SA Cage MSc M Inst Inf Sci
    SM Bradberry BSc MB MRCP
    S Meacham BSc

    National Poisons Information Service
    (Birmingham Centre),
    West Midlands Poisons Unit,
    City Hospital NHS Trust,
    Dudley Road,
    B18 7QH

    This monograph has been produced by staff of a National Poisons
    Information Service Centre in the United Kingdom.  The work was
    commissioned and funded by the UK Departments of Health, and was
    designed as a source of detailed information for use by poisons
    information centres.

    Peer review group: Directors of the UK National Poisons Information


    Toxbase summary


    Dermal and inhalational exposures are associated usually with no or
    only mild adverse effects. Following substantial ingestion, patients
    may develop coma, convulsions and severe muscle fasciculations and may
    take several days, occasionally weeks, to recover.

    Fatalities have occurred rarely after pyrethroid exposure, usually
    following ingestion (He et al, 1989). No fatalities have been reported
    after allethrin exposure.


    Dermal exposure

         -    Tingling and pruritus with blotchy erythema on the face or
              other exposed areas, exacerbated by sweating or touching. 
              Systemic toxicity may ensue following substantial exposure
              (see below).

    Ocular exposure

         -    Lacrimation and transient conjunctivitis may occur.


              Brief exposure:
         -    Respiratory tract irritation with cough, mild dyspnoea,
              sneezing and rhinorrhea.

              Substantial and prolonged exposure:
         -    Systemic toxicity may ensue - see below.


         -    May cause nausea, vomiting and abdominal pain.  Systemic
              toxicity may ensue following substantial ingestion (see

    Systemic toxicity

         -    Systemic symptoms may develop after widespread dermal
              exposure, prolonged inhalation or ingestion. Features
              include headache, dizziness, anorexia and hypersalivation.
         -    Severe poisoning is uncommon.  It usually follows
              substantial ingestion and causes impaired consciousness,
              muscle fasciculations, convulsions and, rarely, non-
              cardiogenic pulmonary oedema.

    Chronic exposure

         -    Long-term exposure is no more hazardous than short-term



         1.   Remove soiled clothing and wash contaminated skin with soap
              and water.
         2.   Institute symptomatic and supportive measures as required.
         3.   Topical vitamin E (tocopherol acetate) has been shown to
              reduce skin irritation if applied soon after exposure
              (Flannagan et al, 1985), but it is not available as a
              pharmaceutical product in the UK.
         4.   Symptoms usually resolve within 24 hours without specific


         1.   Irrigate with lukewarm water or 0.9 per cent saline for at
              least ten minutes.
         2.   A topical anaesthetic may be required for pain relief or to
              overcome blepharospasm.
         3.   Ensure no particles remain in the conjunctival recesses.
         4.   Use fluorescein stain if corneal damage is suspected.
         5.   If symptoms do not resolve following decontamination or if a
              significant abnormality is detected during examination, seek
              an ophthalmological opinion.


         1.   Remove to fresh air.
         2.   Institute symptomatic and supportive measures as required.


         1.   Do not undertake gastric lavage because solvents are present
              in some formulations and lavage may increase risk of
              aspiration pneumonia.
         2.   Institute symptomatic and supportive measures as required.
         3.   Atropine may be of value if hypersalivation is troublesome,
              0.6-1.2 mg for an adult, 0.02 mg/kg for a child.
         4.   Mechanical ventilation should be instituted if non-pulmonary
              oedema develops.
         5.   Isolated brief convulsions do not require treatment but
              intravenous diazepam should be given if seizures are
              prolonged or recur frequently. Rarely, it may be necessary
              to give intravenous phenytoin or to paralyze and ventilate
              the patient.


    Box SA, Lee MR.
    A systemic reaction following exposure to a pyrethroid insecticide.
    Hum Exp Toxicol 1996; 15: 389-90.

    Flannigan SA, Tucker SB, Key MM, Ross CE, Fairchild EJ, Grimes BA,
    Harrist RB.
    Synthetic pyrethroid insecticides: a dermatological evaluation.
    Br J Ind Med 1985; 42: 363-72.

    He F, Wang S, Liu L, Chen S, Zhang Z, Sun J.
    Clinical manifestations and diagnosis of acute pyrethroid poisoning.
    Arch Toxicol 1989; 63: 54-8.

    Lessenger JE.
    Five office workers inadvertently exposed to cypermethrin.
    J Toxicol Environ Health 1992; 35: 261-7.

    O'Malley M.
    Clinical evaluation of pesticide exposure and poisonings.
    Lancet 1997; 349: 1161-6.

    Substance name


    Origin of substance

         Allethrin was the first synthetic pyrethroid discovered to find
         practical application (Davies, 1985).  It was first synthesized
         in 1949 and first marketed in 1952 (IPCS, 1989a).  It is a
         racemic mixture of eight stereoisomers.

