title: Associations between ambient air pollutant mixtures and pediatric asthma emergency department visits in three cities: a classification and regression tree approach.
abstract: BACKGROUND
  Characterizing multipollutant health effects is challenging. We use classification and regression trees to identify multipollutant joint effects associated with pediatric asthma exacerbations and compare these results with those from a multipollutant regression model with continuous joint effects.
  
  
  METHODS
  We investigate the joint effects of ozone, NO2 and PM2.5 on emergency department visits for pediatric asthma in Atlanta (1999-2009), Dallas (2006-2009) and St. Louis (2001-2007). Daily concentrations of each pollutant were categorized into four levels, resulting in 64 different combinations or "Day-Types" that can occur. Days when all pollutants were in the lowest level were withheld as the reference group. Separate regression trees were grown for each city, with partitioning based on Day-Type in a model with control for confounding. Day-Types that appeared together in the same terminal node in all three trees were considered to be mixtures of potential interest and were included as indicator variables in a three-city Poisson generalized linear model with confounding control and rate ratios calculated relative to the reference group. For comparison, we estimated analogous joint effects from a multipollutant Poisson model that included terms for each pollutant, with concentrations modeled continuously.
  
  
  CONCLUSIONS
  No single mixture emerged as the most harmful. Instead, the rate ratios for the mixtures suggest that all three pollutants drive the health association, and that the rate plateaus in the mixtures with the highest concentrations. In contrast, the results from the comparison model are dominated by an association with ozone and suggest that the rate increases with concentration.
  
  
  CONCLUSIONS
  The use of classification and regression trees to identify joint effects may lead to different conclusions than multipollutant models with continuous joint effects and may serve as a complementary approach for understanding health effects of multipollutant mixtures.
authors.affiliation: Department of Epidemiology, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. kgass@emory.edu.
authors.forename: Katherine
authors.initials: K
authors.lastname: Gass
authors.affiliation: Department of Environmental Health, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. mklein@emory.edu.
authors.forename: Mitch
authors.initials: M
authors.lastname: Klein
authors.affiliation: Department of Environmental Health, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. sebelt@emory.edu.
authors.forename: Stefanie E
authors.initials: SE
authors.lastname: Sarnat
authors.affiliation: Department of Environmental Health, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. awinqui@emory.edu.
authors.forename: Andrea
authors.initials: A
authors.lastname: Winquist
authors.affiliation: Department of Epidemiology, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. ldarrow@emory.edu.
authors.forename: Lyndsey A
authors.initials: LA
authors.lastname: Darrow
authors.affiliation: Department of Epidemiology, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. wflande@emory.edu.
authors.forename: W Dana
authors.initials: WD
authors.lastname: Flanders
authors.affiliation: Department of Biostatistics and Bioinformatics, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. howard.chang@emory.edu.
authors.forename: Howard H
authors.initials: HH
authors.lastname: Chang
authors.affiliation: School of Civil and Environmental Engineering, Georgia Institute of Technology, 790 Atlantic Drive, Atlanta, Georgia, 30332, USA. james.mulholand@ce.gatech.edu.
authors.forename: James A
authors.initials: JA
authors.lastname: Mulholland
authors.affiliation: Department of Environmental Health, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. ptolber@emory.ed.
authors.forename: Paige E
authors.initials: PE
authors.lastname: Tolbert
authors.affiliation: Department of Environmental Health, Rollins School of Public Health, Emory University, 1518 Clifton Rd, Atlanta, GA, 30322, USA. mjstric@emory.edu.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Strickland
mesh_terms: D000293:Adolescent; D000393:Air Pollutants / Q000009:adverse effects* / Q000032:analysis; D000397:Air Pollution / Q000009:adverse effects* / Q000032:analysis; D001249:Asthma / Q000139:chemically induced*; D002648:Child; D002675:Child, Preschool; D002947:Cities / Q000706:statistics & numerical data; D004636:Emergency Service, Hospital / Q000706:statistics & numerical data*; D004781:Environmental Exposure / Q000032:analysis; D004784:Environmental Monitoring / Q000379:methods; D005260:Female; D005845:Georgia; D006801:Humans; D016014:Linear Models; D008297:Male; D008923:Missouri; D008962:Models, Theoretical; D009609:Nitrous Oxide / Q000009:adverse effects* / Q000032:analysis; D010126:Ozone / Q000009:adverse effects* / Q000032:analysis; D052638:Particulate Matter / Q000009:adverse effects* / Q000032:analysis; D012621:Seasons; D013781:Texas
chemical_list: D000393:Air Pollutants; D052638:Particulate Matter; D010126:Ozone; D009609:Nitrous Oxide