Seasonal Difference in the Occurrence of Exercise-Induced Bronchospasm in Asthmatics: Dependence on Humidity

Abstract

Background: Most studies on the effects of temperature and humidity on exercise-induced bronchospasm (EIB) in asthmatics have been carried out under indoor conditions. However, any asthmatic patient is exposed to varying climatic conditions. Objective: To investigate whether temperature or relative humidity plays a more important role in determining the degree of EIB in asthmatics under naturally exposed climate conditions. Methods: To exclude the effects of pollen on EIB, we enrolled 69 subjects with perennial asthma (mean ± SD: 20.1 ± 1.5 years). The subjects performed outdoor free running tests. They were divided into winter (n = 25), spring/autumn (n = 22), and summer (n = 22) groups according to the season when they performed the tests. Initial spirometry and skin prick tests were performed. Methacholine bronchial challenge testing and, one day later, the free running tests were done. Results: There were significant differences in temperature and relative humidity among the three groups: However, the relative humidity in winter did not differ from that in spring/autumn. There were no differences in pulmonary functions, airway responsiveness, and atopy score among the three groups. The percentage of cases of positive EIB – fall in forced expiratory volume in 1s FEV₁ of >15% from baseline – in winter (84%, p < 0.05) or spring/autumn (86.4%, p < 0.05) was higher than that in summer (50%). However, the percentage of subjects with a positive EIB in winter did not differ from that in spring/autumn. The maximal percent fall in FEV₁ after exercise in winter did not differ from that in spring/autumn. Conclusions: The occurrence of EIB is associated with environmental temperature and humidity. Under such climatic conditions as in Korea, relative humidity may be a more important factor than temperature in contributing to EIB in patients with perennial asthma.