Occupational asthma continues to be one of the most common occupational lung diseases in industrialized areas. Primary and secondary preventive measures have been well described, but there are relatively few studies to support the effectiveness of such measures, although the benefits of tertiary measures such as early recognition and removal from further exposure to a causative sensitizing agent are well recognized. In Ontario, a combined approach of preventive measures has shown effectiveness in allergy and asthma from occupational exposure to natural rubber latex. In addition, a program to reduce exposure to diisocyanates and introduce medical surveillance was associated with earlier diagnosis and fewer cases in a compensation population. However there remain barriers to the early diagnosis of occupational asthma in Ontario, especially in workers of lower education and lower income. In addition, there is recognized need for further physician education to allow early suspicion and diagnosis of occupational asthma.