Monday, November 21, 2005

Important Risk Assessment for Silica Shows Huge Risks at the Current OSHA PEL

Once in a while a paper appears in Regulatory Toxicology and Pharmacology which departs from its pervasive money tox outlook. This paper on silica, from California's EPA, aims to protect health. BrooklynDodger fears it leaves behind too much risk. The Dodger wonders whether this was introduced by regulators' limitations, was pressed on the authors by reviewers, or is a consequence of reviewers and editors asleep at the switch [latter unlikely.]

Before launching into critique, the Dodger calls the readers' attention to the 3 microgram per cubic meter "limit" - compared to the 100 micrograms permitted by OSHA [inherited from the middle '60's], the 50 micrograms recommended by NIOSH [back in the early '70's].

Exposures in this range persist in the best controlled foundries, probably underground mines, sand pits, etc; the reference limit suggests hazards in agriculture and sweeping the streets.

Also, the Dodger applauds California, and CalEPA, for continuing work to derive exposure limits and do risk assessements, now that OSHA and US EPA have stopped.

"Reference" level is kind of weasel-word for health-based exposure not-quite limit.

First, the endpoint is radiologically diagnosed silicosis, which is only a small part of the picture. The reference level in the title should be "silicosis" not silica. Silica exposure also causes reduced lung function [both restrictive and obstructive] and symptoms. At least twice as many additional workers are afflicted as have radiological silicosis.

Second, radiological silicosis has an occult period - enough dose has been inhaled to cause effects that are not patent on x-ray. The occult period - might be called latency or lag - is 10 years or more. The Dodger didn't study the underlying studies to see how this was accounted for. By definition, 10 years of exposure includes 10 years of lag from first exposure, but the full bloom of the silica would only emerge after 20 years lag from the 10 years of exposure, or 30 years since hire.

Third, mortality from lung cancer and mortality from non-malignant respiratory disease is equivalent in silica cohorts - about the same SMR and about the same number of observed deaths [Dodger will supply references in subsequent post]. However, none of the NMRD deaths on death certificate are silica. Lots are pneumonia. Hypothetically, silica potentiates other acute infectious lung disease just as it does tuberculosis. The risk assessment doesn't account for these non-pneumoconiotic endpoints.

Fourth, the risk assessment doesn't take lung cancer into account.

Fifth, the risk assessment doesn't justify the attack level for the benchmark dose. The reference level is the Benchmark Concentration Level 01 - a 1% attack rate statistically modeled from the data sets.

Notably, because it's human data and chronic exposure data, CalEPA gives up those two uncertainty factors. CalEPA gives us a 3 fold uncertainty factor for intra species variability, which is definitely not risk averse.


Regul Toxicol Pharmacol.
2005 Sep 22; [Epub ahead of print]

Development of a chronic inhalation reference level for respirable crystalline silica.

Collins JF, Salmon AG, Brown JP, Marty MA, Alexeeff GV.

Air Toxicology and Epidemiology Branch, Office of Environmental Health Hazard Assessment, California Environmental Protection Agency, 1515 Clay Street, 16th Floor, Oakland, CA 94612, USA.

Chronic inhalation exposure of workers to crystalline silica can result in silicosis. The general public can also be exposed to lower levels of crystalline silica from quarries, sand blasting, and entrained fines particles from surface soil. We have derived an inhalation chronic reference exposure level for silica, a level below which no adverse effects due to prolonged exposure would be expected in the general public. Incidence of silicosis and silica exposure data from a cohort of 2235 white South African gold miners yielded a reference level of 3mug/m(3) for respirable silica (particle size as defined occupationally) using a benchmark concentration approach. Data from cohorts of American gold miners, Chinese tin miners, diatomaceous earth workers, and black South African gold miners yielded similar results with a range of 3-10mug/m(3). Strengths of the chronic reference exposure level include the availability of several large long-term studies of inhalation in workers at varying exposure concentrations, adequate histopathological and radiologic analysis, adequate follow-up of exposed workers, a dose-response effect in several studies, observation of a No Observed Adverse Effect Level in the key study, and the power of the key study to detect a small effect. Uncertainties include the general underestimation of silicosis by radiography alone and the uncertainties in exposure estimation.

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