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PT Notes

EPA RFI - Threshold Quantities and Off-Site Consequence Analysis Endpoints

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On July 24, 2014, the U.S. Environmental Protection Agency (EPA) announced a Request for Information (RFI) seeking comments on potential revisions to its Risk Management Program (RMP) regulations and related programs to modernize its regulations. Multiple issues were addressed in the RFI.

One of the issues addresses threshold quantities (TQs) and the off-site consequence analysis (OCA) endpoints for regulated substances. EPA is considering the use of Acute Exposure Guideline Levels (AEGLs) to recalculate RMP reporting thresholds and toxic endpoints for OCAs in order to reflect better the potential for adverse effects of an accidental release upon a community.

Current RMP TQs are based on Immediately Dangerous to Life and Health (IDLH) values and EPA is considering recalculating them because IDLH values:

  • Are based upon the response of healthy male workers and do not take into account the exposure of more sensitive individuals, such as the elderly, pregnant women, children, or people with various health problems.
  • Are based upon a maximum 30-minute exposure period which may not reflect, and may underestimate, actual exposures to accidental airborne releases.
  • May not reflect concentrations that could result in serious but reversible injury because IDLH values were designed only to protect workers against concentrations that would prevent death or irreversible health effects, or other deleterious effects (e.g. disorientation or lack of coordination) that would prevent escape.

AEGL values represent threshold exposure limits (exposure levels below which adverse health effects are not likely to occur) for the general public and are applicable to emergency exposures ranging from 10 minutes to 8 hours. AEGL values are designed to protect the general population, including susceptible sub-populations, such as infants, children, the elderly, persons with asthma, and those with other illnesses, which are groups not generally considered in the development of workplace exposure levels. A chemical may have up to three AEGL values, each of which corresponds to a specific tier of health effects.

EPA believes that the use of AEGL values to recalculate RMP reporting thresholds would better reflect the potential for adverse effects of an accidental release upon individuals in a community compared to IDLH values because AEGL values take into account the potential exposure of more sensitive individuals, the potential for longer periods of exposure, and the potential for serious but reversible injuries.

In situations where no AEGL value exists for a chemical, EPA would use Emergency Response Planning Guidelines (ERPGs), if available, to recalculate reporting thresholds. ERPG values estimate the concentrations at which most people will begin to experience health effects if they are exposed to a hazardous airborne chemical for 1 hour. However, sensitive members of the public such as old, sick, or very young people, are not covered and may experience adverse effects at concentrations below the ERPG values. A chemical may have up to three ERPG values, each of which corresponds to a specific tier of health effects.

EPA previously used ERPG values to establish toxic endpoints (i.e. air concentrations) for each RMP toxic chemical to be used when conducting OCAs. However, one consequence of using separate toxicity values for TQs (based upon IDLHs) and toxic endpoints (based upon EPRG-2 values) is some inconsistency in the representation of the relative toxicities of certain substances compared to others. To remedy such issues, EPA is considering the use of AEGL values as the basis for determining both the RMP reporting thresholds and the toxic endpoints.

Furthermore, in those cases where an AEGL value is not available, using EPRG-2 values to calculate both the reporting threshold and toxic endpoint also will remove such inconsistencies. Finally, using AEGL values, when available, to recalculate current toxic endpoints also will take into account the potential exposure of more sensitive individuals, which is not addressed when using ERPG-2 values.

TQs would be expected to decrease when based on AEGL-2 values. However, that is not necessarily the case due to the use of a range of calculated index values to assign a TQ. If EPA used the new index values to assign TQs based on current ranges, then TQs for substances that currently have higher TQs would tend to drop, while TQs for substances with lower TQs would generally remain unchanged. Therefore, under this scenario, most substances would be grouped into the lower TQs.

Alternatively, if EPA established TQs by disregarding the old index ranges and redefining the index value ranges for each TQ according to the new range of index values alone, then the change would have the effect of reshuffling substances into new TQs. In this scenario, incorporating AEGL values likely would result in reducing the TQ for some substances while raising it, or causing no change, for others. As the purpose of assigning TQs according to a distribution of index values was to assign lower TQs to the more toxic and easily dispersed substances, and higher TQs to less toxic and less easily dispersed substances, EPA believes this approach may be more appropriate.

Adopting AEGL-2 values in place of ERPG-2 values to establish new toxic endpoints would have a more direct effect. AEGL-2 values are often, but not always, lower than the existing toxic endpoints. Where the AEGL value is lower than the current toxic endpoint for a particular substance, the new toxic endpoint would likewise be lower. The practical effect of changing toxic endpoints would be to change the OCA distance to endpoint for a given substance and release quantity. For all processes containing substances with new lower toxic endpoints, larger worst-case and alternative release scenario zones would result, whereas processes containing substances with new higher toxic endpoints would have smaller OCA zones. If most toxic endpoints were either to decrease or to remain the same, another result likely would be that fewer regulated processes would be eligible for Program 1.

Questions posed by EPA on this issue include:

  1. Would revising the RMP rule to incorporate AEGL-2, and ERPG-2 values when an AEGL is not available, as the basis for TQs and toxic endpoints make the RMP rule more protective of human health and the environment? Would it result in significant changes to the universe of RMP-regulated facilities due to potential changes in TQs? If so, what number and types of facilities would be most affected and what changes would occur?
  2. The IDLH values used for setting the existing TQs are based on an exposure period of 30 minutes. If the IDLH value was not available, the acute toxicity data used to determine the equivalent IDLH value varied depending on the chemical and actual study, and these numbers typically ranged from 1 to 8 hours. The ERPG-2 values used for the toxic endpoints represent an exposure period of 1 hour. Given that AEGL values are established with five different exposure periods (10 minutes, 30 minutes, 1 hour, 4 hours, and 8 hours), which exposure time should be used if the AEGL value is used to determine the TQs and/or toxic endpoints?
  3. What should be the hierarchy for developing an alternative or equivalent toxicity level of concern (LOC) when an AEGL value has not been established for a toxic substance? Should ERPG values be used instead if they exist? If no ERPG value exists, should an LOC based on the IDLH value be used instead if it exists? If there is no IDLH value, how should the LOC be calculated for either the TQ or toxic endpoint? Is there an alternative method for establishing an equivalent LOC for those chemicals not having an AEGL or ERPG that will result in an appropriate TQ?
  4. Currently, RMP worst-case scenarios can be based on 10-minute or 60-minute release times. Because many AEGL-2 values are established for 1-hour, 4-hour and 8-hour exposure periods, should requirements for determining the worst-case and alternative release scenarios also incorporate four and eight hour release times using the 4-hour and 8-hour AEGL-2 values for a particular toxic chemical?
  5. Should EPA consider using AEGL-1 rather than AEGL-2 values for calculating reporting thresholds and toxic endpoints in order to address acute effects that are transient and reversible (such as discomfort and irritation)?
  6. What would be the economic impacts of recalculating TQs as discussed above? Are there any special circumstances involving small entities that EPA should consider with respect to recalculating TQs?
Further details on this issue are provided in EPA's RFI which can be accessed by clicking here.

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