National Toxicology Program

NTP Chemical Nomination Elements

1. Chemical Identification
   1. Chemical Abstract Service: Bentonite

   2. Common or generic name and synonyms: Wyoming bentonite, swelling (sodium) bentonite, non-swelling (calcium) bentonite

   3. CAS registry number: 1302-78-9

   4. Chemical class and related compounds: montmorillonite clay.

   5. Physical and chemical properties.
      1. Physical description: A clay, usually provided in a finely ground powdered form, but occasionally available as coarse particles i. e. cat litter, or as a moist clay.

      2. Structural and molecular formula and molecular weight: Bentonite is a montmorillonite (also referred to as smectite) mineral. It is a hydrated aluminum silicate formed by the alteration of volcanic ash

      3. Melting and boiling points: not available

      4. Solubility: not available

      5. Stability and reactivity: no reactivity information found.

      6. Other relevant information: Commercial product competition, Bentonite is a specialty clay product. Other clay products such as Kaolin are potential substitutes

   6. References

2. Production, use, occurrences, and analysis
   1. Production
      1. Source and synthesis: Bentonite is mined in Wyoming and in some southern states. European deposits of montmorillonite clay are also occasionally marketed as Bentonite.

      2. Current production: Approximately 4000 tons per year.

   2. Uses: Bentonite is used in drilling muds, foundry sands, and in pelletizing taconite ores. Bentonite is also used as an adsorbent and bleaching clay. Bentonite is also used in grouts.

   3. Occurrence in the environment
      1. Naturally occurring: Bentonite is a naturally occurring clay.

      2. Air water, and soil

   4. Analysis: none found

   5. References: US BOM Minerals handbook, 1985.

3. Toxicology
   1. Human data, case reports, and epidemiological studies: Review of chest films (not a random sample) showed 44% silicosis in bentonite workers (Phipps, Sundin and Mitchell, 1971). A number of other studies report pneumoconiosis in exposed workers after relatively short exposures (7 months to 8 years)(Rombola and Gaurdascione, 1955; Gattner, 1955; Mihajlova-Doceva et. al. 1986)

   2. Experimental animal information: Oral administration of bentonite to rats in an acute, subchronic and 6 month chronic study resulted in no observed adverse effects (Akhundov et. al., 1983)

   3. In vitro and other short-term tests: None found

   4. Other relevant information: None

   5. References:

      BIBRA profile on Bentonite. (available from NIOSH)

      Phipps, B. P., Sundin R. E., and Mitchell R. S., 1971. Silicosis in Wyoming

      bentonite workers. American Review of Respiratory Disease 103: 1 - 17.

      Akhundov V. Y., Akhundov K., Mamedova A., Dzhafarov S. M., Naumova N. N., 1983. Hygenic evaluation of bentonite and filtering material from Azerbaijan deposits. Izv. Akad. Nauk Az. SSR Ser Biol. Nauk 0(2) 115-123. (cited from abstract).

      Mijallova-Doceva et. al. 1986. Probl. Khig. 11, 106 (cited in Bibra)

      Gattner, 1955. Archiveve. Gewebbepath. 18,524 (cited in Bibra)

      Rombola and Guardiscione, 1955. Medna. Lav. 46, 480. (cited in Bibra)

4. Disposition and structure-activity relationship
   1. Absorption: not available
   2. Structure-activity correlations and considerations: not available
   3. References:

5. Ongoing, toxicological and environmental studies in the government, industry, and academe: none

6. Rationale for recommendation and suggested studies:
   Available reports note a relatively high incidence of fibrotic lung disease in workers exposed to Bentonite. Generally this elevated incidence rate has been ascribed to silica, which is present in varying degrees in bentonite. NIOSH reports in Occupational Respiratorv Disease that there is "some evidence that montmorillonite (bentonite) itself can cause pneumoconiosis but its fibrogenic potential is low." In England a United Kingdom expert committee has stated that recent studies indicate that quartz contamination is not the sole cause of bentonite pneumoconiosis (Bibra Tox. profile). Therefore, bentonite is recommended for chronic inhalation studies to determine its potential to cause fibrotic lung disease.