    Synonyms/Proprietary names

         Allyl cinerin 1
         Necarboxylic acid
         Pyresyn                                (RTECS, 1997)

    Chemical group

         Type I synthetic pyrethroid

    Reference numbers

         CAS       584-79-2                     (Pesticide Manual, 1997)
         RTECS     GZ1925000                    (RTECS, 1997)
         UN        2902
         EC        209-542-4                    (DOSE, 1997)

    Physicochemical properties

    Chemical structure

         IUPAC name: (RS)-3-allyl-2-methyl-4-oxocyclopent-2-enyl (1RS)-
         cis, trans- 2,2-dimethyl-3-(2-methyl prop-1-enyl)


    Molecular weight
         302.4                              (Pesticide Manual, 1997)

    Physical state at room temperature
         The technical grade is a liquid.   (Pesticide Manual, 1997)

         Pale yellow                        (Pesticide Manual, 1997)




         Practically insoluble in water
         Hexane 655 g/L (20°C)
         Methanol 71.3 x 103 g/L (20°C)   (Pesticide Manual, 1997)

    Autoignition temperature

    Chemical interactions

    Major products of combustion
         When heated to decomposition, allethrin emits acrid fumes
         potentially containing compounds such as formaldehyde and
         acrolein (Hartzell, 1996).

    Explosive limits
         Above 65.5°C, explosive vapour/air mixtures of bioallethrin may
         be formed.                         (ICSC, 1997)

         Burns with difficulty.             (HSDB, 1997)

    Boiling point
         281.5°C at 760 mm Hg               (Pesticide Manual, 1997)

         1.01 at 20°C                       (Pesticide Manual, 1997)

    Vapour pressure
         0.16 x 10 -2 Pa  at 21°C           (Pesticide Manual, 1997)

    Relative vapour density

    Flash point
         > 60 °C
         Flash points differ for different stereoisomers.    (HSDB, 1997)



         Allethrin is used for the control of flies, mosquitoes, ants and
         other household insect pests.  It is also used as an animal
         ectoparasiticide (Pesticide Manual, 1997).

    Hazard/risk classification

    Risk phrases
         R22 harmful if swallowed.          (DOSE, 1997)

    Safety phrases
         S2,S36 keep out of reach of children.
         Wear suitable protective clothing. (DOSE, 1997)


    Pyrethrins were developed as pesticides from extracts of dried and
    powdered flower heads of  Chrysanthemum cinerariaefolium. The active
    principles of these (see Fig. 1) are esters of chrysanthemumic acid
    (R1 = CH3) or pyrethric acid (R1 = CH3O2C) (both cyclopropane
    (three membered ring) carboxylic acids), with one of three
    cyclopentanone alcohols (cinerolone, R2 = CH3; jasomolone, R2 =
    CH2CH3; or pyrethrolone, R2 = CHCH2), giving six possible
    structures.  These natural pyrethrins have the disadvantage that they
    are rapidly decomposed by light.

    FIGURE 1

    Once the basic structure of the pyrethrins had been discovered, 
    synthetic analogues, pyrethroids, were developed and tested. Initially
    esters were produced using the same cyclopropane carboxylic acids,
    with variations in the alcohol portion of the compounds.

    Allethrin was produced in 1949 and was the first commercial synthetic
    pyrethroid,  followed in the 1960s by others including dimethrin,
    tetramethrin and resmethrin.  3-Phenoxybenzyl esters were also found
    to be active as pesticides (e.g. phenothrin, permethrin).  Synthetic
    pyrethroids with this basic cyclopropane carboxylic ester structure
    (and no cyano group substitution) are known as type I pyrethroids. In
    animal studies type I pyrethroids have been shown generally to produce
    a typical toxic syndrome (see below).

    The insecticidal activity of synthetic pyrethroids was enhanced
    further by the addition of a cyano group at the benzylic carbon atom
    to give alpha-cyano (type II) pyrethroids.  In animal studies type II
    pyrethroids have been shown generally to produce a typical toxic
    syndrome (see below).

    Despite the lack of the cyclopropane ring, similar insecticidal
    activity was found in a group of phenylacetic 3-phenoxybenzyl esters. 
    This led to the development of fenvalerate, an
    alpha-cyano-3-phenoxy-benzyl ester, and other related compounds such
    as fluvalinate.  These all contain the alpha-cyano group and hence are
    type II pyrethroids.

    Animal studies suggest that the two structural types of pyrethroids
    give rise generally to distinct patterns of systemic toxic effects. 
    Type I pyrethroids produce in animals the so-called "T (tremor)
    syndrome", characterized by tremors, prostration and altered "startle"
    reflexes. Type II (alpha-cyano) pyrethroids produce the so-called "CS
    (choreoathetosis/salivation) syndrome" with ataxia, convulsions,
    hyperactivity, choreoathetosis and profuse salivation being obserbed
    in experimental studies.

