How Accurate Are the RSEI Data on Toxic Air Pollution?

The Risk-Screening Environmental Indicators (RSEI) of the U.S. Environmental Protection Agency provide a screening measure of risks to human health, building on data on releases of toxic chemicals into the environment from the Toxics Release Inventory (TRI). On the RSEI website, the EPA characterizes the database as a tool “to identify risk-related situations of high potential concern, and which warrant further evaluation.”

In addition to the mass (pounds) of releases reported in the TRI, the RSEI data incorporate information on toxicity, fate and transport, and population densities. The RSEI data thereby provide a more robust measure of human health risks than the simple mass of releases reported by the TRI. But the RSEI data are not perfect – no data are.

Potential sources of error

1. Incorrect data submitted by facilities. Facilities sometimes provide incorrect data on pounds of releases on the ‘Form R’ reports that they submit annually for the EPA’s Toxics Release Inventory (TRI). These data are used to construct the RSEI measures for these facilities. In several cases, firms have contacted PERI to explain that they incorrectly reported numbers that were substantially higher than the true values. Where these new numbers have been reported to and accepted by the EPA, we are able to revise RSEI scores (on the assumption of a linear relation between pounds released of a specific chemical and that chemical’s contribution to the facility’s RSEI score). In principle, facilities also could error on the side of underreporting true values. No firms have contacted us to report mistakes in this direction. However, the Environmental Integrity Project has argued (in this .pdf document) that there is a systematic bias toward underreporting in the TRI data.

2. Imprecise data submitted by facilities. For chemicals other than persistent and bioaccumulative toxics (PBTs), firms can choose to report a range of pounds released (1-10, 11-499, 500-999) rather than the precise quantity, as long as the release is less than 1,000 pounds. In the 2006 air release data, such reporting accounted for approximately 5,000 releases, or 6.1 % of non-zero release reports (2.8 percent were reported as 1-10 pounds, 2.6 percent as 11-499 pounds, and less than 1 percent as 500-999 pounds). In these cases, the EPA uses the midpoint of the range (5, 250, and 750 pounds, respectively) to calculate RSEI scores. If the chemicals in question have large toxicity weights, the resulting imprecision could substantially affect the facility’s RSEI score. More precise data could change the results in either direction, depending on whether the true value is higher or lower than the range midpoint.

3. Inadequate information on stack heights. Where facility-specific information on stack heights is not available to EPA, the RSEI uses estimated stack heights based, for example, on industrial-sector averages. Insofar as actual stack heights differ from those used in the EPA’s calculations, this may affect the final scores. In general, higher-than-average stack heights would be expected to result in lower RSEI scores by virtue of ‘solution by dilution.’

4. Toxicity weights for chemical groups. In some cases, TRI groups together several related chemicals or species of the same chemical, which are reported as a category. In such cases, the RSEI assigns a toxicity weight to the group as a whole. However, the toxicity of individual compounds within the group may vary widely. Depending on which compound is used as the basis for the RSEI toxicity weight, and which compounds are released by the facility, the resulting RSEI score may overstate or understate true risks. In most cases, the most toxic chemical of each category, based on the calculated toxicity weight, is selected, and the toxicity weight for that chemical is assigned to the entire chemical category.

5. Lack of toxicity weights for some chemicals. The EPA has not yet assigned toxicity weights to all chemicals in the TRI database. In version 2.2.0 of the RSEI data, 430 of the 611 chemicals and chemical groups in the TRI database have toxicity weights and thus are included in calculating the RSEI air scores. Chemicals to which toxicity weights have been assigned account for more than 99% of the reported pounds for all on-site releases in 2006. The chemicals that have not yet been assigned a toxicity weight are not included in calculating RSEI scores. If these omitted chemicals are highly toxic, or if they represent a substantial fraction of the releases at a facility, their omission could substantially understate risk-related scores.

6. Geographical limits of modeled dispersion. In the case of facilities with tall stack heights, high exit velocities, and strong prevailing winds, the fact that the RSEI air-release model only accounts for impacts within an area that extends 50 kilometers in each direction from the one square kilometer cell that contains the facility (i.e., within a territory measuring 101 km x 101 km, yielding a total area of 10,201 square kilometers) means that the RSEI score misses impacts beyond this area. This problem may be most relevant in the case of electric utilities.

