Silica: déjà vu all over again?

Brown and Rushton1 have conducted a retrospective cohort mortality study of 2700 workers in the industrial sand industry. Work in the industrial sand industry results in exposure to crystalline silica, and the focus of the study was whether exposure to silica causes lung cancer. Retrospective exposure assessment, based on air measurements since 1978, and some assumptions about exposure before then, was used to estimate exposure levels for different jobs in the industry over time. The resulting job-exposure matrix was used to assign estimated exposure levels to each worker and to estimate cumulative silica exposure, which is commonly the exposure measure of interest for chronic diseases such as lung cancer.

Brown and Rushton did not find an excess of lung cancer in this cohort compared to the general population (lung cancer SMR 0.99, 77 deaths), nor did they find any excess silicosis (only two silicosis deaths were observed). Furthermore, they did not find a positive exposure-response trend for lung cancer by cumulative exposure category (rate ratios of 1.0, 1.24, 1.42, and 0.88 by increasing exposure).

Should this negative result be considered surprising? After all, the International Agency for Research on Cancer (IARC) declared in 1997 that crystalline silica was a group I (definite) human carcinogen, based on lung cancer findings across a large number of existing occupational studies and positive animal studies.2 The National Toxicology Program (NTP) (www.ntp.niehs.nih.gov/ntp/roc/toc11.html) in the USA followed this by declaring silica a known human carcinogen in 2000. Our own subsequent pooled analysis of 10 …