Toxics 100 Technical Notes

The Toxics Release Inventory (TRI), compiled by the U.S. Environmental Protection Agency (EPA) in accordance with the Emergency Planning and Community Right-to-Know Act of 1986, annually reports the weight (in pounds) of each of approximately 600 toxic chemicals released into the environment by major industrial facilities in the United States.

Our analysis uses year 2005 releases of toxic chemicals into air nationwide. We combine fugitive, stack, and incinerator releases. In all, TRI-reporting facilities released more than 1.7 billion pounds of toxic chemicals into the air in 2005.

The EPA Office of Pollution Prevention and Toxics processes the raw TRI reports to create the Risk Screening Environmental Indicators (RSEI). The EPA combines three variables to assess the human health risks posed by toxic releases:

• fate and transport, or how the chemical spreads from the point of release to the surrounding area;
• toxicity, or how dangerous the chemical is on a per-pound basis; and
• population, or how many people live in the affected areas.

Each release begins at a smokestack, leaking valve, open canister, or other point source within the facility or at the stack of an offsite incineration facility. Using the Industrial Source Complex—Long Term (ISCLT3) model, EPA combines data on local wind patterns, temperature, and topography with information on the smokestack height and the exit velocity of released gases and information about each chemical (molecular weight and rate of decay in sunlight and air) to determine the concentrations of releases in each square kilometer within a 101 km x 101 km grid around the release site.

EPA matches each chemical to a toxicity weight that expresses the relative toxicity of the chemical per pound. Although all TRI chemicals are hazardous, their toxicities vary greatly. For example, one pound of asbestos is equivalent, in terms of toxicity, to 27 million pounds of the chemical chlorodifluoromethane (HCFC-22). The enormous variation in toxicity limits the usefulness of comparisons on the basis of the simple mass (pounds) of chemicals released. By multiplying the mass of each toxic release by its toxicity weight, EPA can compare the toxic significance of releases of different chemicals.

The EPA's toxicity-weighting system is based on peer-reviewed databases including those of the EPA's Integrated Risk Information System (IRIS), the EPA's Office of Pesticide Programs (OPP) Reference Dose Tracking Reports, the U.S. Department of Health and Human Services Agency for Toxic Substances and Disease Registry (ATSDR), the California Environmental Protection Agency (CalEPA) Office of Environmental Health Hazard and Assessment (OEHHA), and the EPA's Health Effects Assessment Tables (HEAST). For some of the chemicals listed in the TRI, no consensus has been reached regarding the appropriate toxicity weight, and these chemicals are excluded from the analysis. In the TRI data for the year 2005, approximately 1 percent of the total mass of toxic releases lacked toxicity weights. Further details on the toxicity weights are available from the EPA (.pdf), or download a spreadsheet of the toxicity weights here (.xls).

After accounting for the quantity, dispersion, and toxicity of the release, EPA multiplies toxicity-weighted concentrations by the number of people living in each of the square-kilometer grid cells to measure the population health risk. (EPA slightly modifies the simple head count to account for differential uptake of chemicals depending on the age and sex composition of the exposed population.) A facility located in an urban area with high population density thus generates more risk than a facility with identical releases in a less populous rural area. To obtain the RSEI score for the facility, EPA aggregates the population-weighted, toxicity-weighted impacts for the entire area around the facility.

Using information on company ownership of facilities from the TRI reports, Dun & Bradstreet's Million Dollar Database, Mergent Online, Hoover's, company websites, printed reports, and telephone calls, we matched each facility to its parent company. Individual facilities are assigned to corporate parents on the basis of ownership structure in the year of 2007. We then aggregated the RSEI scores for air releases of toxics by the facilities owned by each parent company, and ranked companies on this basis. The Toxic 100 reports the top polluters among the companies that appeared on the Fortune 500, Fortune Global 500, and S&P 500 lists, and among the largest 500 U.S. and 500 foreign-owned corporations on the Forbes Global 2000 list in 2007.