    These observations are consistent with some differences in the
    mechanisms of toxicity between type I and type II pyrethroids (see
    below) but the division of reactions by chemical structure is not
    exclusive.  Some compounds produce a combination of the two syndromes,
    and different stereoisomeric forms can produce different syndromes
    (Dorman and Beasley, 1991).  The classification into "T" and "CS"
    syndromes is not used clinically.

    Allethrin is manufactured as a racemic mixture of eight stereoisomers. 
    Some of these (d-allethrin, bioallethrin, esbiothrin and
    S-bioallethrin) are also available separately, reflecting their
    commercial importance.

    Further details are given in the pyrethroid generic monograph.


    In 1989-1990, world-wide annual production of pyrethroids was at least
    2000 tonnes (IPCS, 1989a; IPCS, 1989b; IPCS, 1989c; IPCS, 1990a; IPCS,
    1990b; IPCS, 1990c; IPCS, 1990d; IPCS, 1990e; IPCS, 1990f) including
    several hundred tonnes of allethrin and its isomers (IPCS, 1989a).

    In spite of their long history of use, there are relatively few
    reports of pyrethroid, and specifically allethrin, toxicity.  Less
    than ten deaths have been reported from ingestion or occupational
    (primarily dermal/inhalational) pyrethroid exposure with no deaths
    from allethrin exposure (He et al, 1989; Peter et al, 1996).


    In neuronal cells the generation of an action potential by membrane
    depolarization involves the opening of cell membrane sodium channels
    and a rapid increase in sodium influx.  The closure of sodium channels
    begins the process of action potential inactivation.  Delayed sodium
    channel closure thus increases cell membrane excitability.

    Pyrethroids modify the gating characteristics of voltage-sensitive
    sodium channels in mammalian and invertebrate neuronal membranes
    (Eells et al, 1992; Narahashi, 1989) to delay their closure. They are
    dissolved in the lipid phase of the membrane (Narahashi, 1996) and
    bind to a receptor site on the alpha sub-unit of the sodium channel
    (Trainer et al, 1997). This binding is to a different site from local
    anaesthetics, batrachotoxin, grayanotoxin, and tetrodotoxin
    (Narahashi, 1996).

    The interaction of pyrethroids with sodium channels is highly
    stereospecific (Soderlund and Bloomquist, 1989), with the 1R and 1S
     cis isomers binding competitively to one site and the 1R and 1S
     trans isomers binding non-competitively to another. The 1S forms do
    not modify channel function but do block the effect of the 1R isomers
    (Ray, 1991).

    The prolonged opening of sodium channels by the neurotoxic isomers of
    pyrethroids produces a protracted sodium influx which is referred to
    as a sodium "tail current" (Miyamoto et al, 1995; Soderlund and
    Bloomquist, 1989; Vijverberg and van den Bercken, 1982).  This lowers
    the threshold of sensory nerve fibres for the activation of further
    action potentials, leading to repetitive firing of sensory nerve
    endings (Vijverberg and van den Bercken, 1990) which may progress to
    hyperexcitation of the entire nervous system (Narahashi et al, 1995).
    At high pyrethroid concentrations, the sodium "tail current" may be
    sufficiently great to depolarize the nerve membrane completely,
    generating more open sodium channels (Eells et al, 1992) and
    eventually causing conduction block.

    Only low pyrethroid concentrations are necessary to modify sensory
    neurone function.  For example when tetramethrin was added to a
    preparation of rat cerebellar Purkinje neurons, only about 0.6-1 per
    cent of sodium channels needed to be modified to produce:

    (i)   Repetitive discharges in nerve fibres and nerve terminals;

    (ii)  An increase in discharges from sensory neurons (due to membrane
          depolarization) and

    (iii) Severe disturbances of synaptic transmission (Narahashi, 1989;
          Narahashi et al, 1995; Song and Narahashi, 1996).

    These effects on sodium channels are common to all pyrethroids
    although specific effects of type I pyrethroids, such as allethrin,
    have been clarified in experimental studies. These show that type I

    (i)   Keep sodium channels open (Narahashi, 1989);

    (ii)  Produce repetitive firing of sensory nerve endings (Soderlund
          and Bloomquist, 1989; Vijverberg and van den Bercken, 1982);

    (iii) Modify sodium channels in the resting or closed state so that
          they subsequently open more slowly (Dorman and Beasley, 1991);

    (iv)  Show a more pronounced positive temperature-dependent capacity
          for developing repetitive discharges (more likely to occur at
          higher temperatures) and negative temperature dependence for
          nerve-blocking action (more likely to occur at lower
          temperatures) (Clark and Marion, 1989; Dorman and Beasley, 1991;
          Narahashi, 1989);

    (v)   Produce effects on cultured neurons that are easily reversed by
          washing with a pyrethroid-free solution (Song et al, 1996).

