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EPA-Expo-Box (A Toolbox for Exposure Assessors)

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Fate and Transport

Fate and transport processes “link” the release of contaminants at a source with the resultant environmental concentrations to which receptors can be exposed. When a contaminant is released from a source, it is subject to transport and transformation in the environment. Compounds can also transfer from an environmental medium to biota.

Migration Process Examples Relevant to Air
  • Dispersion of a contaminant through ambient air (transport within a medium)
  • Deposition and resuspension of a contaminant between air and soil (transport between media)
  • Chemical breakdown in air due to photolysis (chemical change)
  • Contaminant adsorbs to particulates in air (physical change)
Transfer – Environment to Biota
  • Contaminants in air deposit on vegetation that is consumed by grazing or foraging animals. Animals bioaccumulate the contaminants in their tissues.
  • Fish exposed to chemicals that deposit from air to water. Exposure could occur via direct uptake from water through gills or by indirect food chain uptake.

For additional information on the uptake of contaminants from plants to animals used as a source of food, see the Food Module in the Media Tool Set of EPA-Expo-Box. For additional information on the uptake of contaminants from water to fish, refer to the Aquatic Biota Module in the Media Tool Set.

Indoor or outdoor dust that is airborne represents a potential inhalation exposure and is therefore relevant to the discussion of air contaminants provided in this module. Settled dust that represents potential dermal and ingestion exposures is discussed as part of the Soil and Dust Module in the Media Tool Set, and readers are referred to that module for more information. It is important to recognize the cycle of deposition and resuspension for some contaminants, in which the inhalation, dermal, and ingestion exposure pathways might all be relevant.

Several information sources, models, and calculation tools are available for characterizing air dispersion and transport of contaminants to other media, and many of these resources are described within this module. Consideration of other media affected by air contaminants is important when evaluating aggregate exposure from multiple exposure pathways.

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Air—Some Important Physicochemical Factors
  • Vapor pressure—an indicator of how likely a compound will remain in the gaseous phase; the higher a chemical’s vapor pressure, the more likely that it will be found in the gas phase.
  • Air-water partition coefficient (i.e., Henry's Law constant [KH])—provides an index of partitioning for a compound between atmospheric and aqueous phases; higher values of KH are associated with compounds that preferentially partition to air rather than to water.
  • Octanol/air partition coefficient (Koa)—describes partitioning between air and aerosol particles, air and foliage, and air and soil and is an indicator of chemical mobility in the atmosphere; higher values of Koa indicate a tendency to sorb onto solid surfaces—vegetation, soils, aerosol particles, etc.
  • Molecular diffusivity—represents the propensity of a chemical to move through air and is a function of the chemical (primarily molecular weight) and the air (e.g., atmospheric temperature, pressure); useful in calculating dry deposition velocities.

After pollutants are released to the atmosphere, their transport, dispersion, and transformation are governed by several factors including:

  • Meteorological factors (e.g., wind, temperature, precipitation)
  • Terrain characteristics (e.g., amount and type of vegetation, presence of water bodies)
  • Wet and dry deposition rates
  • Certain physicochemical properties of the air pollutant (see box on right)
Physicochemical properties data will help determine whether a chemical is likely to remain in the air, partition to other media, or transform physically, chemically, or biologically after release. Physicochemical properties data may be determined by measurements or estimated based on chemical structure; use of chemical-specific measured values is typically preferred. Variability in the indoor air concentrations can be influenced by indoor/outdoor exchange rates, heating and air conditioning systems, and building ventilation rates.

There are a number of sources that provide data that are useful in predicting fate and transport of contaminants in air.

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Fate and transport models can be used to estimate concentrations of contaminants at the point-of-contact for a receptor population. Models might also be used to describe the multimedia transport and fate of pollutants from air to other media (e.g., deposition on soil that may be taken up by plants that are consumed by animals used a source of food; deposition on water that may be used as a source of drinking water, recreation such as fishing or swimming, or crop irrigation; deposition onto indoor surfaces).

Some ambient air models describe dispersion and deposition only; others also estimate exposure concentrations and allow users to characterize risk to potential receptors. Likewise, some indoor air models estimate concentrations based on fate and transport processes as well as quantify individual exposure and risk based on activity patterns and source use.

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