Asthma is a chronic respiratory disease that is increasing in both prevalence and mortality in developed countries around the world. Occupational exposures to sensitizers and irritants are causes of both asthma cases and asthma exacerbations in adults. The determination of how many cases of asthma may be caused or worsened by occupational exposures is highly dependent on how asthma is defined, what constitutes work-relatedness, and what specific methodology is employed. Surveillance-based methods generally have found the lowest proportion of work-related asthma, ranging from 1-8% of cases. Other types of studies, using exposed-unexposed methodology or interviews of incident asthma cases, have determined that 10-25% of cases are occupationally related. Ultimate determination of how much asthma may be related to occupational or environmental exposures will require better surveillance of asthma, along with a better understanding of this disease and its natural history. PubMed: http://www.ncbi.nlm.nih.gov/pubmed/10769342