    In human investigations, maximal conduction velocity in sensory nerve
    fibres of the sural nerve showed some increase in subjects exposed to
    pyrethroids, but there were no abnormal neurological signs, and other
    electrophysiological studies were normal in the arms and legs (Le
    Quesne et al, 1980). He et al (1991) assessed nerve excitability using
    an electromyograph and pairs of stimuli at variable intervals.  They
    showed a prolongation of the "supernormal period" in the median nerve
    in individuals who had been exposed to pyrethroids occupationally for
    three days.  The "supernormal period" was even more prolonged two days
    after cessation of exposure. (Note: the "supernormal period" is the
    period for which the action potential induced by a second stimulus is
    greater than the action potential produced by an initial stimulus).

    Pyrethroids are some 2250 times more toxic to insects than mammals. 
    This can be explained in terms of differences in their potency as
    neuronal toxins and differences in rates of detoxification between
    invertebrates and vertebrates (Narahashi, 1996; Narahashi et al, 1995;
    Song and Narahashi, 1996).

    The sensitivity of invertebrate neuronal sodium channels to
    pyrethroids is ten times greater than in mammals (Song and Narahashi,
    1996).  Furthermore, invertebrates typically have body temperatures
    some 10°C lower than mammals and  in vitro studies show tetramethrin
    to be more potent at evoking repetitive neuronal discharges at lower
    temperatures (Song and Narahashi, 1996).  In these experiments it was
    noted that the recovery of sodium channels from tetramethrin
    intoxication after washing was some five times faster in mammals than
    invertebrates.  In addition pyrethroid hepatic metabolism
    (detoxification) is faster in mammals.  Finally small insect size
    increases the likelihood of end-organ (neuronal) toxicity prior to
    detoxification (Song and Narahashi, 1996).


    In addition to the important differences between invertebrates and
    vertebrates outlined above, the low toxicity of pyrethroid
    insecticides in mammals is due to poor dermal absorption (the main
    route of exposure) and metabolism to non-toxic metabolites (Bradbury
    and Coats, 1989).



    Based on excretion studies for other pyrethroids (Nassif et al, 1980;
    Chester et al, 1987; Eadsforth et al, 1988; van der Rhee et al, 1989;
    IPCS 1990f; Woollen et al, 1991; Woollen et al, 1992), dermal
    absorption of allethrin is likely to be low (less than 1.5 per cent),
    though there are no human data specific to allethrin.


    Between 19 and 57 per cent of orally administered cypermethrin (a type
    II pyrethroid) was absorbed in human studies (Woollen et al, 1991;
    Woollen et al, 1992).  There are no human data specific to allethrin.


    Pyrethroids are hydrolyzed rapidly in the liver to their inactive acid
    and alcohol components (Hutson, 1979; Ray, 1991), probably by
    microsomal carboxylesterase (Hutson, 1979). Further degradation and
    hydroxylation of the alcohol at the 4' position then occurs, and
    oxidation produces a wide range of metabolites (Hutson, 1979; Leahey,

    There is some stereospecificity in metabolism, with  trans-isomers
    being hydrolyzed more rapidly than the  cis-isomers, for which
    oxidation is the more important metabolic pathway (Soderlund and
    Casida, 1977).

    The pattern of metabolites varies between oral and dermal dosing in
    humans (Wilkes et al, 1993). For example, following dermal dosing with
    cypermethrin (a type II pyrethroid) the ratio of  trans/cis 
    cyclopropane acids excreted was approximately 1:1, compared to 2:1
    after oral administration. Such measurements might be useful in
    determining the route of exposure (Woollen, 1993; Woollen et al, 1991;
    Woollen et al, 1992).

    Animal studies have shown that pyrethroid hydrolysis is inhibited by
    dialkylphosphorylating agents such as organophosphorus insecticides
    (Abou-Donia et al, 1996; He et al, 1990; Hutson, 1979), and urinary
    excretion of unchanged pyrethroid was higher in sprayers using a
    methamidophos/ deltamethrin or methamidophos/fenvalerate mixture than
    from those using the pyrethroid alone (Zhang et al, 1991).

    Experiments with chickens (Abou-Donia et al, 1996) showed that
    pyrethroid (permethrin) toxicity was also enhanced by pyridostigmine
    bromide and by the insect repellent N,N-diethyl-m-toluamide (DEET). 
    The authors hypothesized that competition for hepatic and plasma
    esterases by these compounds led to decreased pyrethroid breakdown and
    increased transport of the pyrethroid to neural tissues.


    Allethrin is excreted mainly as metabolites in urine but a proportion
    is excreted unchanged in faeces. An overview of human pyrethroid
    elimination data is included in the generic pyrethroid monograph
    though there are no human data specific to allethrin.


    Occupationally, the main route of allethrin absorption is through the
    skin; inhalation is much less important (Adamis et al, 1985; Chen et
    al, 1991; Zhang et al, 1991). Inhalation is more likely when allethrin
    is used in confined spaces (Llewellyn et al, 1996).  The use of
    protective clothing can reduce dermal exposure (Chester et al, 1987). 
    The physical formulation also affects exposure, with inhalation being
    more important for dust and powder formulations, and dermal exposure
    more important for liquids (Llewellyn et al, 1996).

