title: An approach to represent a combined exposure to air pollution.
abstract: OBJECTIVE
  The objective of this study was to present a technique for estimating the effect of ambient air pollution mix on health outcomes.
  
  
  METHODS
  We created a technique of indexing air pollution mix as a cause of the increased odds of health problems. As an illustrative example, we analyzed the impact of pollution on the frequency of emergency department (ED) visits due to colitis among young patients (age < 15 years, N = 11 110). Our technique involves 2 steps. First, we considered 6 ambient air pollutants (carbon monoxide, nitrogen dioxide, sulphur dioxide, ozone, and 2 measures of particulate matter) treating each pollutant as a single exposure. Odds ratios (ORs) for ED visits associated with a standard increase (interquartile range - IQR) in the pollutants levels were calculated using the case-crossover technique. The ORs and their 95% confidence intervals (95% CIs) were also found for lagged exposures (for lags 1-9 days). Second, we defined a Health Air Study Index (HASI) to represent the combined impact of the 6 air pollutants.
  
  
  RESULTS
  We obtained positive and statistically significant results for individual air pollutants and among them the following estimations: OR = 1.06 (95% CI: 1.02-1.1, NO2 lag 3, IQR = 12.8 ppb), OR = 1.04 (95% CI: 1.01-1.07, SO2 lag 4, IQR = 2.3 ppb), OR = 1.04 (95% CI: 1-1.06, PM lag 3, IQR = 6.2 μg/m3). Among the re-calculated ORs with the HASI values as an exposure, the highest estimated value was OR = 1.37 (95% CI: 1.12-1.68, for 1 unit of the HASI, lag 3).
  
  
  CONCLUSIONS
  The proposed index (HASI) allows to confirm the pattern of associations for lags obtained for individual air pollutants. In the presented example the used index (HASI) indicates the strongest relation with the exposure lagged by 3 days.
authors.affiliation: Health Canada, Ottawa, Canada (Population Studies Division). mietek.szyszkowicz@hc-sc.gc.ca.
authors.forename: Mieczyslaw
authors.initials: M
authors.lastname: Szyszkowicz
mesh_terms: D000328:Adult; D000397:Air Pollution / Q000009:adverse effects* / Q000032:analysis; D002170:Canada / Q000453:epidemiology; D004781:Environmental Exposure / Q000009:adverse effects* / Q000032:analysis; D018876:Environmental Illness / Q000175:diagnosis / Q000453:epidemiology*; D004784:Environmental Monitoring; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D016017:Odds Ratio; D052638:Particulate Matter / Q000032:analysis*; D018570:Risk Assessment / Q000379:methods*; D055815:Young Adult
chemical_list: D052638:Particulate Matter
keywords: air pollution; ambient air pollution; case-crossover; emergency department visit; index; odds ratio