Urbanization and traffic related exposures as risk factors for schizophrenia.
BACKGROUND
Urban birth or upbringing increase schizophrenia risk. Though unknown, the causes of these urban-rural differences have been hypothesized to include, e.g., infections, diet, toxic exposures, social class, or an artefact due to selective migration.


METHODS
We investigated the hypothesis that traffic related exposures affect schizophrenia risk and that this potential effect is responsible for the urban-rural differences. The geographical distance from place of residence to nearest major road was used as a proxy variable for traffic related exposures. We used a large population-based sample of the Danish population (1.89 million people) including information on all permanent addresses linked with geographical information on all roads and house numbers in Denmark. Schizophrenia in cohort members (10,755 people) was identified by linkage with the Danish Psychiatric Central Register.


RESULTS
The geographical distance from place of residence to nearest major road had a significant effect. The highest risk was found in children living 500-1000 metres from nearest major road (RR = 1.30 (95% Confidence Interval: 1.17-1.44). However, when we accounted for the degree of urbanization, the geographical distance to nearest major road had no significant effect.


CONCLUSIONS
The cause(s) or exposure(s) responsible for the urban-rural differences in schizophrenia risk were closer related to the degree of urbanization than to the geographical distance to nearest major road. Traffic related exposures might thus be less likely explanations for the urban-rural differences in schizophrenia risk.
Pedersen
Carsten Bøcker
CB
National Centre for Register-based Research, University of Aarhus, Taasingegade 1, 8000 Aarhus C, Denmark. cbp@ncrr.dk
Mortensen
Preben Bo
PB
D000293:Adolescent; D000328:Adult; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D004781:Environmental Exposure / Q000009:adverse effects*; D005260:Female; D005843:Geography; D006801:Humans; D008297:Male; D018986:Motor Vehicles / Q000706:statistics & numerical data*; D012042:Registries; D012111:Residence Characteristics; D012307:Risk Factors; D012424:Rural Population / Q000706:statistics & numerical data; D012559:Schizophrenia / Q000453:epidemiology*; D014505:Urban Population / Q000706:statistics & numerical data; D014507:Urbanization / Q000639:trends*