OSHA considers lung cancer, an irreversible and frequently fatal disease, to be a clear material impairment of health (see Homer et al., 2009, Document ID 1343). According to the National Cancer Institute (SEER Cancer Statistics Review, 2006, Document ID 1343), the five-year survival rate for all forms of lung cancer is only 15.6 percent, a rate that has not improved in nearly two decades. After reviewing the record as a whole, OSHA finds that respirable crystalline silica exposure substantially increases the risk of lung cancer. This finding is based on the best available toxicological and epidemiological data, reflects substantial supportive evidence from animal and mechanistic research, and is consistent with the conclusions of other government and public health organizations, including the International Agency for Research on Cancer (1997, Document ID 1062; 2012, Document ID 1473), the HHS National Toxicology Program (2000, Document ID 1417), the CDC's National Institute for Occupational Safety and Health (2002, Document ID 1110), the American Thoracic Society (1997, Document ID 0283), and the American Conference of Governmental Industrial Hygienists (2010, Document ID 0515).
Checkoway et al. (1997) investigated the risk of lung cancer among diatomaceous earth (DE) workers exposed to respirable cristobalite (a type of silica found in DE) (Document ID 0326; 1711, pp. 139-143). Exposure samples were collected primarily at one of the two plants in the study by plant industrial hygienists over a 40-year timeframe from 1948 to 1988 and used to estimate exposure for each individual in the cohort (Seixas et al., 1997, Document ID 0431, p. 593). Based on 77 deaths from cancer of the trachea, lung, and bronchus, the standardized mortality ratios (SMR) were 129 (95% CI 101-161) and 144 (95% CI 114-180) based on rates for U.S. and local county males, respectively (Document ID 0326, pp. 683-684). The authors found a positive, but not monotonic, exposure-response trend for lung cancer. The risk ratios for lung cancer with increasing quintiles of respirable crystalline silica exposure were 1.00, 0.96, 0.77, 1.26 and 2.15 with a 15-year exposure lag. Lung cancer mortality was thus elevated for workers with cumulative exposures greater than 2.1 mg/m3-yrs, but was only statistically significantly elevated for the highest exposure category (RR = 2.15; 95% CI 1.08-4.28) (Document ID 0326, p. 686). OSHA notes that this highest exposure category includes cumulative exposures only slightly higher than 4.5 mg/m3-yrs, the level of cumulative exposure resulting from a 45-year working lifetime at the previous PEL of 100 μg/m3. OSHA does not believe that the appearance of a statistically significantly elevated lung cancer risk in the highest category should be interpreted as evidence of an exposure-response threshold, especially in light of the somewhat elevated risk seen at lower exposure levels. OSHA believes it is more likely to reflect limited power to detect excess risk at lower exposure levels, a common issue in epidemiological studies which was emphasized by peer reviewer Dr. Andrew Salmon in relation to purported thresholds (Document ID 3574, p. 37).
OSHA finds all of Dr. Rosenman's responses to Dr. Hessel's criticisms to be reasonable. And based on Dr. Rosenman's comments and testimony, OSHA continues to believe that the Rosenman et al. (2003) analysis and resulting estimates of the number of new silicosis cases that arise each year are reasonable. Additionally, Dr. Rosenman, in updating his data for his testimony for this rulemaking, found that the ratio had increased from 6.44 in the published study to 15.2 times in more recent years (Document ID 3577, Tr. 854). The study supports OSHA's hypothesis that silicosis is a much more widespread problem than the surveillance data suggest and that OSHA's estimates of the non-fatal illnesses that will be avoided as a result of this new silica standard are not unreasonable. Regardless, even assuming commenters' criticisms have merit, they do not significantly affect OSHA's own estimates from the epidemiological evidence of the risks of silicosis.