An epidemiologist at a tertiary centre in Mumbai is reviewing a published study that investigated the association between occupational asbestos exposure and lung cancer. The study enrolled 200 workers with confirmed lung cancer and 200 workers without lung cancer, then retrospectively assessed their past asbestos exposure using employment records and interviews. The researchers calculated an odds ratio of 8.5 (95% CI: 5.2–13.8). A colleague questions whether this design was appropriate for establishing causation. What is the key limitation of this study design that must be acknowledged when interpreting the findings?