Castano et al1 applied specific inhalational challenge testing to examine the important relationship between occupational rhinitis (OR) and occupational asthma (OA) in 43 subjects. Using this approach they found that OR occurred in 76.4% of confirmed cases of OA, and this provides strong evidence that the united airways disease model does apply to occupational causation overall. However, as they readily acknowledge, an attempt to stratify this relationship according to whether the causative agent is of high molecular weight (HMW) or low molecular weight (LMW) is limited by the small size of the study population. An epidemiological approach offers …