This testimony before the House Committee on Education and Labor, Subcommittee on Labor Standards, discussed the use of epidemiological data in the assessment of occupational respiratory respiratory disease, using cotton dust exposure as an example. Most occupational respiratory diseases are caused by more than one risk factor. It is important to determine whether adequate criteria exist to make reliable and valid diagnoses and whether these diagnoses may be linked to an occupational origin. Allocation of risk may be assessed only from epidemiological data gathered on a reference population reasonably representative of the subject's exposure. Apart from assessment of the specific diagnosis and its associations with industrial exposure is the assessment of impairment. The evidence which links cotton dust exposure and cigarette smoking to chronic obstructive lung disease includes pathological studies which reveal significantly more chronic bronchitis among cotton dust exposed workers, consistent increases in disability claim rates, many cross sectional epidemiological studies, prospective studies of cotton textile workers, and a unique community study. For a just and equitable system of assessing impairment it is necessary that significant occupational exposure to cotton dust be documented, that objective evidence of the impairment be obtained, and that evidence of no other lung disease be found.