The purpose of this discussion is to examine the suitability, selection, and application of health-based exposure guidelines for the general population, which includes a variety of potentially susceptible subpopulations such as children, the elderly, those with genetic or existing disease traits (e.g., asthmatic or immuno-compromised persons), or populations experiencing high and adverse human effects or environmental events, practices or programs.
The exposure guidelines appropriate to each phase of an incident involving hazardous chemicals should reflect the nature of the activity and population to protect, with the objective being the eventual return to normal operations and permanent resumed use/re-occupancy of the affected facilities. Some values will reflect clearance goals, whereas others will be useful in determining acceptable exposure levels during the response effort (i.e., characterization, decontamination and cleanup operations).
For carcinogens, an effect level may be based on a toxicity value, such as a “unit risk” level or a “cancer slope factor” (CSF). Since cancer effects are considered non-threshold (any exposure will increase risk), a risk management determination would be necessary to establish a concentration criteria.124 When both cancer and noncancer effects are caused by a chemical, a risk management decision must be made as to which effect will drive the decision on a cleanup goal or clearance decision. Often, both are mitigated to a large extent by addressing noncarcinogenic effects, but depending on the level of risk the stakeholders consider acceptable, the cancer endpoint could be a much-lower value.
The following sections discuss specific considerations in selecting health-based exposure guidelines for different media: air, water and soil, and surfaces.
4.5.1. HEALTH-BASED EXPOSURE GUIDELINES FOR AIR
Table A-1 provides a list (should not be interpreted as an exhaustive list) of available inhalation exposure guidelines for the general population matched to each phase of a incident response and recovery.
Table A-1: Existing Inhalation Exposure Guidelines Applicable to the General Public
| Duration of exposure | Is exposure expected to be repeated?a | Exposure guidelines to be considered |
|---|---|---|
| < 8 hours | No | AEGLb, ERPGb |
| < 8 hours | Yes | 24-h PAL, Acute CA-REL, Occupational TWA valuesc |
| < 24 hour (repeated) | N/A | 24-h PAL, Acute CA-REL, Occupational TWA valuesc |
| 1–30 days (repeated) | N/A | 30-day PAL, Acute MRL, Occupational TWA valuesb, c |
| 30–90 days (repeated) | N/A | 90-day, Intermediate MRL, RfC, IURd |
| Chronic exposure | N/A | RfC, IURd, Chronic MRL, Chronic CA-REL c |
aIn the early phase of an incident (i.e., in the first few days), there may be a potential for a repetition of exposure to a toxic airborne chemical. When a single exposure event occurs, but the potential for a subsequent exposure event to occur in the next few days due to recurrence or remediation activities exists, different hierarchy needs to be applied.
b Emergency response exposure guidelines such as AEGLs and ERPGs are derived using an assumption of a “once-in-a-lifetime” exposure event. As such, the chemical-specific and incident-specific details should be considered by qualified toxicologists and/or risk assessors prior to using these types of values in instances other than the immediate response to a chemical release (e.g., a single, non-repeated release of duration less than 8 hours).
cOccupational values should be used cautiously and only if no, more appropriate values for the general public are available. Expert scientific judgment should be consulted before using these values outside the occupational setting.
dThe RfC and health-based exposure guidelines derived from an IUR for cancer from IRIS are essentially equivalent in ranking, and the use of the value with a lower concentration should be the first consideration.
4.5.2. HEALTH-BASED EXPOSURE GUIDELINES FOR WATER AND SOIL
Pre-calculated, health-based exposure guidelines for materials and surfaces other than air are not as prevalent and primarily address long-term, chronic exposures. ATSDR MRLs are available for acute, intermediate, and chronic exposure durations, but for materials and surfaces other than air, MRLs are not presented as concentrations that can be directly compared to environmental data. Instead, they are presented as doses (e.g., mg/kg-day), which may be used in the derivation of health-based exposure guidelines. Similarly, EPA RfDs and CSFs are not directly comparable to environmental concentrations but may be used in equations to derive health-based exposure guidelines expressed as environmental concentrations.
For drinking water, a list of drinking water exposure guidelines is available from EPA’s “2018 Edition of the Drinking Water Standards and Health Advisories.”125 Examples of drinking water exposure guidelines include Maximum Concentration Levels, Health Advisories (1-day, 10-day and Lifetime values), and Drinking Water Equivalent Levels. For other materials and surfaces and for those chemicals without Maximum Concentration Levels or other drinking water exposure guidelines, EPA’s Superfund program has developed tools for calculating risk-based screening levels using chronic toxicity values and a set of default exposure assumptions for residential and nonresidential land uses. EPA has developed screening levels for chronic exposure to soil, water, and air for both residential and occupational exposures. These values are available to risk assessors for use in site-specific decision-making.
State-adopted and EPA-approved water quality standards are Applicable or Relevant and Appropriate Requirements for remedial cleanup. Most states have more than 100 water quality standards for toxins and conventional pollutants. They may include, depending on the state, chemicals that could be involved in a large-scale, nationally significant chemical incident.
In addition, DOD has developed screening levels specifically for chemical warfare agents. The U.S. Army Public Health Center has developed a list of environmental screening levels for CWAs including in water, soil, and waste.126
Table A-2 provides a list of available soil, surfaces, and drinking water exposure guidelines for the general population matched to each phase of an incident response.
| Duration of exposure | Media | Exposure guidelines to be considered |
|---|---|---|
| Acute | All | 24-h Oral PAL, Acute CA-REL, Occupational TWA valuesa |
| 1–30 days (repeated) | All | 30-day Oral PAL, Acute Oral MRL |
| 30s–90-days (repeated) | All | 90-day, Intermediate Oral MRL, RfD, CSFb |
| Chronic exposure | All | RfD, CSFb, Chronic Oral MRL |
| Chronic exposure | Drinking Water | EPA Maximum Concentration Levels, EPA Lifetime Health Advisory |
aOccupational values should be used cautiously and only if no, more appropriate values for the general public are available. Expert scientific judgment should be consulted before using these values outside the occupational setting.
bThe RfC and health-based exposure guidelines derived from an IUR for cancer from IRIS are essentially equivalent in ranking, and the use of the value with a lower concentration should be the first consideration.
4.5.3. CONTAMINATED SURFACES
There are few peer-reviewed, published values for short- or long-term dermal toxicities. Quantitative risk- based methods apply oral toxicity values to assess risks from dermal exposure. Depending on the studies from which a chemical’s toxicity value were derived, one may need to adjust the oral toxicity value from an administered dose to an absorbed dose. The methodology is provided in EPA’s “Risk Assessment Guidance for Superfund (RAGS).”127
The RAGS Part B provides methodologies to calculate cleanup goals for environmental materials and surfaces such as soil and water.128 More recently, the Agency recognized the need to expand its efforts to include building surfaces. Subsequent to the attack on the World Trade Center, EPA became involved in efforts to develop risk-based surface cleanup goals using methodology similar to that provided by RAGS Part B.129 Other available methods for the derivation of surface cleanup goals have been developed by CalEPA that incorporate EPA’s Stochastic Human Exposure and Dose Simulation Model130 and the U.S. Army Public Health Center.131