High levels of air pollution are harmful, especially to persons with heart or lung disease. The severity of reactions ranges from minor symptoms to premature death. But extremely high levels of pollution are rarely encountered, and the more important and unsettled question is whether longterm exposure to low levels of pollutants has a significant effect on health.
Cigarette smoking is also firmly established as a major risk factor for COPD (Table 1). Data from longitudinal, crosssectional, and case-control studies show that in comparison with nonsmokers, cigarette smokers have higher death rates for chronic bronchitis and emphysema; higher prevalence and incidence rates for chronic bronchitis, emphysema, and obstructive airways disease; and higher frequencies of respiratory symptoms and lung function abnormalities. They also have a greater average annual rate of decline in FEVvDifferences between cigarette smokers and nonsmokers increase as cigarette consumption increases. Pipe and cigar smokers have higher morbidity and mortality rates for COPD than nonsmokers, but lower rates than cigarette smokers.
The main purpose of epidemiology is to identify causes and risk factors associated with the development of disease. Other purposes are: to measure the frequency and describe the distribution, range of severity, and course of disease; to estimate risk for subgroups within the population and to identify persons at highest risk. This information is relevant to preventing disease, to screening and early detection, to predicting onset, course, and outcome of disease, and to planning and evaluating preventive and therapeutic measures.