canada, 1999). The maximum concentration of NDMA detected (i.e., 329 mg/kg) was in latex disposable protective gloves in the USA. However, only a small proportion of the total nitrosamines in the gloves would be expected to be leached out and dermally absorbed (Fiddler et al., 1985). N-Nitrosamines have been detected in baby bottle rubber nipples and pacifiers in Canada. The maximum concentrations of NDMA reported in the published literature were 25 mg/kg in baby bottle rubber nipples and 8.6 mg/kg in rubber pacifiers (Sen et al., 1984).

The nitrosation of natural constituents of tobacco during curing and fermentation results in the formation of three major classes of N-nitroso compounds in tobacco and tobacco products — volatile, non-volatile, and tobacco-specific N-nitrosamines (Hoffmann et al., 1984; Tricker et al., 1991b). In addition, the combustion of cigarette tobacco results in the pyrolytic formation of volatile N-nitrosamines, including NDMA (Tricker & Preussmann, 1992). The yields of these volatile N-nitrosamines in cigarette smoke from combustion of tobacco depend on many chemical and physical parameters, including the amounts of organic nitrogen and nitrate present (Hoffmann et al., 1987). Furthermore, nicotine serves as a specific precursor for formation of NDMA (Hoffmann et al., 1987).

The NDMA content of cigarette and oral tobacco and the amounts of NDMA in mainstream smoke, sidestream smoke, and ETS have been assessed in several studies (Health Canada, 1999). The levels of preformed volatile N-nitrosamines in the cigarette tobacco are considerably lower than the corresponding levels in the mainstream smoke (Tricker et al., 1991b), and the levels of NDMA in sidestream smoke are generally 1 or 2 orders of magnitude greater than in the mainstream smoke from the same cigarette (Health Canada, 1999).

The average ETS emission factor for NDMA for six US commercial cigarette brands was 570 ± 120 ng/ cigarette (Daisey et al., 1994; Mahanama & Daisey, 1996). These data have been extrapolated to estimate the concentration of NDMA in indoor air spaces of defined volume and air exchange rates. The predicted concentrations of NDMA in indoor air ranged from 0.002 to 0.005 mg/m³ (Mahanama & Daisey, 1996). Predicted concentrations based on data from other studies ranged from 0.011 to 0.037 mg/m³ (Mahanama & Daisey, 1996). These modelled concentrations are similar to the measured concentrations of NDMA in indoor air contaminated with ETS, summarized in section 6.1.2.

Point estimates of daily intake (per kilogram body weight), based on available data that are limited in both spatial and temporal scope and reference values for body weight, inhalation volumes, and amounts of food and drinking-water consumed daily, are presented for six age groups in Table 2. These are ranges of reasonable worst-case estimates of daily intake, based on historic data, and indicate that daily intake of NDMA may be as high as 0.03 µg/kg body weight per day. It is not possible to develop defensible estimates of the current average daily intakes of NDMA for the general population due to the limitations of the (particularly recent) available Canadian data. If, despite these limitations, the lower ends of the ranges of reasonable worst-case estimates are considered upper bounds of average population exposure estimates, the daily intake of NDMA from outdoor air (in the vicinity of point sources), water, and food for the general population is unlikely to exceed 0.008 µg/kg body weight per day. Based on the assumptions underlying the reasonable worst-case estimates, most of the daily intake can be attributed to consumption of food contaminated with NDMA during processing, preservation, and/or preparation. It should be noted, though, that the data on which the estimates in food are based may not be representative of the situation today, due to the impact of subsequent introduction of changes in food processing and controls to limit formation in food. Intake of NDMA due to inhalation of air contaminated by atmospheric discharges from industrial point sources contributes somewhat less to the total daily intake,⁶ and an even smaller contribution is attributed to consumption of drinking-water containing NDMA, based on a survey of water treatment plants in Ontario. However, although possibly unrepresentative, available data indicate that contaminated groundwater in the vicinity of industrial point sources can, in some cases, lead to intakes that are greater than those from all other media combined.

If it is assumed that the population is exposed to the maximum concentration of NDMA in ETS-contaminated indoor air (0.24 µg/m³) for 21 h/day (EHD, 1998), the upper-bounding estimates of intake by inhalation range from 0.04 to 0.13 µg/kg body weight per day. If it is assumed that an average adult smoker consumes 20 cigarettes a day and that the mainstream smoke contains between 4 and 278 ng/cigarette (Adams et al., 1987; Kataoka et al., 1997), the estimated intake of NDMA is 0.080–5.6 µg/smoker per day, or 0.001– 0.08 µg/kg body weight per day. The upper end of this range of estimates of daily intake for smokers (i.e., 0.08 µg/kg body weight per day) is 5 times greater than the upper end of the range of reasonable worst-case estimates of intakes for adults from air, water, and food (i.e., 0.016 µg/kg body weight per day, as summarized in Table 2).

