The U.S. Environmental Protection Agency (EPA) in May 2012 released the updated Regional Screening Levels (RSLs) that were developed using risk assessment guidance from the EPA Superfund program. Although the RSLs are developed for the Superfund program, most Federal and State risk-based environmental cleanup programs reference the RSLs. The RSLs are used as risk-based screening tools to determine if there is a potential need for further investigation or whether a site-specific risk assessment is required for cleanup of a site. Based on the current revisions, changes to the RSLs were both chemical specific and based on individual chemical toxicities and risk drivers (i.e., carcinogenic versus noncarcinogenic), and incorporate changes in several major potential exposure pathways.
What are the significant changes?
Eleven chemicals had their volatile organic compound (VOC) status changed due to updates in Henry’s Law constants. The chemicals that were previously classified as semi-volatile organic compounds (SVOCs), but are now VOCs, are: Dihydrosafrole, 2-Chloroacetaldehyde, Propylene, Ethyleneimine, 1,4-Dithiane, Methyl Isocyanate, Mineral Oils, and Dimethylvinylchloride. Chemicals that were once VOCs that are now SVOCs are: Isobutyl Alcohol, Cresols, and Propylene Glycol Dinitrate. The reclassification of these chemicals may affect projects where vapor intrusion is a potential complete exposure pathway because constituents of potential concern (COPCs) that may not have been included in the vapor intrusion exposure pathway previously may now be a potential risk-driver for your project. If you have a project that may have a vapor intrusion pathway, a review of the potential COPCs that may be affected by these reclassifications should be implemented, as it may affect future sampling and risk assessment tasks.
Projects that included lead as a COPC may be affected by the May 2012 update that now includes the National Ambient Air Quality Standard (NAAQS) for lead of 0.15 micrograms per cubic meter (ug/m3), which has been adopted as the residential air RSL. In previous RSL tables, there were no air screening values listed for lead.
Not all changes to the RSLs result in more stringent levels. For example, Methylene Chloride (i.e., Dichloromethane) was re-evaluated for health risks from exposure to concentrations that may be detected in indoor air due to various sources. Methylene Chloride is now listed as a mutagen in the RSL table because it is thought to cause cancer via DNA damage. Therefore, an additional safety factor was applied to take into account increased cancer susceptibility of children. Although one would expect that the RSLs would be more stringent (that is, set at a lower screening level), this is actually not the case. For indoor air concerns, the carcinogenic RSL is raised from 5.2 to 96 ug/m3 in residential buildings, and raised from 26 to 1,200 ug/m3 in commercial or industrial buildings.
Another example is tetrachloroethylene (PCE), which has a new Integrated Risk Information System (IRIS) value. The carcinogenic and non-carcinogenic RSLs typically define the EPA’s health-protective range. For indoor air, EPA’s new PCE health range for residential buildings is 9.4 to 42 ug/m3, and for commercial or industrial buildings, the new range is 47 to 180 ug/m3. Therefore, unlike PCE’s primary counterpart for chlorinated hydrocarbon cleanups, trichloroethene (TCE), the cancer RSL for PCE has actually increased (is less conservative) by about a factor of 20. It is important to note that the difference in carcinogenic versus non-carcinogenic RSLs for PCE is less than a factor of ten. Therefore, if a 1 x 10-5 risk level is used for screening, the non-carcinogenic RSL of 180 ug/m3 should be used. The RSL table identifies chemicals that are subject to this rule by including “**” in the RSL table next to the exposure pathway-specific RSL.
What were the other changes that were made to RSL chemicals?
The recent IRIS activity relative to dioxin included a new non-carcinogenic toxicity value for dioxin (2,3,7,8-TCDD). However, this update only changes the non-carcinogenic RSLs slightly (less than two fold). For example, the residential soil non-carcinogenic RSL changes from 72 to 51 parts per trillion (ppt). However, the carcinogenic RSL for residential soils of 4.5 ppt dioxin remained unchanged.
Other additions and revisions include:
- Uranium (Soluble Salts) Agency for Toxic Substances and Disease Registry (ATSDR) Reference Concentration (RfC) was removed because it was draft.
- Cyanides, Sodium Cyanide, Potassium Cyanide, Potassium Silver Cyanide, and Calcium Cyanide have new IRIS Reference Doses (RfDs).
- Cresol, p-chloro-m- and Cresol, -p now have the ATSDR RfD for Cresols.
- Tetrahydrofuran (IRIS), Hexamethylphosphoramide (Provisional Peer Reviewed Toxicity Values – PPRTV) and Sulfalone (PPRTV) are new chemicals to the RSLs, and their source for toxicity criteria referenced as IRIS or PPRTV.
Where can I get more information?
Where can I find the RSLs?