Research Note 95-5 - Toxic Air Pollutants, Environmental Media, Air, Water, Soil, Food

No. 95-5
February 1995
California Environmental Protection Agency Brief Reports to the Scientific and Technical
Air Resources Board Community

Research Division, John R. Holmes, Ph.D., Chief P.O. Box 2815, Sacramento CA 98512

Transfer of Toxic Air Pollutants Among Environmental Media

The literature concerning seven toxic air pollutants was critically reviewed to obtain estimates of factors affecting the transfer of these substances between media to which humans are exposed, or which are involved in exposure. Estimates were provided where data were deficient. The pollutants reviewed are representative of chemical classes frequently encountered as air pollutants that are especially toxic. The study was performed by the University of California, Los Angeles.


California's regulations for air toxics require the assessment of human health risk from exposures to these pollutants as found in air, water, soil, food, and other media to which people are exposed, or which serve as intermediates in the path from source to exposure. This requires an understanding of the processes that govern the movement of pollutants between the media. Using toxics levels in the ambient air as a starting point, algorithms have been developed to estimate the extent of transfer of the pollutants from one medium to another. This approach requires knowledge of intermedia transfer factors (such as the ratio at which a pollutant is distributed between two contiguous media). Default values must often be used due to a lack of data. The objective of this project was to develop a critically reviewed database of intermedia transfer factors for seven toxic air pollutants that are of concern to the ARB. The pollutants studied were 2,3,7,8-tetrachlorodibenzo-p-dioxin, hexavalent chromium, benzo[a]pyrene, methylene chloride, benzene, mercury, and formaldehyde.
Methods: From reference materials the researchers obtained ranges of values for certain physical and chemical properties, such as vapor pressure and water solubility, for each of the seven pollutants. With this information they determined the most reliable values of input parameters for use in Cal/EPA's Health Risk Assessment model. An example of such an input parameter is amount of cow's milk consumed per day by a 6-year-old child. Based on a critical review of literature values for physical, chemical, and partitioning processes that are important in determining intermedia transfer factors, the researchers assembled coefficients for these processes and reviewed their reliability and accuracy. Using models that were applicable over the range of values of conditions of interest such as anticipated wind speeds and soil properties, the investigators used the data obtained in this study to estimate physical, chemical, and transport properties for the seven pollutants of interest.
Results: Each intermedia transfer factor summary is intended as a self-contained report for the pollutant evaluated. Listed below are some of the important findings of the review.

  • Air-to-leaf uptake may be a significant exposure pathway for vegetation ingestion.
  • The most significant degradation process for benzene in the atmosphere is its reaction with hydroxyl radical during daylight hours.
  • Biodegradation is the most significant loss mechanism in soil and water.
Hexavalent chromium, Cr(VI)
  • Inhalation is the most important exposure route.
  • Cr(VI) only exists in compounds or complexes, and the bioavailability is a function of the solubility and particle size, which in turn depends on the emission source and environmental matrix.
  • The main transformation product of Cr(VI) is to Cr(III), by reduction. But in oxidizing soils, Cr(III) may be oxidized to Cr(VI).
  • Resuspension of dust may be a significant exposure pathway.
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)
  • The important processes that are responsible for the removal of TCDD from the atmosphere are photochemical degradation, hydroxyl radical attack, and wet and dry deposition.
  • The distribution of TCDD between gas- and particle-bound phases (partitioning) and the mass distribution of TCDD as a function of particle size is critical in determining the relative importance of various transport mechanisms.
  • Resuspension of particle-bound TCDD, which may be a significant exposure pathway, is currently not considered in most exposure models.
  • Bioavailability of TCDD from soil and water is limited by its strong binding properties and very low solubility.
Methylene chloride (MC)
  • Commercial and industrial uses of MC will probably continue to increase.
  • Reaction with the hydroxyl radical is the main removal mechanism from the air, while evaporation is the main removal mechanism from water.
  • Indoor air exposure from use of consumer products is a significant exposure pathway.
  • Some bacterial strains mineralize MC; this is a degradation pathway that should be considered.
Benzo[a]pyrene (BaP)
  • The most effective degradation processes for BaP are photochemical breakdown, reaction with hydroxyl radicals, and wet and dry deposition.
  • Depending on the soil microflora, biodegradation can be a significant breakdown route for BaP.
  • Since BaP is not volatile or water soluble, the most important transport mechanism is through adsorption onto particles.
  • The most significant intermedia transfer of BaP is to the soil compartment. Dry deposition and rain wash-out of the particle-bound BaP dominates the transfer of BaP from air to soil.
Mercury (Hg)
  • Most of Hg emissions are in the form of elemental Hg. Emissions from human activities account for 10-30 percent of total emissions, mainly as by-products of combustion. Most of these emissions are to the air.
  • The processes that chiefly determine the transport of Hg in the environment are: microbial methylation and demethylation, oxidation-reduction reactions, solubility and precipitation equilibria, uptake by biota, atmospheric dry and wet deposition, volatilization, and both sorption and desorption.
  • The most significant exposure pathway is ingestion of fish contaminated with methyl mercury.
Formaldehyde (HCHO)
  • Photolysis and reaction with hydroxyl radical are the main removal mechanisms from the air, while biodegradation is the main removal process from water.HCHO is not expected to bioaccumulate, since its octanol/water partition coefficient is low. However, the value for this coefficient is known only approximately.
  • HCHO is not expected to adsorb strongly onto soil; it therefore would easily leach into groundwater.
Significance and Application: The California Health and Safety Code requires Cal/EPA to conduct multipathway exposure assessments of toxic air pollutants according to specified methodologies. The information developed in this study will support Cal/EPA's assessments of the seven pollutants studied. The reviews of the intermedia transfer factors provide useful and scientifically robust information that will assist the ARB, local air districts, and private companies in meeting the requirements of the air toxics "hot spots" regulations. ARB staff will use this information as they consider possible airborne toxic control measures, and other Cal/EPA staff will use it for their risk assessment needs.
Related Projects: ARB Contract No. 92-344 is underway to develop a general purpose predictor of intermedia transfer factors that would be a companion module to the Cal/EPA health risk assessment model contained in State guidelines.

This research was conducted under contract with the University of California, Los Angeles (ARB Contract No. A032-170). Comments or questions can be directed to the contract manager, Ralph Propper, by mail, FAX (916) 322-4357, phone (916) 323-1535, or e-mail: For an index of Research Notes, call (916) 445-0753 or FAX (916) 322-4357.
Copies of the research report upon which this Note is based can be ordered from:
National Technical Information Service
5285 Port Royal Rd
Springfield VA 22161
Request NTIS No. PB95-261376
Title: Development of Intermedia Transfer Factors for Toxic Air Pollutants
Author(s): Yoram Cohen and Arthur M. Winer