    Dermal exposure

    This is the most common route of pyrethroid exposure. Adverse effects
    manifest primarily as peripheral neurotoxicity with reversible
    hyperactivity of sensory nerve fibres (paraesthesiae), though erythema
    and pruritus are also described (see below).

    Peripheral neurotoxicity

    Paraesthesiae have been reported usually following inappropriate
    pyrethroid handling though there are no reports specific to allethrin.
    Paraesthesiae occur most commonly on the face (He et al, 1989).  It
    seems probable that paraesthesiae are related to the repetitive firing
    of sensory nerve endings in contaminated skin (Aldridge, 1990) and not
    to inflammation as there is little effect on neurogenic vasodilatation
    (Flannigan and Tucker, 1985b). The symptoms are exacerbated by sensory
    stimulation (heat, sun, scratching (Aldridge, 1990), sweating or
    application of water and may prevent sleep (Tucker and Flannigan,

    Paraesthesiae generally start 30 minutes to two hours after exposure
    and peak after about six hours. Recovery is usually complete within 24
    hours (Aldridge, 1990; He et al, 1989).

    Dermal toxicity

    When used at recommended doses in the treatment of scabies and lice,
    pyrethroids only rarely produce adverse effects.  Pruritus is the
    side-effect reported most frequently (Brandenburg et al, 1986;
    DiNapoli et al, 1988), although this may also be caused by the skin
    infestation being treated.

    Skin irritation during occupational pyrethroid exposure may occur in
    up to ten per cent of workers (Kolmodin-Hedman et al, 1982) and may be
    influenced by the ratio of stereoisomers used in the pyrethroid
    formulation being more prevalent with a higher proportion of the
     trans isomer.  In addition to pruritus, erythema, burning and
    blisters have been reported (Brandenburg et al, 1986; Kalter et al,
    1987; IPCS, 1990b; Kolmodin-Hedman et al, 1995).

    There are no reports of skin irritation following allethrin exposure.
    Lisi (1992) assessed 230 volunteers for irritant or delayed contact
    sensitivity reactions to a range of pyrethroids.  Two (non-atopic)
    patients had irritant reactions to five per cent resmethrin and a
    further two had positive patch tests to one per cent fenvalerate. 
    There were no positive reactions to allethrin, deltamethrin, fenothrin
    or permethrin.

    Ocular exposure

    Symptoms of mild eye irritation have been reported following
    occupational pyrethroid exposure (Kolmodin-Hedman et al, 1982; IPCS,
    1990d; Lessenger, 1992) though there are no reports specific to


    Inhalational pyrethroid exposure typically is occupational and
    produces symptoms and signs of pulmonary tract irritation. The
    frequency and severity of symptoms may vary with the ratio of
    different stereoisomers in a formulation, being more prevalent with a
    higher proportion of the  trans isomer. Systemic effects may occur
    following more substantial exposure (He et al, 1989) and are described
    below.  There are no reports of inhalational toxicity specific to
    allethrin exposure.


    Pyrethroid ingestion typically gives rise to nausea, vomiting and
    abdominal pain within minutes.  In one series (He et al, 1989)
    involving some 344 cases, vomiting was a prominent feature in 56.8 per
    cent. In one case, permethrin/pyrethrins accidentally sprayed directly
    into the mouth resulted in a burning sensation which commenced several
    hours after exposure, and only gradually improved over five months,
    with persistent disordered taste sensation (Grant, 1993).

    Substantial pyrethroid ingestion may give rise to neurological
    features and other systemic effects as discussed below.

    There are no reports specific to allethrin ingestion.

    Systemic effects

    Systemic effects generally have occurred after inappropriate
    occupational handling of pyrethroids.  This may involve using too
    concentrated solutions, prolonged exposure, spraying against the wind
    or using unprotected hands or mouth to unblock congested sprayers (He
    et al, 1989).  Most reported cases have involved  dermal, inhalational
    and sometimes also oral exposure to fenveralate, deltamethrin or
    cypermethrin with systemic features occurring between four and 48
    hours after spraying (He et al, 1989).  Intentional ingestion may also
    produce systemic effects (He et al, 1989; Peter et al, 1996).  Most
    patients recover over two to four days with only seven fatalities

    among 573 cases in one review (He et al, 1989). Four of the seven
    fatalities developed convulsions, one patient died from
    non-cardiogenic pulmonary oedema, one from "atropine intoxication" and
    one death followed exposure to a pyrethroid/organophosphorus pesticide

    A further death has been reported recently in a patient who became
    comatose within ten hours of 30 mL deltamethrin ingestion and died
    from aspiration pneumonia complicated by renal failure (Peter et al,

    There are no reports of systemic toxicity following allethrin

    Gastrointestinal toxicity

    As discussed above gastrointestinal irritation is common following
    pyrethroid ingestion.  Vomiting was a prominent symptom also in 16 per
    cent of  occupational cases (He et al, 1989) in whom ingestion was
    not suspected, but where exposure involved deltamethrin, cypermethrin
    or fenvalerate. In this review, which included occupational exposures,
    anorexia occurred in 45 per cent of 573 cases of acute pyrethroid
    poisoning (He et al, 1989).


