Substance-Specific Issues

Diatomaceous earth, calcined; classification with respect to carcinogenicity

Substances:

Calcined diatomaceous earth; CAS No. 91053-39-3
Flux-calcined diatomaceous earth; CAS No. 68855-54-9

Issue:

Does calcined diatomaceous earth and/or flux-calcined diatomaceous earth fall within the WHMIS D2A criteria for carcinogenicity?

Background:

Uncalcined diatomaceous earth (CAS No. 61790-53-2) typically contains around 1% crystalline silica. When diatomaceous earth is subjected to pressure or is processed ("calcined") at temperatures above 1000°C some of the amorphous silica is converted to crystalline silica in the form of cristobalite. Calcined diatomaceous earth can contain anywhere from 1% to 75% cristobalite.

In 1997, IARC issued Volume 68 of their monographs which included an evaluation of silica which states:

"Amorphous silica is found in nature as biogenic silica and as silica glass of volcanic origin. One form of biogenic silica, diatomaceous earth, originates from the skeletons of diatoms deposited on sea floors and contains small amounts of cristobalite and quartz. After calcination (which significantly increases the cristobalite content), diatomaceous earth is used as a filtration agent, carrier for pesticides, filler in paints and paper and as a refractory or abrasive product in a variety of industries. Occupational exposure to both amorphous and crystalline silica may occur during the production and use of diatomaceous earth."

On page 54, the monograph states that "calcination and, even more so, flux calcination yield a considerable amount (up to 65%) of crystalline material (cristobalite)".

(In the 2001 edition of Threshold Limit Values for Chemical Substances and Physical Agents / Biological Exposure Indices, the ACGIH does not have a carcinogenicity notation for uncalcined diatomaceous earth and there is no entry for calcined diatomaceous earth.)

In its April 5, 1999 memorandum to its regional administrators, OSHA's Directorate of Compliance Programs communicated:

"Diatomaceous earth has been tested as a whole and evaluated as a Group 3 carcinogen by IARC. [...] Therefore, there is no requirement under the HCS to state a definitive finding of carcinogenicity on the label or MSDS for diatomaceous earth products containing less than 1% crystalline silica. [...] This enforcement policy does not apply to products containing greater than 1% crystalline silica [...]"

Considerations:

IARC lists flux-calcined diatomaceous earth as a type of amorphous silica. IARC's overall evaluation is: "amorphous silica is not classifiable as to its carcinogenicity to humans (Group 3)". However, this evaluation is based on very limited epidemiological evidence and the animal studies did not include the inhalation route of exposure for flux-calcined diatomaceous earth.

The monograph also summarizes (on page 206) human exposure studies that suggest that exposure to calcined diatomaceous earth results in an elevated risk of lung cancer.

Conclusion:

As calcination can "significantly"increase the cristobalite content, IARC's overall evaluation of uncalcined diatomaceous earth may not be applicable to calcined products whereby some of the amorphous silica has been converted to crystalline silica in the form of cristobalite. Thus the IARC Group 3 classification of amorphous silica would not preclude the classification of calcined diatomaceous earth under the carcinogenicty criteria for an untested mixture specifed in the CPR.

Since calcined (including flux-calcined) diatomaceous earth was not evaluated as to its carcinogenicity it must be considered an untested mixture subject to paragraph 58(a) of the CPR.

Note: The foregoing, last revised in July 2002, supersedes all previous information communicated by WHMIS regulatory agencies on this matter.

References:

IARC Monographs on the Evaluation of Carcinogenic Risks to Humans; Volume 68, "Silica, Some Silicates, Coal Dust and para-Aramid Fibrils", 1997

OSHA, U.S. Department of Labor; "Special Emphasis Program (SEP) for SILICOSIS", May 2, 1996 memorandum from Joseph A. Dear, Assistant Secretary to regional administrators; http://www.osha.gov/Silica/SpecialEmphasis.html

OSHA, U.S. Department of Labor; "Hazard communication requirements for diatomaceous earth" , April 5, 1999 memorandum from Richard E. Fairfax, Director, Directorate of Compliance Programs to regional administrators; http://www.osha.gov/pls/oshaweb/owadisp.show_document? p_table=INTERPRETATIONS&p_id=2 2720&p_text_version=FALSE