Toxicologists and risk assessors evaluate potential chemical exposures at hazardous sites to: (1) determine the potential risks posed by a site and, thus, whether cleanup is warranted and (2) derive cleanup levels that are protective of human health and the environment, typically, with a focus on human health because, at most sites, that is what drives the cleanup (although ecological risks are beginning to be examined more carefully).When examining the equations used by risk assessors to determine the potential risks at hazardous sites, it becomes clear that the toxicity factors used are critically important, and can result in a swing of a cleanup level by several orders of magnitude. However, toxicity factors are under constant review by the U.S. Environmental Protection Agency and state regulatory agencies, as new studies are published and the review of existing studies is completed. An excellent example of this is related to the toxicity factors for chlorinated solvents, notably trichloroethene (TCE), which has undergone intense review and evaluation in the last decade. Recently, the US EPA formed a task group to address the most recent TCE risk assessment and, concurrently, several states are moving forward with developing specific policies that are affecting remediation projects at sites with TCE and other types of chlorinated solvent contamination.
In this Editor’s Perspective, the development of toxicity values for TCE and other chlorinated solvents is explored. However, before delving into a specific discussion regarding the risks from exposure to chlorinated solvents, the overall process of calculating screening levels for carcinogenic chemicals is explained to clarify some of the risk-assessment magic. Then, the discussion focuses on the development of risk-based values for several chlorinated solvents, including the controversial events relative to establishing risk-based values for TCE. The focus of this column is primarily related to inhalation risk in order to not cloud the discussion with too many numerical values; however, the discussion would be remarkably similar for other exposure-related risks.
Understanding how risk factors are used to calculate cleanup levels
From a professional perspective, the risk-assessment process is generally not well understood by engineers, geologists, and environmental scientists involved in site investigation and cleanups. Typically, the concentration of the chemical found at the site is compared to an established table of generic cleanup values, often referred to as “lookup tables.” However, if the regulatory program has not established generic values or if the site conditions do not fit into one of the default scenarios used to develop the generic cleanup values, a risk-assessment scientist performs a risk assessment to develop cleanup levels. The risk assessor examines the property, current and future exposure pathways (including routes of exposure, e.g., ingestion), and the chemicals of concern. Following a process established by the US EPA Superfund program, defined in a document called “RAGS” (Risk Assessment Guidance for Superfund), the risk assessor determines the cleanup levels using some relatively standard equations and accepted toxicity values for the chemicals. This process has a certain magical “Voil´a!” to it as the risk assessor emerges from behind a black curtain with cleanup levels used to guide remediation projects.
The risk-assessment process followed by the risk-assessment scientist is a relatively straightforward, methodical process. Although there is some flexibility in some of the input parameters for the site exposures, for the most part the calculations are routine. However, the toxicity values input into the equations can have a profound affect on the cleanup levels derived from the process. As described below, changes in the toxicity levels established by the US EPA or state regulatory agencies can significantly shift the cleanup levels. For groundwater cleanups driven by drinking-water standards, the industry is not significantly affected because the maximum contaminant levels established by the US EPA under the Safe DrinkingWater Act are generally the default parameters (for most federal and state sites, although some states vary groundwater cleanup levels as well). However, changes in toxicity values that affect soil and indoor-air-based cleanup levels are under way for two key chlorinated solvents, TCE and tetrachloroethene (PCE). Changes in the risk values will result in changes in the cleanup levels established for soil exposures and vapor-intrusion risks, resulting in more of a moving target for remediation professionals.