    He et al (1989) described dizziness in 60.6 per cent, headache in 44.5
    per cent, fatigue in 26 per cent, increased salivation in 20 per cent
    and blurred vision in seven per cent of 573 cases of acute pyrethroid
    poisoning (229 occupational and 344 accidental exposures).  None
    specifically involved allethrin.

    Limb muscle fasciculations, coma and convulsions may complicate severe
    acute pyrethroid poisoning, and have occurred as soon as 20 minutes
    after ingestion (He et al, 1989). "Convulsions" was the stated cause
    of death in four of seven fatalities among 573 cases of acute
    pyrethroid poisoning (He et al, 1989) but further details were not

    An electromyelogram (EMG) in one case of acute pyrethroid (not
    specified) poisoning showed repetitive muscle discharges without
    denervation potentials (He et al, 1989).

    There is animal evidence that the neurotoxicity of permethrin is
    increased by pyridostigmine and by DEET (Abou-Donia et al, 1996;
    McCain et al, 1997).

    Cardiovascular toxicity

    Palpitation was reported in 13.1 per cent of 573 cases of acute
    pyrethroid poisoning involving oral, inhalational and dermal exposure
    to deltamethrin, fenvalerate or cypermethrin (He et al, 1989).  An
    electrocardiogram (ECG) showed ST and T wave changes in eight of 71

    patients.  Other ECG abnormalities included sinus tachycardia,
    ventricular ectopics and (rarely) sinus bradycardia (He et al, 1989).
    All ECG changes resolved in 2-14 days.

    Pulmonary toxicity

    Chest tightness has been described following accidental or deliberate
    pyrethroid ingestion (He et al, 1989).

    Non-cardiogenic pulmonary oedema has been reported rarely following
    substantial pyrethroid ingestion usually in association with severe
    neurological complications, and may contribute to a fatal outcome (He
    et al, 1989).

    Musculoskeletal toxicity

    A case of acute polyarthralgia after skin exposure to flumethrin (a
    type II pyrethroid) has been reported recently (Box and Lee, 1996).


    Among 235 cases of occupational or accidental acute pyrethroid
    poisoning (none specifically involving allethrin) in whom a full blood
    count was performed, 15 per cent showed a leucocytosis (He et al,
    1989); this was probably a non-specific response.


    Urinalysis among 124 patients with acute pyrethroid poisoning
    (involving oral, dermal and inhalational exposure) showed three
    patients with haematuria (He et al, 1989).


    There are no reports involving chronic allethrin exposure.

    Dermal exposure

    Few long-term adverse effects from pyrethroids have been reported
    (IPCS, 1990d; Chen et al, 1991, He, 1994).  There is no confirmed
    evidence that repeated exposure to pyrethroids leads to permanent
    damage to sensory nerve endings (Vijverberg and van den Bercken,

    In a study of 199 workers exposed for several months to deltamethrin,
    fenvalerate and cypermethrin (all type II pyrethroids) in a packaging
    plant, the symptoms described were identical to those following acute
    pyrethroid exposure and did not last more than 24 hours once subjects
    were away from the work environment (He et al, 1988). This suggests
    there are no true  chronic effects from repeated pyrethroid exposure.


    Sixty-four (32 per cent) of the 199 workers described above complained
    of sneezing and increased nasal secretions but these symptoms were
    only present at work, against suggesting no difference in effect
    between chronic or acute pyrethroid exposure.  Systemic symptoms of
    dizziness, fatigue and nausea were mild and reported by only 14, nine
    and 10 per cent of workers respectively.


    Dermal exposure


    Clothes contaminated with allethrin should be removed, and
    contaminated skin washed with soap and water (He, 1994).

    Specific measures

    Topical alpha tocopherol (vitamin E) to treat paraesthesiae

    As paraesthesiae usually resolve in 12-24 hours, specific treatment is
    not generally administered or required.  However, the topical
    application of  dl-alpha tocopherol acetate (vitamin E) has been
    shown to reduce the severity of skin reactions to other pyrethroids
    including fenvalerate (IPCS, 1990c; Tucker et al, 1984; Tucker et al,
    1983), flucythrinate, permethrin and cypermethrin (Flannigan and
    Tucker, 1985a).  The reaction to cypermethrin was completely inhibited
    by vitamin E (Flannigan et al, 1985). Vitamin E appears to be useful
    both prophylactically and therapeutically (Flannigan and Tucker,
    1985a).  In a controlled human volunteer study, a commercial vitamin E
    oil preparation produced 98 per cent inhibition of the cutaneous
    symptoms from fenvalerate when applied immediately (Flannigan et al,
    1985).  At four hours the inhibition was only 50 per cent (Advisory
    Committee on Pesticides, 1992). The mechanism of the effect of topical
    vitamin E has not been clarified, although some  in vitro studies
    suggest vitamin E may block the pyrethroid-induced  sodium "tail
    current" in neuronal membranes (Song and Narahashi, 1995).