Reasonable worst-case estimates of daily intake of NDMA for all age groups from ingestion of contaminated groundwater range from 0.03 to 0.31 µg/kg body weight per day (see Table 2). These estimates are based on the minimum (i.e., 1.3 µg/litre) and maximum (i.e., 2.9 µg/litre) confirmed concentrations of NDMA in supply wells in Elmira, Ontario, in 1989 (Kornelsen et al., 1989). The groundwater was contaminated by discharges from a nearby industrial facility.

Estimates of daily intake of NDMA from ingestion of beer are not included in the reasonable worst-case estimates of intake from food in Table 2. For comparison, the most recent maximum concentration (i.e., 0.59 µg/litre) of NDMA in Canadian beer⁷ (Sen et al., 1996) and average daily rates of consumption of beer (EHD, 1998) are the basis for reasonable worst-case estimates of daily intake, which range from <0.0002 to 0.0009 µg/kg body weight per day.

Based on the limit (i.e., 10 µg/kg) for nitrosamines in cosmetics in Canada (R. Green, personal communication, 1995), the potential dermal uptake of NDMA from a shampoo was estimated based on product use scenarios (ECETOC, 1994). A shampoo was selected for this calculation, as the maximum reported concentration (i.e., 24 µg/kg) of NDMA in personal care products was in a shampoo in Germany (Spiegelhalder & Preussmann, 1984). The estimated uptake of 0.000 02 µg/kg body weight per day resulting from this calculation (Health Canada, 1999) is several orders of magnitude less than the reasonable worst-case estimates of combined daily intakes from air, water, and food that are summarized in Table 2.

Although NDMA is not used directly, workplaces in which there is potential for exposure to NDMA (as a by-product of manufacturing processes) include, but are not necessarily limited to, leather tanneries, rubber and tire industries, rocket fuel industries, dye manufacturers, soap, detergent, and surfactant industries, foundries (core-making), fish processing industries (fish meal production), pesticide manufacturers, and warehouses and sales rooms (especially for rubber products) (ATSDR, 1989). Occupational exposure may result from inhalation or dermal contact (ATSDR, 1989). The National Occupational Exposure Survey (1981–1983) indicated that 747 workers, including 299 women, were potentially exposed to NDMA (NIOSH, 1984) in the USA. US Occupational Safety and Health Administration regulations concerning NDMA (OSHA, 1993) designate strict procedures to avoid worker contact. Mixtures containing >1.0% NDMA must be maintained in isolated or closed systems, workers must observe special hygiene rules, and certain procedures must be followed for movement of the material and in case of accidental spills or emergencies. Synthetic cutting fluids, semisynthetic cutting oils, and soluble cutting oils may contain nitrosamines, either as contaminants in amines or as products from reactions between amines and nitrite. Concentrations of nitrosamines ranging from 1 to 1000 mg/litre have been determined in certain synthetic cutting oils. There are approximately 8–12 additives that could be responsible for nitrosamine formation in cutting oils. Approximately 750 000–780 000 workers employed by more than 1000 cutting fluid manufacturing firms are potentially exposed to nitrosamines in cutting oils. In addition, there is potential exposure of an undetermined number of machine shop workers who use these fluids. Kauppinen et al. (2000) estimated that in the early 1990s, about 14 000 workers in the European Union likely had occupational exposure to NDMA. Based upon monitoring studies conducted in a number of rubber manufacturing facilities in Europe, reported maximum concentrations of NDMA in workplace air have ranged from about 1 µg/m³ into the hundreds of micrograms per cubic metre (Ducos & Gaudin, 1986; Daubourg et al., 1992; Solionova et al., 1992; Rogaczewska & Wróblewska-Jakubowska, 1996; Oury et al., 1997; Straif et al., 2000).

While quantitative data in humans have not been identified, on the basis of studies conducted with laboratory animals, ingested NDMA is absorbed rapidly and extensively (i.e., >90%) (Daugherty & Clapp, 1976; Diaz Gomez et al., 1977; Kunisaki et al., 1978), primarily from the lower intestinal tract (Phillips et al., 1975; Hashimoto et al., 1976; Agrelo et al., 1978; Pegg & Perry, 1981). Detection of NDMA in the urine of rats and dogs exposed by inhalation indicates that the nitrosamine is absorbed through the lungs; however, reliable quantitative information on the absorption of NDMA following inhalation was not identified. Although quantitative data were not identified, absorption through the skin may be inferred from the results of a study in which small amounts (i.e., 0.03%) of NDMA were detected in the urine of rats following epicutaneous (dermal) administration of a solution containing 350 µg NDMA (Spiegelhalder et al., 1982).

