Chamber of Commerce, Chemistry Council Cast Doubt on OSHA’s Science on Silica

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By Robert Iafolla  

March 19 --The Occupational Safety and Health Administration has failed to provide a scientific justification for lowering the permissible exposure limit for silica, scientists and researchers hired by industry groups said during hearings on the agency's proposal March 19.

Scientists and researchers testifying on behalf of the U.S. Chamber of Commerce and the American Chemistry Council said OSHA didn't properly consider potential errors in existing studies, the extremely long latency period of silicosis, the downward trend of worker exposure, the requirements of proving causality or an alternative model for how silica damages lungs.

Yale School of Public Health professor Jonathan Borak, who represented the Chamber, even criticized OSHA's definition of respirable crystalline silica for being ambiguous.

The industry attack on the science underpinning OSHA's silica proposal came on the second day of the agency's hearings, when the peer reviewers who analyzed the agency's risk assessment report were present. The hearings, which are scheduled to continue for three weeks, are a part of building the rulemaking record for the agency's proposed standard (see related story).

The rulemaking record will inform OSHA's explanation for its final rule and is likely to be cited in the seemingly inevitable legal challenge against the rule.

'Not Ready for Prime Time.'

OSHA's proposal would reduce the permissible exposure limit to 50 micrograms per cubic meter of air, down from about 100 micrograms for general industry and 250 micrograms for construction and shipyards. While the plan also calls for a number of protective measures, the testimony from researchers hired by the Chamber, as well as a risk assessment specialist tapped by the American Chemistry Council, primarily focused on the permissible exposure limit.

“My best assessment is preliminary quantitative risk assessment is not ready for use; it's not ready for prime time,” Tony Cox, a risk assessment specialist representing the council, said. “The answers and estimates it delivers are not trustworthy. You wouldn't want to bet money on them. They might not be right.”

Cox claimed that OSHA cherry-picked the research it used in its risk analysis, including studies that supported its assumptions about the risks of silica and ignored research that didn't. Cox also said the agency failed to examine the crucial question of causality, except to say silica causes silicosis without supporting that claim.

'We're Not Stupid.'

During questioning of Cox, National Institute for Occupational Safety and Health epidemiologist Robert Park said it was “ludicrous” to question the conclusion that silica causes silicosis and other lung diseases. Park said a significant number of good studies support that judgment on causality.

“We're not stupid,” Park said.

Cox said that he doesn't question that exposure to silica at high levels for a sufficient period of time causes harmful health effects, but that it takes rigorous examination rather than simple judgment to conclude that lowering the exposure limit will further protect workers.

Borak, who heads a consulting firm in addition to teaching at Yale, said there's no evidence that the current exposure limit isn't adequately protective. Borak said studies show the latency period for silicosis is around 30 years, longer than the 10- to 30-year range that OSHA claims. Due to that latency period, he said, the bulk of people who have died of silicosis in a multi-year Centers for Disease Control and Prevention study were exposed prior to 1971, when OSHA established its exposure limit.

Measurement Errors, Threshold Model

Christopher Long, a risk assessment specialist representing the Chamber, said that studies on the effects of silica exposure suffer from exposure measurement errors. While OSHA accounts for three types of errors, it doesn't account for an additional 10 types of errors, Long said.

Under questioning from an OSHA official, Long said he couldn't think of a single silica study that properly accounted for all types of exposure measurement errors.

Peter Valberg, an environmental, safety and health consultant representing the Chamber, said that OSHA seemed to dismiss the notion that there is a threshold level for when silica affects lungs, in favor of a linear model that correlates exposure level with harmful effects. A threshold model says that silica exposure isn't harmful until it exceeds a specific level.

Valberg said in response to an OSHA question that the threshold level could vary significantly among individuals, which might make the threshold level appear linear.

In total, the Chamber sent eight witnesses with expertise in industrial hygiene, occupational health, statistics, epidemiology and risk assessment to cast doubt on the science undergirding OSHA's proposed silica rule. It will send four more witnesses to critique the proposed rule March 21. The American Chemistry Council sent one witness and will send another nine March 26.


To contact the reporter on this story: Robert Iafolla in Washington at

To contact the editor responsible for this story: Jim Stimson at

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