title: Estimating risk of emergency room visits for asthma from personal versus fixed site measurements of NO2.
abstract: BACKGROUND
  We examined the impact of data source and exposure measurement error for ambient NO2 on risk estimates derived from a case-crossover study of emergency room visits for asthma in Windsor, Canada between 2002 and 2009.
  
  
  METHODS
  Paired personal and fixed-site NO2 data were available from an independent population (47 children and 48 adults) in Windsor between 2005 and 2006. We used linear regression to estimate the relationship and measurement error variance induced between fixed site and personal measurements of NO2, and through a series of simulations, evaluated the potential for a Bayesian model to adjust for this change in scale and measurement error. Finally, we re-analyzed data from the previous case-crossover study adjusting for the estimated change in slope and measurement error.
  
  
  RESULTS
  Correlations between paired NO2 measurements were weak (R(2)≤0.08) and slopes were far from unity (0.0029≤β≤0.30). Adjusting the previous case-crossover analysis suggested a much stronger association between personal NO2 (per 1ppb) (Odds Ratio (OR)=1.276, 95% Credible Interval (CrI): 1.034, 1.569) and emergency room visits for asthma among children relative to the fixed-site estimate (OR=1.024, 95% CrI 1.004-1.045).
  
  
  CONCLUSIONS
  Our findings suggest that risk estimates based on fixed-site NO2 concentrations may differ substantially from estimates based on personal exposures if the change in scale and/or measurement error is large. In practice, one must always keep the scale being used in mind when interpreting risk estimates and not assume that coefficients for ambient concentrations reflect risks at the personal level.
authors.affiliation: Air Health Effects Science Division, Health Canada, Ottawa, Canada; Department of Environmental and Occupational Health, University of Montreal, Montreal, Canada. Electronic address: scott.weichenthal@hc-sc.gc.ca.
authors.forename: Scott
authors.initials: S
authors.lastname: Weichenthal
authors.affiliation: McGill University Health Center, Division of Clinical Epidemiology, Montreal, Canada.
authors.forename: Patrick
authors.initials: P
authors.lastname: Bélisle
authors.affiliation: Air Health Effects Science Division, Health Canada, Ottawa, Canada.
authors.forename: Eric
authors.initials: E
authors.lastname: Lavigne
authors.affiliation: Institute of Health: Science, Technology and Policy, Carleton University, Ottawa, Ontario, Canada.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Villeneuve
authors.affiliation: Air Health Effects Science Division, Health Canada, Ottawa, Canada.
authors.forename: Amanda
authors.initials: A
authors.lastname: Wheeler
authors.affiliation: Department of Civil and Environmental Engineering, University of Windsor, Windsor, Canada.
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Xu
authors.affiliation: McGill University Health Center, Division of Clinical Epidemiology, Montreal, Canada; McGill University, Department of Epidemiology, Biostatistics and Occupational Health, Montreal, Canada.
authors.forename: Lawrence
authors.initials: L
authors.lastname: Joseph
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000393:Air Pollutants / Q000032:analysis / Q000633:toxicity*; D001249:Asthma / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D018592:Cross-Over Studies; D004636:Emergency Service, Hospital / Q000706:statistics & numerical data; D004781:Environmental Exposure; D004784:Environmental Monitoring; D005260:Female; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D009585:Nitrogen Dioxide / Q000032:analysis / Q000633:toxicity*; D009864:Ontario / Q000453:epidemiology; D012621:Seasons; D055815:Young Adult
chemical_list: D000393:Air Pollutants; D009585:Nitrogen Dioxide
keywords: Asthma; Case-crossover studies; Exposure measurement error; NO(2)