Once absorbed, NDMA and its metabolites are distributed widely (Daugherty & Clapp, 1976; Anderson et al., 1986) and likely passed to offspring through mothers’ milk (Diaz Gomez et al., 1986). The nitrosamine and its metabolites have been detected in the fetuses of pregnant rodents injected with the substance (Althoff et al., 1977; Johansson-Brittebo & Tjälve, 1979). Pharmacokinetic analyses of NDMA injected intravenously into a number of laboratory species have revealed that the nitrosamine is cleared rapidly from the blood, with metabolism involving both hepatic and extrahepatic components. NDMA and its metabolites may be excreted in the urine or exhaled as carbon dioxide.

Quantitative information from studies on the metabolism of NDMA in individuals was not identified. However, based upon a few studies in which the metabolic conversion of NDMA in human liver preparations has been examined, there appear to be no qualitative differences in the metabolism of NDMA between humans and laboratory animals. The metabolism of NDMA involves either the alpha-hydroxylation or denitrosation of the nitrosamine (Figure 2). Both pathways are considered to proceed through a common intermediate radical [CH₃(CH₂)N–N=O], generated by the action of the cytochrome P450 [CYP2E1]-dependent mixed-function oxidase system (Haggerty & Holsapple, 1990; Lee et al., 1996). Along the alpha-hydroxylation pathway, the hydroxymethylnitrosamine (HOCH₂CH₃N–N=O) formed from the intermediate radical decomposes to formaldehyde (itself ultimately converting to carbon dioxide) and monomethylnitrosamine (CH₃NHN=O); the monomethylnitrosamine, owing to its instability, under goes rearrangement to the strongly methylating methyldiazonium ion (CH₃N⁺ N), which alkylates biological macromolecules such as DNA, RNA, and proteins. Metabolic conversion of the intermediate radical via denitrosation may lead to the formation of methylamine (CH₃NH₂) and formaldehyde.

NDMA has been consistently potently carcinogenic in all experimental species examined. Since exposure to NDMA occurs principally through its occurrence as a contaminant in media to which the general population is exposed, this end-point is expected to be limiting; hence, the focus of testing and, as a result, assessment has been carcinogenicity. Other end-points have not been well investigated; available data are considered inadequate as a basis for their meaningful characterization. In addition, exposure in available studies has been restricted primarily to ingestion; hence, meaningful dose–response analyses for other routes of exposure, even for the critical end-point (e.g., carcinogenicity), are precluded.

NDMA is highly acutely toxic after oral administration to rats, with LD₅₀s ranging from 23 to 40 mg/kg body weight. It is also highly acutely toxic via inhalation; 4-h LC₅₀s are 78 ppm (240 mg/m³) for rats and 57 ppm (176 mg/m³) for mice. One day after three dogs were exposed (via inhalation) to 16 ppm (49 mg/m³) NDMA for 4 h, one had died, and the others were moribund (ATSDR, 1989). In all three species, acute inhalation exposure produced haemorrhagic necrosis in the liver; an increased blood clotting time was reported for the NDMA-exposed dogs (ATSDR, 1989). Following intraperitoneal exposure, LD₅₀s of 43 mg/kg body weight in rats and 20 mg/kg body weight in mice have been reported (IARC, 1978). In other laboratory species, acute exposure to NDMA produced effects in the liver (hepatotoxicity), kidney (tumours), and testes (necrosis of the seminiferous epithelium) (Magee & Barnes, 1962; Schmidt & Murphy, 1966; Hard & Butler, 1970a,b; McLean & Magee, 1970; OME, 1991).

Data on the potential of NDMA to induce sensitization and/or irritation were not identified.

Hepatic effects (i.e., hepatocyte vacuolization, portal venopathy, and necrosis/haemorrhage), often associated with reduced survival, have been observed in a number of mammalian species exposed orally under various conditions (e.g., in rats receiving 1, 3.8, or 5 mg NDMA/kg body weight per day for 30, 7–28, or 5–11 days, respectively; in mice receiving 5 mg/kg body weight per day for 7–28 days; in hamsters receiving 4 mg/kg body weight per day for 1–28 days; in guinea- pigs, cats, and monkeys receiving 1 mg/kg body weight per day for 30 days or 5 mg/kg body weight per day for 5–11 days; in dogs receiving 2.5 mg/kg body weight per day, 2 days/week, for 3 weeks; and in mink receiving 0.32 mg/kg body weight per day for 23–34 days) (summarized from IARC, 1978; ATSDR, 1989).

