Acid fog and hospital visits for asthma: an epidemiological study.

The aim of this study was to elucidate the adverse respiratory effects of naturally occurring acid fog. In total, 102 adult asthmatic patients (44 nonatopic and 58 atopic) were studied for a 2 yr period (January 1992 to December 1993) in Kushiro, a city with only a small industrial area, located in the northern-most island in Japan. Fog occurred on 378 out of 731 days, and the acidity of the fog ranged from pH 3.32 to 6.91 (mean pH 4.95). The association between hospital visits for asthma and meteorological factors or air pollutants was investigated. In nonatopic patients, fog, high ozone and water vapour pressure, low day-to-day temperature differences, low concentrations of atmospheric NO and NO2 contributed significantly (p<0.05) to increasing hospital visits. In atopic subjects, fog, high water vapour pressure, low levels of atmospheric NO2 and SO2 contributed significantly to hospital visits (p<0.05). In Poisson regression analysis the remaining factors of significance (p<0.01) for nonatopic asthma were fog and low NO and for atopic asthma were high water vapour pressure and low SO2 (p<0.05). A weak but significant correlation was observed between the number of hospital visits and the mean pH of the foggy day (r=-0.38, p<0.05) in nonatopic asthmatic patients, not in atopic asthma. On foggy days, gaseous air pollutant levels were significantly (p<0.01) lower than on fog-free days. It was concluded that, naturally occurring acid fog may have a weak bronchoconstrictive effect which appears to be more influential in nonatopic asthmatic subjects than in atopic subjects.