    Vitamin E is not included in the British National Formulary but is
    available from health food or alternative medicine sources.

    Other agents to treat paraesthesiae

    Various other topical therapies have been tested for treatment of
    pyrethroid-induced paraesthesiae: in human trials mineral oil, corn
    oil and "A&D ointment" (Tucker et al, 1984; Tucker et al, 1983) were
    almost as effective as Vitamin E cream (but the oils may lead to
    defatting of skin). Butylated hydroxyanisole and an industrial barrier
    cream (Tucker et al, 1984) and topical indomethacin (Flannigan and
    Tucker, 1984), were of little therapeutic benefit and in two studies
    zinc oxide paste exacerbated paraesthesiae (Tucker et al, 1984; Tucker
    et al, 1983) .

    Ocular exposure

    Irrigate the affected eye with lukewarm water or 0.9 per cent saline
    for at least ten minutes.  A topical anaesthetic may be required for
    pain relief or to overcome blepharospasm.  Ensure no particles remain
    in the conjunctival recesses.  Use fluorescein if corneal damage is
    suspected.  If symptoms do not resolve following decontamination or if
    a significant abnormality is detected during examination, seek an
    ophthalmological opinion.


    Removal from exposure is the priority.  Mild symptoms of rhinitis
    respond to oral antihistamines.  Other symptomatic and supportive
    measures should be dictated by the patientśs condition.


    Gut decontamination

    Gastric lavage should be avoided since solvents present in allethrin
    formulations may increase the risk of aspiration pneumonia.

    Systemic toxicity

    Most patients exposed to allethrin require only simple supportive
    care.  Systemic toxicity is rare but in such patients the presence of
    excess salivation, muscle fasciculations and pulmonary oedema may
    present diagnostic difficulty since similar features are typical also
    of severe organophosphorus pesticide poisoning. Measurement of the red
    cell cholinesterase activity (which is reduced in acute
    organophosphorus poisoning but not in pyrethroid intoxication) allows
    clarification but may not be available rapidly.

    Isolated brief convulsions do not require treatment but intravenous
    diazepam 5-10 mg should be given if seizures are prolonged.  Rarely it
    may be necessary to give intravenous phenytoin, or to paralyze and
    ventilate the patient.  Diazepam is useful also in the treatment of
    muscle fasciculations. The role of atropine is discussed below.

    Several experimental studies have investigated the role of
    pharmaceuticals in the management of the neurological complications of
    severe pyrethroid poisoning. However, these should be interpreted with
    caution, not only because they usually have involved high-dose
    parenteral pyrethroid administration, but also because there is
    considerable interspecies variation with regard to therapeutic
    efficacy (Casida et al, 1983; Vijverberg and van den Bercken, 1990).

    Atropine for hypersalivation and pulmonary oedema

    In experimental studies atropine sulphate (25 mg/kg subcutaneously)
    reduced hypersalivation produced by oral fenvalerate or cypermethrin
    (each at a dose exceeding the LD50), but did not increase survival
    (Hiromori et al, 1986).

    Intravenous atropine (0.6-1.2 mg in an adult) may be useful to control
    excess salivation but care should be taken to avoid excess
    administration.  In a review of pyrethroid poisoning cases reported
    from China (He et al, 1989), 189 of 573 patients were treated with
    atropine which led to an improvement in salivation and pulmonary
    oedema in a few severe cases, but eight patients developed atropine
    intoxication following intravenous administration of 12-75 mg. One
    patient, probably misdiagnosed as having acute organophosphorus
    insecticide poisoning, died of atropine intoxication after a total
    dose of 510 mg, and one patient acutely intoxicated with a
    fenvalerate/dimethoate mixture could not be revived despite a total
    atropine dose of 170 mg.

    Atropine and ethylcarbamate

    In a French study a combination of intravenous atropine 3 mg/kg and
    ethylcarbamate 1000 mg/kg effectively protected rodents against the
    lethal effects of intravenous deltamethrin, increasing the LD50 by a
    factor of 3.48 (Leclercq et al, 1986).

    Diazepam and phenobarbital for convulsions

    In mice (n=10) pre-treatment with intraperitoneal diazepam (1 mg/kg),
    but not phenobarbital (10-30 mg/kg), significantly increased the time
    to onset of convulsions caused by the intracerebroventricular
    administration of deltamethrin (p< 0.005) and fenvalerate (p<0.05)
    (Gammon et al, 1982).  Under the same conditions diazepam was not
    effective in preventing permethrin- or allethrin-induced seizures.

