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The SYNERGY project is a giant case-control database for lung cancer, with 13,300 cases and 16,000 controls pooled from 11 studies done in Europe and Canada. In addition to detailed data on smoking exposure, it also contains occupational history.
Susan Peters et al sliced and diced the data and determined that the quartile with the highest lifetime occupational exposure to organic dusts had an odds ratio for lung cancer of 1.24 (independent of smoking history and other covariates). The majority of people affected as "cases" worked as farmers and woodworkers (cabinetmakers, sawyers, loggers, etc).
This is the first study to suggest this association so strongly. Although it's a persuasive cross sectional case-control study, longitudinal cohort studies are the gold standard methodology to establish causation.
Peters S et al. Occupational exposure to organic dust increases lung cancer risk in the general population. Thorax 2012;67:111-116.