Clinical features and natural history of occupational asthma due to western red cedar (Thuja plicata).

After an average follow-up interval of three and a half years (range one to nine years), 125 patients with occupational asthma due to red cedar exposure were re-examined. Fifty patients remained in the same job. All of them continued to have asthmatic attacks requiring regular medication for relief of symptoms. They had worse lung function and ther bronchial reactivity to methacholine increased. Seventy-five patients left the industry; half of them became asymptomatic, whereas the remaining half continued to have recurrent attacks of asthma. Several factors were of prognostic significance. Those with a shorter duration of exposure as well as a shorter duration of symptoms prior to diagnosis and removal from exposure showed improvement. Those who remained symptomatic tended to be older; they had longer duration of exposure and a longer duration of symptoms prior to diagnosis. They tended to have more abnormal results of lung function studied and more marked bronchial hyper-reactivity to methacholine at the time of diagnosis. They also tended to have dual asthmatic reaction rather than late asthmatic reaction to inhalation challenge with red cedar extract. Smoking, race and degree of peripheral blood eosinophilia did not play a role in determining the outcome. Since none of these patients had symptomatic asthma before employment and since they reacted to inhalation challenge of red cedar, it could be assumed that persistent asthma in those who failed to recover is the result of their previous occupational exposure. Early diagnosis and removal from exposure were found to be associated with recovery.