Environmental risk factors for Parkinson's disease: the epidemiologic evidence.

Descriptive data from several studies suggest variations in the frequency of Parkinson's disease in different population groups. Door-to-door surveys were carried out among a biracial U.S. population (blacks and whites) and in communities in Nigeria and the People's Republic of China. The U.S. investigation revealed no substantial differences in the age-adjusted prevalence ratios by race. However, blacks in Nigeria have a much lower prevalence ratio than blacks in the U.S., suggesting an environmental etiologic factor. Prevalence ratios derived from China are also lower than the U.S. figures. Studies of temporal trends in the incidence rates in one U.S. population (Rochester, Minnesota) show virtually no change over 35 years, indicating that the primary cause(s) of Parkinson's disease must have been present in this nonindustrialized community for many years. Analytic studies generally reveal an inverse association between Parkinson's disease and cigarette smoking, although epidemiologic evidence does not support a direct protective effect of smoking. Preliminary investigations suggest an increased risk associated with the rural environment and the consumption of well water. Further studies are required to discover as yet unknown environmental factors that heighten the risk of Parkinson's disease.