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The California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA), as the lead agency for the implementation of the Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65 or the Act), has received a request from the Sorptive Minerals Institute (SMI, Washington, DC) that OEHHA grant a safe use determination for packaged sorptive mineral-based pet litter containing crystalline silica, pursuant to its authority under Section 12104 of Title 22 of the California Code of Regulations (22 CCR). The products that are the subject of the request are clay-based pet litters that are purchased and poured by the consumer, used by the pet, disposed of by the consumer, and replenished by the consumer. Crystalline silica (airborne particles of respirable size) has been listed under Proposition 65 as a chemical known to the State to cause cancer since October 1, 1988.

In accordance with the process set forth in Section 12104(f) of 22 CCR, public comment on this request was solicited on November 27, 1998, particularly with respect to the potential for exposure to airborne particles of crystalline silica of respirable size that may result from the use of pet litter commercially produced from clay-containing mineral deposits as in packaged sorptive mineral-based pet litter. A public hearing was convened on the last day of the 30-day comment period, January 6, 1999, in Sacramento, California.

Based on the results of a screening level assessment using testing data submitted by SMI on the amounts of respirable crystalline silica produced from the normal use of both conventional and scoopable pet litter products, and a review of the comments submitted by the public, OEHHA has decided to issue a safe use determination to SMI for crystalline silica in mineral-based pet litter. The essential elements and results of this screening level assessment are described below.

SMI provided testing data on 12 conventional pet litters and nine scoopable pet litters originating from clay deposits from different parts of the country (termed East and West Coast litters by SMI). Briefly, the key data consisted of laboratory measurements over time of respirable dust (<10 µm) generated from activities related to the use of pet litter including initial pouring, clump removal (in the case of scoopable pet litter) and replenishment. Respirable dust collected from the different activities was further evaluated by SMI for quartz content. Quartz was assumed to be the only form of crystalline silica in pet litter. Data provided by the requester also included parameters on normal use in terms of time and amount used by the average consumer. In accordance with the process set forth in Section 12104(f) of 22 CCR, OEHHA has previously deemed the submitted data adequate and appropriate for performing this screening level assessment.

The primary sources of exposure to dust containing crystalline silica from the use of conventional pet litter are from the filling of the litter pan and subsequent disposal following use. Using the data generated during 10 pound pour tests measuring the respirable dust generated over time (up to three minutes), the quartz content of the respirable dust, and the annual use and frequency of activities related to the use of conventional pet litters, average yearly exposure levels were estimated for each of the 12 products for which testing data were submitted. The testing data indicate that the average concentration of exposure to respirable crystalline silica from the use of conventional litter ranges from 0.0007 to 0.01 µg/m3.

The primary sources of exposure to dust containing crystalline silica from the use of scoopable pet litter are from the filling of the litter pan, clump removal, replenishment of removed litter, and subsequent disposal following use. Data provided by SMI for scoopable pet litters included measurements over time (up to ~2˝ minutes) of respirable dust generated from 10 pound pour tests, clump removal tests and tests of clump removal with replenishment of litter. Parameters on the annual use and frequency of activities related to the use of scoopable pet litters were also provided. Using this information, average yearly exposure levels were estimated for each of the nine products for which testing data were submitted. The testing data indicate that the average concentration of exposure to respirable crystalline silica from the use of scoopable litter ranges from 0.002 to 0.06 µg/m3.

A number of factors may tend to increase or decrease estimates of exposure relative to the approach used to develop the exposure levels described above. We believe, on the whole, that the assumptions made are likely to have resulted in overestimates of exposure levels for the average user of pet litter.

For the estimation of risk of cancer from the exposure to crystalline silica, readily available potency estimates in the scientific literature were used. Estimates derived from epidemiological studies were considered to be most appropriate for this screening evaluation of carcinogenic risk. Goldsmith et al. (1995; Scand J Work Environ Health 21(Suppl 2):104-7) has developed estimates of risk based upon the increase in lung cancers observed in epidemiological studies of gold miners in South Africa and diatomaceous earth workers in California. Cancer slope factors derived from these studies ranged from 6.8 ´ 10-7 to 1.85 ´ 10-5 for exposure to 1 µg/m3 silica dust and were derived using occupational exposure assumptions of 40 years of employment, eight hour workshifts, 50 hour workweeks, and 50 workweeks per year. Based on these estimates, concentrations associated with excess cancer risk of one in 100,000 would range from 0.54 to 15 µg/m3 silica dust. Active research is being conducted with respect to the relationship between silicosis and lung cancer in humans, the contribution of reactive oxygen species to the development of malignancy, and the "biological activity" of crystalline silica. Additional data and increased confidence in information regarding crystalline silica’s mode of action in the induction of human malignancy will likely lead to reductions in cancer potency estimates. Thus we expect that the upper end of the screening cancer slope factors presented above represent "worst case" estimates of the true low dose cancer slope factor for crystalline silica present in clay-based pet litter.

With the assumption that a consumer uses a single product throughout his/her lifetime, the highest estimate produced is an annual time weighted average exposure of 0.06 µg/m3 quartz dust for a scoopable litter product. This exposure level is approximately nine-fold lower than the lowest concentration (0.54 µg/m3) associated with one in 100,000 persons exposed derived from occupational epidemiological studies. Thus, the estimated exposure of the average user of either conventional or scoopable pet litter to crystalline silica under the conditions used to produce this assessment corresponds to an excess cancer risk of less than one in 100,000, and would not trigger the Proposition 65 warning requirement.

As stated in 22 CCR, Section 12104 (a) and (k), this safe use determination is specific to the requester, SMI, and for crystalline silica in packaged sorptive mineral-based pet litter as used in accordance with the facts presented in the request, and is advisory only. Moreover, the issuance of a safe use determination does not affect the authority of the Attorney General, district attorneys, certain city attorneys and any other person in the public interest to prosecute violations of the Act pursuant to Health and Safety Code Section 25249.7 nor does it affect the responsibility of courts to interpret the Act and apply the provisions of the Act to particular facts.

Follow this link to download a copy of this notice as a pdf file.

Follow this link to OEHHA's May 20, 1999 response.

Questions regarding this notice should be directed to:

Cynthia Oshita
Office of Environmental Health Hazard Assessment
Post Office Box 4010
Sacramento, CA 95812-4010
FAX: (916) 327-1097
Telephone: (916) 445-6900

In order to be considered, comments must be postmarked (if sent by mail) or received at OEHHA (if hand-delivered or sent by FAX) by 5:00 p.m., January 6, 1999.

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For questions regarding Proposition 65, please contact the Cynthia Oshita in the Proposition 65 Implementation Program

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