Most historical and present-day evidence suggests that there can be both acute and chronic effects of the strongly acidic component of PM, i.e., the hydrogen ion (H+), concentration when it is below pH 4.0 (Koutrakis et al. 1988; Speizer 1999). Increased hospital admissions for respiratory causes were documented during the London fog episode of 1952, and this association has now been observed under present-day conditions. Thurston et al. (1992, 1994) have noted associations between ambient acidic aerosols and summertime respiratory hospital admissions in both New York state and Toronto, Canada, even after controlling for potentially confounding temperature effects. In the 1994 report, statistically significant independent H+ effects remained even after the other major copollutant, in the regression model, ozone was considered. H+ effects were estimated to be largest during acid-aerosol episodes (H+ = 10 µg/m3 as sulfuric acid or H+ at ˜200 nmol/m3), which occur roughly 2 or 3 times per year in eastern North America. The studies provide evidence that present-day strongly acidic aerosols might represent a portion of PM that is contributing to the significant acute respiratory health effects noted for PM in the general public.
Results of recent symptom studies of healthy children indicate the potential for acute acidic PM effects in this population. Although the “6-city Study” of parent diaries of children's respiratory and other illness did not demonstrate H+ associations with lower respiratory symptoms except at H+ above 110 nmol/m3 (Schwartz et al. 1994), upper respiratory symptoms in two of the cities were found to be most-strongly associated with high concentrations of H2SO4 (Schwartz et al. 1991). Two recent summer-camp (and schoolchildren) studies of lung function have indicated a statistically significant association between acute exposures to acidic PM and decreases in the lung function of children, independent of those associated with O3 (Neas et al. 1995; Studnicka et al. 1995).
Reported associations between chronic H+ exposures and children's respiratory health and lung function are generally consistent with adverse effects as a result of chronic H+ exposure. Preliminary bronchitis prevalence rates reported in the “6-city Study” locales were found to be more-closely associated with average H + concentrations than with PM in general (Speizer 1989). Follow-up studies of those cities (and a seventh) that controlled for maternal smoking, education, and race suggested associations between summertime average H+ and chronic bronchitic and related symptoms (Damokosh et al. 1993). Bronchitic symptoms were observed 2.4 times more frequently (95% confidence interval, 1.9-3.2) at the highest acid concentration (H + at 58 nmol/m3) than the lowest concentration (16 nmol/m3). Furthermore, in a followup study of children in 24 United States and Canadian communities (Dockery et al. 1996) in which the analysis was adjusted for the effects of sex, age, parental asthma, parental education, and parental allergies, bronchitic symptoms were confirmed to be statisti-