In addition to effects in the liver, "congestion" in a variety of organs (i.e., kidneys, lung, spleen, and myocardium) has been reported following examination of rats receiving 3.8 mg NDMA/kg body weight per day in the diet for 1–12 weeks (Khanna & Puri, 1966). Gastrointestinal haemorrhage has been observed in rats receiving dietary doses of 10 mg NDMA/kg body weight per day for 34–37 days (Barnes & Magee, 1954) and in mink receiving 0.3 or 0.6 mg NDMA/kg body weight per day in the diet for 23–34 days (Carter et al., 1969). Effects in the kidneys (including glomerulus dilatation and slight thickening of the Bowman’s capsule) were observed in mink receiving 0.2 mg NDMA/ kg body weight per day from the diet (period not specified) (Martino et al., 1988).

Although most studies would be considered limited by current standards (e.g., small group sizes, single dose levels, limited histopathological examination), there has been clear, consistent evidence of carcinogenicity in a number of studies in which rodents (i.e., rats, mice, hamsters) were exposed to NDMA orally, via inhalation, or by intratracheal instillation. NDMA increased the incidence of liver and Leydig cell tumours in rats ingesting this nitrosamine from drinking- water or the diet (Terao et al., 1978; Arai et al., 1979; Ito et al., 1982; Lijinsky & Reuber, 1984); increased tumour incidences were noted at concentrations of NDMA of about 5 mg/litre in drinking-water and 10 mg/kg in the diet. Increased incidences of nasal, hepatic, pulmonary, and renal tumours were observed in rats exposed to NDMA via inhalation (Moiseev & Benemanskii, 1975; Klein et al., 1991); increases in the incidence of hepatic, pulmonary, and renal tumours were observed following exposure to NDMA at a concentration of 0.2 mg/m³ (Moiseev & Benemanskii, 1975). Hepatic, pulmonary, and renal carcinogenicity was observed in mice admin istered NDMA via drinking-water (Terracini et al., 1966; Clapp & Toya, 1970; Anderson et al., 1979, 1986, 1992) or through inhalation (Moiseev & Benemanskii, 1975); increases in tumour incidence were observed at concentrations of NDMA in drinking-water ranging from 0.01 to 5 mg/litre. Moreover, in some cases (e.g., Terracini et al., 1966), the period of exposure to NDMA was relatively short (i.e., 3 weeks). NDMA increased the incidence of liver tumours in hamsters exposed intratracheally (Tanaka et al., 1988). The administration of NDMA to pregnant rats (by intraperitoneal injection) or mice (by stomach tube) increased the frequency of hepatic and renal tumours in the offspring (Alexandrov, 1968; Anderson et al., 1989). An increased incidence of renal tumours has also been observed in rats administered either a single oral (Magee & Barnes, 1962) or intraperitoneal (Hard & Butler, 1970a; McLean & Magee, 1970) dose of NDMA (at levels of 30–60 mg/kg body weight).

In a more recently conducted comprehensive carcinogenicity bioassay (designed to provide detailed information on exposure–response) involving lifetime exposure, 15 dose groups of 60 male and 60 female Colworth-Wistar rats were provided with drinking-water containing a wide range of concentrations of NDMA⁸ (Tables 3 and 4) (Brantom, 1983; Peto et al., 1991a,b). The estimated daily intakes of NDMA ranged from 0.001 to 0.697 mg/kg body weight in the males and from 0.002 to 1.224 mg/kg body weight in the females. A control group of 120 males and 120 females received drinking-water without NDMA (Brantom, 1983; Peto et al., 1991a,b). Groups of animals were taken for interim sacrifice after 12 and 18 months of study. Survival of the animals was reduced with increasing dose; animals in the highest dose group did not survive longer than 1 year. There were no significant differences in body weight between the exposed animals and the controls. Dose-related increases in tumour incidence were observed only in the liver in both males and females (see Tables 3 and 4). The increase in tumour incidence was greatest for hepatocellular carcinoma and biliary cystadenoma. Non-neoplastic effects observed in the liver included hyperplastic nodules and the shrinkage of hepatocytes.

In numerous studies conducted in vitro in bacterial and mammalian cells, there has been overwhelming evidence that NDMA is mutagenic and clastogenic (reviewed in IARC, 1978; ATSDR, 1989). Increased frequencies of gene mutations, chromosomal damage, sister chromatid exchange, and unscheduled DNA synthesis have been observed in a wide variety of cell types, in assays conducted in the presence or absence of metabolic activation. Positive results have been observed in human as well as rodent cells.

