High levels of urban air pollution
are harmful to individuals with existing heart or lung disease.
The role of outdoor air pollution in causing COPD is unclear,
but appears to be small when compared with that of cigarette
smoking. It has also been difficult to assess the effects
of single pollutants in long-term exposure to atmospheric
pollution. However, air pollution from fossil fuel combustion,
primarily from motor vehicle emissions in cities, is associated
with decrements of respiratory function37. The relative
effects of short-term, high-peak exposures and long-term,
low-level exposures is a question yet to be resolved.