7. Focus on long-term health effects. The RSEI model only addresses chronic human toxicity (cancer and non-cancer effects, such as developmental toxicity, reproductive toxicity, neurotoxicity, etc.) associated with long-term exposure. It does not address acute health effects associated with short-term, periodic exposures to higher levels of these same chemicals, and does not address ecological effects.

8. Products of decay not modeled. Some chemicals may react and decay in the environment, and these products of decay are not included in the model. The EPA’s AERMOD model incorporates chemical-specific first-order decay rates, but does not model products of decay. Exclusion of these decay products from the model may underestimate the risk impact of releases if the decay product is toxic.

9. Imprecise location of facilities. In the RSEI model, the U.S. is divided into a grid of 1-km by 1-km square grid cells. Facilities are assigned to a specific cell in this grid according to their latitude and longitude coordinates. For modeling purposes, the facility is then assumed to be located at the center of the grid cell, regardless of where its latitude and longitude coordinates place it within the cell.

10. Releases assumed to be constant over time. A facility is assumed to release its annual discharge at a constant rate throughout the year. Annual stack air releases as reported to TRI are converted to an equivalent constant emission rate in grams per second. These assumptions ignore the possibility that relatively few high-concentration discharges per year could have different human health impacts.

11. The TRI data capture the largest point-source air pollution emissions in the United States, but they do not report emissions from mobile sources, such as trucks, automobiles, ships, and aircraft. The TRI also excludes facilities that are not required to report by virtue of small size or belonging to non-listed industrial sectors. Potentially significant air polluters not covered for these reasons include agriculture, airlines, trucking firms, gas stations, dry cleaners, and auto-body shops.

12. The chemicals in the TRI do not include some bulk pollutants that pose significant health and environmental risks, including sulfur dioxide, nitrogen oxides, ozone, carbon monoxide, particulate matter, and carbon dioxide. Nor is every toxic chemical listed. A complete picture of air pollution and the attendant health risks would include these other chemicals.

Ways to improve RSEI accuracy

The first problem – incorrect data on the mass of releases – can best be addressed by the firms themselves. Greater public visibility for TRI and RSEI data will strengthen the firms’ incentives to submit accurate data and to submit timely revisions in case of erroneous reporting. This incentive is asymmetric, however: firms have stronger motives to correct over-reporting than under-reporting. How to strengthen incentives for accurate reporting more symmetrically is a question worth further exploration.

The second problem – imprecise data resulting from reporting of releases as a range of values rather than a precise number – can be resolved by eliminating the range reporting option. This change would not add to reporting burdens since facilities already must calculate the number in order to report the relevant range

The third problem- inadequate information on stack heights- could be addressed by requiring this information to accompany TRI release data reported to the EPA.

The fourth problem – the use of toxicity weights for chemical groups – could be addressed by modifications of TRI reporting requirements to require more detailed breakdowns within important categories, including those with high toxicity weights in the RSEI. The RSEI project has begun to address the problem in the case of chromium compounds by assuming the industrial sector average share of highly toxic hexavalent chromium among total chromium releases as the basis of RSEI modeling a given chromium release.

The fifth problem – lack of toxicity weights for some chemicals – ultimately must be addressed by further toxicological research to provide the necessary information. In the meantime, the accuracy of the RSEI score might be improved by assigning to these chemicals a weight equal to the average toxicity weight for all other chemicals (rather than their current implicit weight of zero).

The sixth problem – geographical limits of modeled dispersion – could be addressed by modifying the RSEI air-release model to extend the range of impacts beyond the current 50-km limit.

The seventh problem – the model’s exclusive focus on long-term health effects - could be remedied by including acute effects in the toxicity weighting, perhaps as a separate score. Similarly, increased research on the ecological effects of these chemicals could produce an additional score based on these outcomes. These extensions would require a major effort to incorporate the results of research on these impacts.

The eighth problem – that products of decay are not modeled – could be addressed by including information on decay products and their toxicity. The model already includes information on rates of decay for TRI chemicals.

The ninth problem – the assumption that a facility is located in the center of a grid square – is could be addressed by altering the model to allow for greater geographical precision.

The tenth problem – that releases are assumed to be constant – could be addressed by improved monitoring and reporting to include information on the timing of releases as well as annual totals.

The eleventh and twelfth problems require expansion of the reporting requirements of the U.S. EPA's Toxics Release Inventory and other right-to-know regulation and legislation.

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