Environmental exposure in relation to occupational asthma (OA) concerns the exposure of subjects in the work-place. There are qualitative and quantitative differences in occupational exposures which may influence the development and/or the course of OA. Although more than 300 different agents have been described causing OA, 5-6 groups of substances are sufficient to explain more than 80% of all cases of OA. In addition, a variant of occupational asthma without latency period may occur after an acute inhalation of massive concentrations of irritants, and exacerbations of a pre-existent asthma may be due to products or conditions in the work-place. Despite our increased knowledge of the etiology and pathophysiology of OA, its prevalence in the industrialised world has been relatively steady over the last decade. Therefore, the transfer of knowledge has not been sufficiently effective in the prevention of the disease. The results of studies on the exposure-response relationship for several high molecular weight chemicals (and fewer low molecular weight) indicate that sensitisation may be prevented in large part by reducing work-place concentrations of these agents. These data give support to the attempt to develop Threshold Limit Values (TLV) for sensitisers.