For definition of Groups, see Preamble Evaluation.
VOL.: 68 (1997) (p. 245)
Chem. Abstr. Name: Palygorskite
Palygorskite is a hydrated magnesium aluminium silicate, which occurs as a fibrous chain-structure mineral in clay deposits in several areas of the world. There is a major deposit of commercial importance in the United States. Palygorskite fibre characteristics vary with the source, but fibre lengths in commercial samples are generally less than 5 mm. Palygorskite has been mined since the 1930s and is used mainly as an absorbent for pet wastes and oils and greases and as a component of drilling muds. Occupational exposure to palygorskite occurs during its mining, milling, production and use. General population exposures also may occur in its use as pet waste absorbent, in fertilizers and pesticides and by ingestion of antidiarrhoeal preparations.
A single cohort study of palygorskite (attapulgite) miners and millers was available. It showed small excesses of mortality from lung cancer and stomach cancer, but no indications of any exposure-response for either cancer.
Samples of palygorskite from different regions vary considerably with regard to their fibre lengths. Results of studies in experimental animals suggest that carcinogenicity is dependent on the proportion of long fibres (> 5 mm) in the samples.
In one inhalation study in rats with palygorskite from Leicester, United Kingdom, in which about 20% of the fibres were longer than 6 mm, bronchoalveolar hyperplasia and a few benign and malignant alveolar tumours and mesotheliomas were observed. The same sample induced a high incidence of pleural mesotheliomas in rats after intrapleural administration. One sample from Torrejon, Spain, in which 0.5% of the fibres were longer than 6 mm, produced a significant increase in the incidence of pleural mesotheliomas after intrapleural injection.
In rats, intraperitoneal injection of a palygorskite sample (of unspecified origin and in which 30% of the fibres were longer than 5 mm) produced a high incidence of malignant abdominal tumours. A sample from Caceres, Spain, in which 3% of the fibres were longer than 5 mm, induced malignant abdominal tumours in rats after intraperitoneal injection.
Several studies involving exposures of rats by inhalation, intrapleural or intraperitoneal injection using samples originating from Lebrija (Spain), Mormoiron (France) and Attapulgus (GA, United States) employed materials with relatively short fibres (less than or 0.5% were longer or equal to 5 mm). In these studies, no significant increase in the incidence of tumours was observed.
Intratracheal instillation studies with palygorskite (attapulgite) fibres in sheep demonstrated significant and sustained inflammatory changes as measured in bronchoalveolar lavage fluids. These effects were mild compared to UICC chrysotile B but comparable to short chrysotile fibres. Intratracheal instillation studies in rats demonstrated that palygorskite (attapulgite) was less active than short chrysotile, UICC chrysotile B or aluminium silicate fibres but was more active than calcium silicate fibres. In-vitro studies have indicated that palygorskite can be toxic to mouse peritoneal and rat and rabbit alveolar macrophages.
In a single study, palygorskite did not show evidence for induction of sister chromatid exchange in rat pleural mesothelial cells.
There is inadequate evidence in humans for the carcinogenicity of palygorskite (attapulgite).
There is sufficient evidence in experimental animals for the carcinogenicity of long palygorskite (attapulgite) fibres (> 5 mm).
There is inadequate evidence in experimental animals for the carcinogenicity of short palygorskite (attapulgite) fibres (< 5 mm).
Long palygorskite (attapulgite) fibres (> 5 mm) are possibly carcinogenic to humans (Group 2B).
Short palygorskite (attapulgite) fibres (< 5 mm) cannot be classified as to their carcinogenicity to humans (Group 3).
For definition of the italicized terms, see Preamble Evaluation.
Previous evaluation: Suppl. 7 (1987) (p. 117)
See Also: Toxicological Abbreviations