Epidemiologic studies have provided contradictory evidence regarding etiologic risks for PD. No area more clearly demonstrates these contradictions than in the area of occupational and environmental risk factors and PD. Numerous studies demonstrate a higher risk of PD for individuals living in a rural environment;12 however, these findings are inconsistent, possibly due to lack of power in the negative studies.3-6 On the other hand, there is some evidence that the prevalence of PD may be higher in industrialized countries. Schoenberg et al. compared the prevalence of PD in Copiah County, Mississippi, (341/100,000 over age 39) to Igbo-Ora, Nigeria (67/100,000 over age 39), using similar methodology and studying genetically similar populations. They concluded that environmental factors may be responsible for the observed higher prevalence in the industrialized US population.7 Similarly, a door-to-door epidemiologic study of PD in China found a prevalence of 57/100,000.8 Presumably, the degree of U.S. industrialization may account for some of this difference. However, a small study based in a health district in Canada found a lower risk of PD in industrialized areas of the district.5 In a population based mortality study, Rybicki et al. demonstrated that counties in Michigan with a higher concentration of industries with potential for heavy-metal exposures (iron, zinc, copper, mercury, magnesium, and manganese) had a higher PD death rates.9 Using levodopa prescription records as a surrogate for PD, two studies have shown an increased risk of PD in areas with prominent employment in wood pulp and steel alloy industries.10 Some potential confounds to the surrogate diagnosis and study methodology include inclusion of non-PD phenocopies and inability to separate working in an environment from living in an environment. If increasing world industrialization is a risk factor for PD, the incidence should be increasing throughout the last century. Only one study has addressed the incidence of PD over time. The yearly incidence of PD has not significantly changed between 1955 and 1970 in Rochester, Minnesota.11 However, it is unlikely that there has been a substantial change in the industrialization of this relatively rural community over that period of time. No preindustrial epidemiology studies of PD exist, and many cases of PD likely went unrecognized in the beginning of industrialization in this country. It may be possible to reconcile these contradictory data with more attention to regional differences in industrial pollution and farming practices.
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