Similarly, clear evidence of genetic effects has also been observed in in vivo studies. Clastogenic effects (e.g., micronuclei, sister chromatid exchange, chromosomal aberrations) in hepatocytes (Tates et al., 1980, 1983, 1986; Mehta et al., 1987; Braithwaite & Ashby, 1988; Cliet et al., 1989; Neft & Conner, 1989; Sawada et al., 1991), bone marrow cells (Bauknecht et al., 1977; Wild, 1978; Neal & Probst, 1983; Collaborative Study Group for the Micronucleus Test, 1986; Neft & Conner, 1989; Krishna et al., 1990; Sato et al., 1992; Morrison & Ashby, 1994), spleen cells (Neft & Conner, 1989; Krishna et al., 1990), and peripheral blood lymphocytes (Tates et al., 1983; Sato et al., 1992), as well as in oesophageal (Mehta et al., 1987) and kidney cells (Robbiano et al., 1997), have been observed in rodents (rats, mice, or hamsters) administered NDMA either orally or by intraperitoneal injection. Increased frequencies of micronucleated cells were observed at doses as low as 5 mg/kg body weight in rats (Trzos et al., 1978; Mehta et al., 1987). Effects in germ cells (i.e., micronucleated spermatids) were observed in mice given 6 or 9 mg NDMA/kg body weight via intraperitoneal injection (Cliet et al., 1993). The inhalation exposure of female mice to 1030 mg NDMA/m³ increased the frequency of micronucleated bone marrow cells (Odagiri et al., 1986). Evidence of genotoxicity (e.g., chromosomal aberrations, micronuclei, gene mutation, DNA strand breaks) has also been observed in the offspring of hamsters (Inui et al., 1979) and mice (Bolognesi et al., 1988) administered NDMA during gestation.

In rodents (rats, mice, or hamsters) administered NDMA either orally or by intraperitoneal injection, evidence of DNA damage has been observed in the liver, kidneys, and lungs (Laishes et al., 1975; Petzold & Swenberg, 1978; Abanobi et al., 1979; Mirsalis & Butterworth, 1980; Brambilla et al., 1981, 1987; Bermudez et al., 1982; Cesarone et al., 1982; Barbin et al., 1983; Doolittle et al., 1984; Kornbrust & Dietz, 1985; Loury et al., 1987; Mirsalis et al., 1989; Pool et al., 1990; Brendler et al., 1992; Jorquera et al., 1993; Asakura et al., 1994; Tinwell et al., 1994; Webster et al., 1996). DNA damage in thymus (Petzold & Swenberg, 1978), sperm (Cesarone et al., 1979), and nasal and tracheal cells (Doolittle et al., 1984) has also been noted. NDMA was mutagenic at the lacI locus (in the liver) in in vivo assays involving transgenic mice (Mirsalis et al., 1993; Tinwell et al., 1994; Butterworth et al., 1998). Effects (i.e., increased unscheduled hepatic DNA synthesis) have been observed in rats at doses as low as 0.1 mg NDMA/ kg body weight (Mirsalis & Butterworth, 1980).

Available data are inadequate as a basis for assessment of the reproductive or developmental toxicity of NDMA. Interpretation of the results of most identified investigations is complicated by the high doses administered, likely to have induced acute or repeated-dose organ toxicity. In a report by Anderson et al. (1978), time to conception in female mice provided with drinking-water containing 0.1 mg NDMA/litre for 75 days prior to mating was about 3 days longer than in unexposed controls; no other reproductive effects were assessed in this study. In a study conducted with male rats, a single intraperitoneal injection of 30 or 60 mg NDMA/kg body weight induced testicular damage (necrosis or degeneration of the seminiferous epithelium) (Hard & Butler, 1970b).

