The effects of air pollution: More bad news, but some good news too

Two studies provide further evidence that relatively low levels of air pollution have adverse respiratory effects. However, one of the studies suggests that improvement in air quality can slow the decline in pulmonary function.

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The effects of exposure to diesel exhaust:

McCreanor and associates found that exposure to diesel exhaust on a busy city street leads to reduced lung function in patients with asthma. The reduced lung function is accompanied by inflammatory changes in sputum and exhaled breath condensate.

Their randomized crossover study included 60 persons with mild or moderate asthma. Participants were asked to walk at a leisurely pace for 2 hours along Oxford Street in London; the only vehicles allowed on this street are diesel-powered buses and taxis. At a different time, the participants walked through a nearby park.

Exposures to fine particles, ultrafine particles, elemental carbon, and nitrogen dioxide were higher on the street than in the park. The reductions in forced expiratory volume in 1 second (FEV₁) and forced vital capacity (FVC) were significantly greater after walking on the street than after walking in the park. The effects were more pronounced in persons with moderate asthma than in those with mild asthma.

The differences in FEV₁ and FVC were not accompanied by significant differences in symptoms. However, they were associated with increases in biomarkers of neutrophilic inflammation (sputum myeloperoxidase level) and airway acidification. Ultrafine particles and elemental carbon were the most relevant exposures in this regard.

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The effects of improved air quality:

Downs and colleagues found that reducing exposure to airborne particulates can attenuate the decline in pulmonary function associated with such exposures.

In a study conducted in Switzerland, 9651 randomly selected adults were assessed in 1990 and 8047 were assessed again in 2002. The variables measured included exposure to particulate matter less than 10 μg in aerodynamic diameter (PM₁₀), FEV₁, FVC, and forced expiratory flow between 25% and 75% of FVC (FEF_25-75).

Over the 11-year period, the overall exposure to individual home outdoor PM₁₀ declined. Regression analysis, which controlled for smoking and other variables that predict respiratory health, revealed a negative association between the decrease in PM₁₀ and the rate of decline in FEV₁ as a percentage of FVC. A reduction of 10 μg of PM₁₀ per cubic meter had a net effect of reducing the annual decline in FEV₁ by 9% and the annual decline in FEF_25-75 by 16%. A similar pattern was observed for cumulative exposure.