title: The impact of daily mobility on exposure to traffic-related air pollution and health effect estimates.
abstract: Epidemiological studies of traffic-related air pollution typically estimate exposures at residential locations only; however, if study subjects spend time away from home, exposure measurement error, and therefore bias, may be introduced into epidemiological analyses. For two study areas (Vancouver, British Columbia, and Southern California), we use paired residence- and mobility-based estimates of individual exposure to ambient nitrogen dioxide, and apply error theory to calculate bias for scenarios when mobility is not considered. In Vancouver, the mean bias was 0.84 (range: 0.79-0.89; SD: 0.01), indicating potential bias of an effect estimate toward the null by ~16% when using residence-based exposure estimates. Bias was more strongly negative (mean: 0.70, range: 0.63-0.77, SD: 0.02) when the underlying pollution estimates had higher spatial variation (land-use regression versus monitor interpolation). In Southern California, bias was seen to become more strongly negative with increasing time and distance spent away from home (e.g., 0.99 for 0-2 h spent at least 10 km away, 0.66 for ≥ 10 h spent at least 40 km away). Our results suggest that ignoring daily mobility patterns can contribute to bias toward the null hypothesis in epidemiological studies using individual-level exposure estimates.
authors.affiliation: Geography Department, University of Victoria, Victoria, British Columbia, Canada. elsetton@uvic.ca
authors.forename: Eleanor
authors.initials: E
authors.lastname: Setton
authors.forename: Julian D
authors.initials: JD
authors.lastname: Marshall
authors.forename: Michael
authors.initials: M
authors.lastname: Brauer
authors.forename: Kathryn R
authors.initials: KR
authors.lastname: Lundquist
authors.forename: Perry
authors.initials: P
authors.lastname: Hystad
authors.forename: Peter
authors.initials: P
authors.lastname: Keller
authors.forename: Denise
authors.initials: D
authors.lastname: Cloutier-Fisher
mesh_terms: D000393:Air Pollutants / Q000032:analysis; D000397:Air Pollution / Q000032:analysis* / Q000706:statistics & numerical data; D015982:Bias; D001955:British Columbia; D002140:California; D004781:Environmental Exposure / Q000032:analysis* / Q000706:statistics & numerical data; D004784:Environmental Monitoring / Q000379:methods; D006801:Humans; D009585:Nitrogen Dioxide / Q000032:analysis; D012111:Residence Characteristics; D013997:Time Factors; D014504:Urban Health; D001335:Vehicle Emissions / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D001335:Vehicle Emissions; D009585:Nitrogen Dioxide