Air pollutants can exhibit both acute and chronic effects. Acute effects mainly occur during smog situations in industrial areas when atmospheric concentrations of hazardous substances are extremely high. This leads to an increase of morbidity and mortality especially in old and ill people. Chronic effects are the consequence of a longlasting exposition to low doses of noxious substances. Epidemiological studies have shown that frequency of respiratory diseases is higher in air polluted areas; this effect is particularly evident in smokers. Biological monitoring is used to determine the individual heavy metal burden. Lead is a danger especially for the developing central nervous system. Cadmium accumulates in the renal cortex during lifetime and may cause disturbances of renal function if a certain concentration is reached. The risk groups for heavy metals are young children and pregnant women in the case of lead and old people in the case of cadmium, respectively. Lung cancer incidence is higher in large cities than in rural areas. Carcinogenicity of city smog extracts was proved in vitro and in vivo. These findings suggest that urban air pollution may play a role as a causal factor in respiratory cancer.