In a single-generation study (Anderson et al., 1978) in which the reproductive effects of a number of substances were examined, groups of 20 female mice were provided with drinking-water containing 0 or 0.1 mg NDMA/litre for 75 days prior to mating and throughout pregnancy and lactation (estimated daily and total intakes of 0.02 mg/kg body weight per day and 2 mg/kg body weight, respectively). The proportion of deaths (based upon the total number of stillborn and neonatal deaths) was increased (P < 0.05) 2-fold in the NDMA-exposed animals compared with controls (i.e., 20% and 9.9%, respectively), due in large part to an increase in the number of stillborn animals. Exposure to NDMA had no effect upon maternal fluid consumption, litter size, or average body weight of the weanlings, and no consistent gross or histopathological abnormalities were observed in the stillborn fetuses or dead neonates to account for the increased mortality. In a somewhat more recent study with mice administered higher doses of the nitrosamine, a single intraperitoneal injection of 37 mg NDMA/kg body weight on day 16 or 19 of gestation resulted in the deaths of all fetuses in exposed dams; information on maternal toxicity was not provided (Anderson et al., 1989). Notably, this dose is greater than the LD₅₀ for this route in these animals of 20 mg/kg body weight (IARC, 1978). In the same study, lethality was not observed following the administration of 7.4 mg NDMA/kg body weight (Anderson et al., 1989).

Fetal body weight was significantly (P < 0.05) reduced after a single oral dose of 20 mg NDMA/kg body weight was administered to pregnant rats on day 15 or 20 of gestation (Nishie, 1983). Although information on fetal survival or teratogenicity was not provided, toxic effects (reduced weight gain, hepatotoxicity, and death) were observed among the dams. Fetal deaths were noted in a number of studies (cited in ATSDR, 1989) conducted with rats in which NDMA was admin istered to pregnant dams 1) as a single oral dose (30 mg/kg body weight) on one of days 1–12 (Alexan drov, 1974) or 1–15 (Napalkov & Alexandrov, 1968) of gestation; 2) as repeated gavage doses of 1.4–2.9 mg/kg body weight per day for 7 or more days during gestation (Napalkov & Alexandrov, 1968); or 3) in the diet (intake of 5 mg/kg body weight per day) from an unspecified day of pregnancy to sacrifice on day 20 of gestation (Bhattacharyya, 1965). Although no teratogenic effects were reported in these studies, interpretation of these investigations is difficult owing to insufficient information on experimental design and results, lack of controls, and lack of information on maternal toxicity (ATSDR, 1989). The doses administered in some of these studies were close to the LD₅₀.

Data concerning effects on the brain or central nervous system in animals exposed to NDMA were not identified.

Similarly, available data are inadequate as a basis for assessment of the immunological effects of NDMA. Interpretation of the results of most identified investigations is complicated by likely toxicity associated with the high doses administered. In studies in which B6C3F₁ female mice were administered repeated intraperitoneal injections of 1.5, 3, or 5 mg NDMA/kg body weight per day for 14 days, observed effects on the immune system included suppression of humoral immunity with declines in the IgM antibody-forming cell response to sheep red blood cells and reductions in splenocyte proliferation in response to lipopolysaccharide (reviewed in Haggerty & Holsapple, 1990). Also observed were reductions in T-lymphocyte function (i.e., reduced cell-mediated immunity) with a decline in proliferative responses to various T-cell mitogenic stimuli, suppression of the mixed lymphocyte response, and selected delayed hypersensitivity responses, as well as significant reductions in host resistance to infection with Listeria monocytogenes, Streptococcus zooepidemicus, or the influenza virus or to challenge with B16F10 tumour cells. Reductions in antibody formation and in vitro lymphoproliferative responses were observed in male BALB/c mice admin istered 5 mg NDMA/kg body weight intraperitoneally for 14 days (Jeong & Lee, 1998).

Female CD-1 mice provided with drinking-water containing 5 or 10 mg NDMA/litre for 30–120 days exhibited marked suppression of humoral- and cell- mediated immunity (Desjardins et al., 1992); however, effects were reversible within 30 days of cessation of exposure. No effects were observed in animals consuming drinking-water containing 1 mg NDMA/litre.

There is strong evidence that the toxicological effects of NDMA are directly dependent upon the CYP2E1-dependent metabolic conversion of this nitrosamine to highly reactive species. Lee et al. (1996) attributed the hepatotoxicity of NDMA to the methyldiazonium ion formed via the alpha-hydroxylation pathway; denitrosation was considered to make little contribution to the overall hepatotoxic effect of this nitrosamine in rats. The principal DNA adduct formed following exposure to NDMA is N⁷-methylguanine (representing about 65% of all adducts formed initially upon exposure); O⁶-methylguanine is a secondary adduct (representing about 7% of all adducts formed initially). Other DNA adducts formed in smaller amounts include N³-methyladenine and O⁴-methylthymine.

N⁷-Methylguanine may undergo depurination yielding apurinic sites, which, if not repaired prior to DNA replication, can result in guanine to thymine transversions (Swenberg et al., 1991). O⁶-Methylguanine and O⁴-methylthymine (formed at about 1% of the amount of O⁶-methylguanine) are strongly promutagenic by direct mispairing. O⁶-Methylguanine gives rise to guanine:cyto sine to adenine:thymine (i.e., G:C to A:T) transitions, while O⁴-methylthymine causes A:T to G:C transitions (Swenberg et al., 1991; Souliotis et al., 1995).

