Office of Environmental Health Hazard Assessment

Air Toxicology and Epidemiology

Air Toxic Hot Spots Program - Chronic Toxicity Summary: Silica (Crystalline, Respirable).  Materials for Review by the Scientific Review Panel for Air Toxics

Finalized Document Available: Chronic Reference exposure for Silica (Crystalline, respirable)

Please note: the next SRP meeting, previously scheduled for April 9th in Oakland, has been changed to Wednesday, May 19 (Location TBA)

The Office of Environmental Health Hazard Assessment (OEHHA) has released a draft document, Chronic Toxicity Summary for Silica (Crystalline, Respirable) to solicit public comment. This draft document has been developed by OEHHA for use in implementing the Air Toxics Hot Spots Program (Health and Safety Code Section 44300 et seq.). OEHHA developed four Technical Support Documents (TSDs) and a Guidance Manual pursuant to Health and Safety Code Section 44360, which provided the scientific basis for values used in assessing risk from exposure to facility emissions. The TSD for the Determination of Chronic Reference Exposure Levels (Part III of the TSD series), which has already undergone public and peer review and been adopted by OEHHA, describes the methodology for risk assessment for materials having adverse health impacts following chronic exposures. The present document presents a toxicity summary and chronic REL for crystalline silica, particularly when this material is composed of particles of respirable size ( ≤10 µm). If approved by the Scientific Review Panel on Toxic Air Contaminants, and adopted by the Director of OEHHA, this document will form a further addendum to the Part III TSD.

The Chronic Toxicity Summary for Silica became available on the OEHHA Home Page at on April 25, 2003. The availability of the document on this site commenced a public review period that included two public workshops, and was originally scheduled to end on May 23, 2003. This  was later extended to end on June 22, 2003.

OEHHA has received a number of comments on this document, and has amended to reflect these comments, and consultation with staff of the California Air Resources Board and the California Department of Health Services.  The modified Summary, along with the comments received and OEHHA's responses to them, will be presented to the Scientific Review Panel at their next meeting scheduled for April 9, 2004 May 19th, 2004 .  The materials, including response to comments, to be presented to the Panel are available as pdf files.  Further information about dates, locations and agenda for meetings of the Scientific Review Panel can be obtained from the ARB web page at

Please direct any inquiries concerning technical matters, or availability of the Chronic Toxicity Summary for Silica (Crystalline, Respirable), to Dr. Andrew G. Salmon (Chief, Air Toxicology and Risk Assessment Unit), Office of Environmental Health Hazard Assessment, 1515 Clay St., 16th Floor, Oakland, CA 94612, telephone (510) 622-3191 or by e-mail to

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