exposure (American Thoracic Society, 1996a). Short-duration exposures of asthmatics to SO2 also reduce lung function; in the protocols, delivering the gas during exercise increases the amount of SO2 received (American Thoracic Society, 1996b).

Short periods of moderately elevated particle concentrations have been associated with pulmonary-function deficits (Dassen et al., 1986; Brunekreef et al., 1991). Subsequent studies that have examined daily time series, rather than episodes, have also associated ambient PM10 concentrations with short-term variation in peak expiratory flow rate at concentrations below current national ambient-air quality standards (Pope and Dockery, 1992; Pope et al., 1991).

Daily symptom incidence and duration of respiratory illness have been linked to exposure to airborne particles. Supporting evidence includes results from the Six Cities Study conducted by Harvard investigators (Schwartz et al., 1989), in which particle concentrations never exceeded 75% of the current air-quality standard. In this study, daily diary responses concerning respiratory symptoms were significantly associated with particle concentrations. Similar findings come from Switzerland (Braun-Fahrlander et al., 1992) and Provo, Utah (Pope and Dockery, 1992; Pope et al., 1991). Acid-aerosol exposure has been associated with increased symptoms in a diary study of asthmatics (Ostro et al., 1991). For several of the respiratory symptoms, the odds ratios from Utah, the Six Cities Study, and Switzerland are similar.

Exposure to airborne particles has been associated with increased rates of bronchitis in children (Dockery et al., 1989) at concentrations below current standards and also with increased rates of croup attacks in children (Schwartz et al., 1991). These effects do not appear to be limited to children. For example, Ostro and Rothschild (1989) have reported an association between airborne particles and ozone and respiratory illness severe enough to restrict activity in adults. A meta-analysis has reported a significant association between NO2 exposure and respiratory illness in children (Hasselblad et al., 1992).

Bates and Sizto (1987) reported that exposures to both ozone and sulfate were associated with increased incidences of hospitalization for respiratory illness in Ontario. Pope (1989), as mentioned above, found sharp changes in hospitalization rates for children when a steel mill closed and then reopened. Hospital admissions were also increased in the German smog episode of 1985 (Wichmann et al., 1989) at a time of sharp increases in both total suspended particles and SO2. Sunyer et al. (1991) reported that airborne particles and SO2 were associated with hospitalization for respiratory illness in Barcelona, Spain. Hospital emergency-room visits were associated with sulfates and SO2 in a study in Vancouver, Canada (Bates et al., 1990), and respirable particles were associated with hospital

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