Available data are consistent with the formation and persistence of the secondary adduct, O⁶-methyl guanine, being associated with both the carcinogenicity and mutagenicity of NDMA (reviewed in Haggerty & Holsapple, 1990; Swenberg et al., 1991; Souliotis et al., 1995). The ability of cells to repair DNA adducts (by removing O⁶-methylguanine through the action of a specific O⁶-methylguanine DNA-methyltransferase) prior to cell division likely plays a critical role in determining the susceptibility of tissues to tumour development.

In monkeys administered (orally) 0.1 mg NDMA/ kg body weight, O⁶-methylguanine was detected in 32 tissues examined (Anderson et al., 1996). The highest levels were in the gastric mucosa and liver, but elevated levels were also present in white blood cells, the oesophagus, ovaries, pancreas, bladder, and uterus. O⁶-Methylguanine DNA-methyltransferase activity varied over a 30-fold range; the highest activities were in the gastric mucosa, liver, kidneys, and lungs. The formation of O⁶- methylguanine was detected in fetal liver, lung, kidney, spleen, and brain in a study in which pregnant patas monkeys were administered (intragastrically) a single dose of 1 mg NDMA/kg body weight (Chhabra et al., 1995).

The greater persistence of O⁶-methylguanine DNA adducts in the kidney compared with the liver in rats administered a single oral dose of 20 mg NDMA/kg body weight parallels earlier findings in which the acute oral or intraperitoneal administration of NDMA to rats at such dose levels increased the incidence of kidney but not liver tumours (Magee & Barnes, 1962; Schmidt & Murphy, 1966; Hard & Butler, 1970a; McLean & Magee, 1970). In contrast, the long-term oral administration of low doses of NDMA (i.e., <2 mg/kg body weight per day) increased the incidence of liver but not kidney tumours in these animals (Brantom, 1983; Lijinsky & Reuber, 1984; Peto et al., 1991a,b), a finding attributed to the first-pass metabolism of NDMA in the liver (Swenberg et al., 1991).

There are quantitative age- and species-related differences in hepatic O⁶-methylguanine, possibly associated with variations in the activity of the transferase, consistent with observed variations in the carcinogenicity of the compound among species and strains exposed under various conditions. These include greater hepatic activity in adults versus newborn mice (Coccia et al., 1988), in rats versus mice (Lindamood et al., 1984), and between strains of mice (greater in C3H than in C57BL) (Lindamood et al., 1984).

Evidence supporting a role for O⁶-methylguanine formation in tumour development following exposure to NDMA was recently reviewed by Souliotis et al. (1995). G:C to A:T transitions have been observed in the ras oncogene in mouse lung tumours induced by NDMA (Devereux et al., 1991), in the livers of lacI transgenic mice administered a single dose of 4 mg NDMA/kg body weight (Mirsalis et al., 1993), and in the liver, kidney, and lung of lacI transgenic mice administered five daily doses of 1 mg NDMA/kg body weight (Wang et al., 1998). Moreover, transgenic mice expressing high levels of O⁶-methylguanine DNA-methyltransferase in the liver were less susceptible than normal controls to NDMA-induced hepatocarcinogenesis (Nakatsuru et al., 1993). However, Souliotis et al. (1995) also reported that the dose–response relationship for the accumulation of O⁶-methylguanine in hepatic DNA in rats administered drinking-water (for 28 days) containing concentrations of NDMA similar to those used in the study conducted at BIBRA Toxicology International (Brantom, 1983; Peto et al., 1991a,b) did not strictly parallel the dose–response for the development of hepatic tumours in the carcinogenicity bioassay.

Two deaths linked to the acute ingestion of NDMA, as well as a third attributed to the consumption of at least four doses of approximately 250–300 mg NDMA over a 2-year period, have been reported (Fussgänger & Ditschuneit, 1980; Pedal et al., 1982). Liver failure was observed in all three cases; the two acutely exposed decedents also exhibited cerebral haemorrhage. In two fatalities involving exposure to unknown concentrations of NDMA fumes, a tender and enlarged liver, splenic enlargement, abdominal distension, and the accumulation of yellow fluid in the peritoneal cavity were observed in one man prior to death (Freund, 1937); in the other death, liver cirrhosis was observed at autopsy (Hamilton & Hardy, 1974). In two other non-fatal cases involving exposure to NDMA fumes, effects included jaundice, the accumulation of fluid in the peritoneal cavity, exhaustion, headaches, abdominal cramps, soreness on the left side, nausea, and vomiting (Freund, 1937; Hamilton & Hardy, 1974).

