LOWER THRESHOLD AND GREATER BRONCHOMOTOR RESPONSIVENESS OF ASTHMATIC SUBJECTS TO SULFUR-DIOXIDE

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Abstract
To determine whether human subjects with mild asthma or seasonal rhinitis have greater bronchomotor responses to SO2 than normal subjects, a study was undertaken in 7 asthmatic, 7 atopic and 7 normal subjects, 23-37 yr of age. Change in specific airways resistance (SRaw) provoked by 10 min of breathing 1, 3 and 5 ppm of SO2 delivered by mouthpiece was measured on separate days at least 48 h apart. To assess the significance of parasympathetic pathways in the bronchomotor responses to SO2, the change in SRaw provoked by 5 ppm of SO2 was measured in 17 of the subjects after they inhaled atropine sulfate aerosol (0.1 mg/kg). In the asthmatic subjects, SRaw increased significantly at all concentrations of SO2; in the normal and atopic subjects, SRaw increased only at 5 ppm. In the asthmatic group, SRaw increased more than in either of the other groups at 5 ppm (P < 0.005) and was sometimes associated with marked dyspnea requiring bronchodilator therapy. The increases in SRaw produced by inhalation of SO2 were prevented by treatment with atropine in asthmatic and nonasthmatic subjects, which suggested the involvement of parasympathetic pathways. Apparently, subjects with mild asthma develop bronchoconstriction after exposure to concentrations of SO2 well below currently accepted standards for occupational exposure. SO2-induced bronchoconstriction may be mediated by parasympathetic pathways.