    Propranolol and procainamide for tremor

    Pre-treatment with intravenous propranolol or procainamide (each 15
    µmol/kg) reduced the severity of tremor or writhing induced in rats by
    the intravenous administration of deltamethrin (10 µmol/kg) (Bradbury
    et al, 1983).

    Ivermectin and pentobarbital for choreoathetosis

    In rodents administered 2 mg/kg intravenous deltamethrin,
    pre-treatment with 4 mg/kg intravenous ivermectin reduced
    choreoathetosis from 3.9 to 3.2 (as graded on a scale of 1-4) (p =
    0.023), and reduced salivation by 72 per cent. Pentobarbital (15 mg/kg
    i.p.) reduced choreoathetosis produced by 1.5 mg/kg intravenous
    deltamethrin from 3.0 to 1.3 (p = 0.004). An equi-sedative dose of
    phenobarbital produced a non-significant fall to 2.4 (p = 0.11)
    (Forshaw and Ray, 1997).

    Mephenesin and methocarbamol

    The skeletal muscle relaxant mephenesin 22 µmol/kg prevented all motor
    symptoms induced in rats by the intravenous administration of
    deltamethrin (10 µmol/kg) (n=4-20 in different treatment groups)
    (Bradbury et al, 1983).

    Mephenesin has a short half-life in vivo, but intraperitoneal
    methocarbamol (a mephenesin derivative) (400 mg/kg intraperitoneally
    followed by 200 mg/kg whenever tremor was observed) significantly
    (p<0.01) reduced mortality in rats administered more than the oral
    LD50 of fenvalerate, fenpropathrin, cypermethrin or permethrin (n=10
    in each treatment group) (Hiromori et al, 1986).

    There are insufficient data to advocate a clinical role for
    methocarbamol in systemic pyrethroid toxicity.

    Sodium-channel blockers (local anaesthetics)

     In vitro studies suggest local anaesthetics may be useful as
    antagonists of the effect of deltamethrin on sodium channels
    (Oortgiesen et al, 1990). The relevance to human poisoning is not


    Avoiding dermal and inhalational exposure via adequate self-protection
    and sensible use is the most important requirement to reduce adverse
    effects from occupational use of allethrin.


    Maximum exposure limit

    International Standards Organization (ISO) limits for natural
    pyrethrins: long-term exposure limit (8 hour TWA reference period) 5
    mg/m3; short-term exposure limit (15 min reference period) 10 mg/m3 
    (Health and Safety Executive, 1995).


    Endocrine toxicity

     In vitro studies show that several other pyrethroids interact
    competitively with human skin fibroblast androgen receptors and with
    sex hormone binding globulin (Eil and Nisula, 1990).  A possible
    anti-androgenic effect of pyrethroids in humans was suggested
    following an outbreak of gynaecomastia in refugees exposed to
    fenothrin, but there was insufficient evidence to confirm this (Eil
    and Nisula, 1990).

    Animal studies evaluating other endocrine effects of pyrethroids have
    produced conflicting results (Akhtar et al, 1996; Kaul et al, 1996).


    In oral dosing studies in rodents, several pyrethroids suppressed the
    cellular immune response (Blaylock et al, 1995; Tulinská et al, 1995)
    or produced thymus atrophy (Madsen et al, 1996).

    The significance of these immunological studies to man is not known
    and there are no data specific to allethrin.


    In animal studies no oncogenic effects were observed at any dose level
    for allethrin and its isomers (IPCS, 1989a).


    In animal studies there was no evidence that allethrin (and isomers)
    was teratogenic, embryotoxic or fetotoxic (IPCS, 1989a).


    Data regarding the potential genotoxicity of pyrethroids provide
    conflicting results although in a review of available  in vitro and
     in vivo data it was concluded that allethrin was not mutagenic or
    genotoxic (IPCS, 1989a).

    Fish toxicity

    Allethrin is toxic to fish.  It is more toxic at cooler temperatures,
    and thus more toxic to cold than warm water fish, but the toxicity of
    pyrethroids is little affected by pH or water hardness (Mauck et al,

    LC50 (96 hr) for channel catfish, bluegill sunfish, rainbow trout,
    steelhead and salmon are between 10 and 56 µg/L allethrin (DOSE,

    EC Directive on Drinking Water Quality 80/778/EEC

    Maximum admissible concentration (any pesticide) 0.1 µg/L (EC
    Directive, 1980).

    SA Cage MSc M Inst Inf Sci
    SM Bradberry BSc MB MRCP
    S Meacham BSc

    National Poisons Information Service (Birmingham Centre),
    West Midlands Poisons Unit,
    City Hospital NHS Trust,
    Dudley Road,
    B18 7QH

    This monograph was produced by the staff of the Birmingham Centre of
    the National Poisons Information Service in the United Kingdom.  The
    work was commissioned and funded by the UK Departments of Health, and
    was designed as a source of detailed information for use by poisons
    information centres.

    Date of last revision


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