title: Prioritizing chemicals and data requirements for screening-level exposure and risk assessment.
abstract: BACKGROUND
  Scientists and regulatory agencies strive to identify chemicals that may cause harmful effects to humans and the environment; however, prioritization is challenging because of the large number of chemicals requiring evaluation and limited data and resources.
  
  
  OBJECTIVE
  We aimed to prioritize chemicals for exposure and exposure potential and obtain a quantitative perspective on research needs to better address uncertainty in screening assessments.
  
  
  METHODS
  We used a multimedia mass balance model to prioritize > 12,000 organic chemicals using four far-field human exposure metrics. The propagation of variance (uncertainty) in key chemical information used as model input for calculating exposure metrics was quantified.
  
  
  RESULTS
  Modeled human concentrations and intake rates span approximately 17 and 15 orders of magnitude, respectively. Estimates of exposure potential using human concentrations and a unit emission rate span approximately 13 orders of magnitude, and intake fractions span 7 orders of magnitude. The actual chemical emission rate contributes the greatest variance (uncertainty) in exposure estimates. The human biotransformation half-life is the second greatest source of uncertainty in estimated concentrations. In general, biotransformation and biodegradation half-lives are greater sources of uncertainty in modeled exposure and exposure potential than chemical partition coefficients.
  
  
  CONCLUSIONS
  Mechanistic exposure modeling is suitable for screening and prioritizing large numbers of chemicals. By including uncertainty analysis and uncertainty in chemical information in the exposure estimates, these methods can help identify and address the important sources of uncertainty in human exposure and risk assessment in a systematic manner.
authors.affiliation: Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada. jon.arnot@utoronto.ca
authors.forename: Jon A
authors.initials: JA
authors.lastname: Arnot
authors.forename: Trevor N
authors.initials: TN
authors.lastname: Brown
authors.forename: Frank
authors.initials: F
authors.lastname: Wania
authors.forename: Knut
authors.initials: K
authors.lastname: Breivik
authors.forename: Michael S
authors.initials: MS
authors.lastname: McLachlan
mesh_terms: D004781:Environmental Exposure; D004784:Environmental Monitoring / Q000379:methods*; D004785:Environmental Pollutants / Q000032:analysis / Q000633:toxicity*; D006801:Humans; D008956:Models, Chemical; D009930:Organic Chemicals / Q000032:analysis / Q000633:toxicity*; D018570:Risk Assessment; D035501:Uncertainty
chemical_list: D004785:Environmental Pollutants; D009930:Organic Chemicals