Relevant epidemiological studies include case– control investigations in which the potential risks of cancer of the stomach (Risch et al., 1985; González et al., 1994; La Vecchia et al., 1995; Pobel et al., 1995), upper digestive tract (Rogers et al., 1995), and lung (Goodman et al., 1992; De Stefani et al., 1996) associated with the ingestion of NDMA have been assessed. In some of these reports (Goodman et al., 1992; González et al., 1994; Pobel et al., 1995), the estimated intake of NDMA was based upon recollection of an individual’s typical diet consumed in the year preceding the onset of illness, as well as the reported levels of this nitrosamine in the foodstuffs consumed, derived from other studies. In the studies conducted by De Stefani et al. (1996) and Rogers et al. (1995), subjects were asked to recall their typical diet in the 5 and 10 years, respectively, prior to the onset of illness.

In three of four case–control studies,⁹ there was a positive relationship with evidence of exposure–response for the intake of NDMA and gastric cancer (González et al., 1994; La Vecchia et al., 1995; Pobel et al., 1995), although not in an additional study in which oral, laryngeal, and oesophageal cancers were investigated separately (Rogers et al., 1995). In two case–control studies¹⁰ in which matching or control for confounders was rather more extensive than that for the investigations of gastric cancer mentioned above, there were clear exposure–response relationships for NDMA and lung cancer (Goodman et al., 1992; De Stefani et al., 1996). In almost all studies, associations between the cancers of interest and nitrate, nitrite, and NDMA were examined; results were relatively consistent in this regard, with there being an association with cancer most commonly with NDMA; results for nitrite were mixed, and there was an inverse association with nitrate.

In a population-based cohort study that assessed the risks of head and neck, stomach, and colorectal cancer associated with the dietary intake of NDMA, the relative risk (RR) of colorectal cancer was increased for the group having the highest intake of NDMA¹¹ (Knekt et al., 1999). The highest intake group had increased and reduced RRs of head and neck (RR = 1.37; 95% CI = 0.5–3.74) and stomach cancer (RR = 0.75; 95% CI = 0.37–1.51), respectively, compared with the lowest quartile (reference group).

There appears to be no qualitative difference between rodents and humans in the formation of DNA adducts following exposure to NDMA. In a case of suspected NDMA poisoning in a human male, methylation of liver DNA was evident at both the N⁷ and O⁶ positions of guanine (Herron & Shank, 1980). Using an immunohistochemical technique, Parsa et al. (1987) detected the formation of O⁶-methylguanine in human pancreatic explants incubated in vitro with NDMA.

Green algae (Selenastrum capricornutum) and blue-green algae (Anabaena flos-aqua) were exposed to NDMA over a 13-day period in static systems. The test was conducted to determine effects on algal growth rate, cell number, maximum standing crop, and dry weight. The 13-day EC₅₀s for growth were 4 mg/litre and 5.1 mg/litre for the green and blue-green algae, respectively (Draper & Brewer, 1979).

Draper & Brewer (1979) reported a 96-h LC₅₀ of 940 mg/litre for fathead minnow (Pimephales promelas) and a 96-h LC₅₀ of 1365 mg/litre for flatworms (Dugesia dorotocephala). For scud (Gammarus limnaeus), 96-h LC₅₀ values ranged from 280 to 445 mg/litre (Draper & Fisher, 1980). Both studies were conducted in static renewal systems.

The LC₅₀ values for a saltwater fish, the common mummichog (Fundulus heteroclitus), in a static non- renewal system were 8300 mg/litre at 24 h, 5500 mg/litre at 48 h, 4700 mg/litre at 72 h, 3300 mg/litre at 96 h, and 2700 mg/litre at 120 h (Ferraro et al., 1977).

Grieco et al. (1978) reported a dose-related increase in hepatocellular carcinomas in a study in which rainbow trout (Oncorhynchus mykiss) received 3, 200, 400, or 800 mg NDMA/kg in the diet over 52 weeks. Tumours did not form in trout receiving 3 mg/kg, although body weight was reduced. OME (1998) observed that growth reduction in rainbow trout was a more sensitive response than tumour induction.

Frogs (Rana temporaria) were exposed to 5 mg NDMA/litre in water for 63 days and 203 days. In both studies, the frogs developed hepatocellular carcinomas as well as adenomas and tumours of the haematopoietic system. Approximately 44% of th