title: Potential confounding of particulate matter on the short-term association between ozone and mortality in multisite time-series studies.
abstract: BACKGROUND
  A critical question regarding the association between short-term exposure to ozone and mortality is the extent to which this relationship is confounded by ambient exposure to particles.
  
  
  OBJECTIVE
  We investigated whether particulate matter < 10 and < 2.5 microm in aerodynamic diameter (PM(10) and PM(2.5)) is a confounder of the ozone and mortality association using data for 98 U.S. urban communities from 1987 to 2000.
  
  
  METHODS
  We a) estimated correlations between daily ozone and daily PM concentrations stratified by ozone or PM levels; b) included PM as a covariate in time-series models; and c) included PM as a covariate as in d), but within a subset approach considering only days with ozone below a specified value.
  
  
  RESULTS
  Analysis was hindered by data availability. In the 93 communities with PM(10) data, only 25.0% of study days had data on both ozone and PM(10). In the 91 communities with PM(2.5) data, only 9.2% of days in the study period had data on ozone and PM(2.5). Neither PM measure was highly correlated with ozone at any level of ozone or PM. National and community-specific effect estimates of the short-term effects of ozone on mortality were robust to inclusion of PM(10) or PM(2.5) in time-series models. The robustness remains even at low ozone levels (< 10 ppb) using a subset approach.
  
  
  CONCLUSIONS
  Results provide evidence that neither PM(10) nor PM(2.5) is a likely confounder of observed ozone and mortality relationships. Further investigation is needed to investigate potential confounding of the short-term effects of ozone on mortality by PM chemical composition.
authors.affiliation: School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA. michelle.bell@yale.edu
authors.forename: Michelle L
authors.initials: ML
authors.lastname: Bell
authors.forename: Jee Young
authors.initials: JY
authors.lastname: Kim
authors.forename: Francesca
authors.initials: F
authors.lastname: Dominici
mesh_terms: D000397:Air Pollution / Q000032:analysis*; D015986:Confounding Factors, Epidemiologic; D006801:Humans; D009026:Mortality; D010126:Ozone / Q000032:analysis / Q000633:toxicity*; D052638:Particulate Matter / Q000032:analysis*; D013997:Time Factors; D014481:United States / Q000453:epidemiology; D014505:Urban Population
chemical_list: D052638:Particulate Matter; D010126:Ozone