3.3.1. ACUTE EXPOSURE GUIDELINE LEVELS (AEGLS) – U.S. ENVIRONMENTAL PROTECTION AGENCY
The AEGLs are developed through an EPA Federal Advisory Committee and reviewed and published by the NRC, as specified in the Standard Operating Procedures (SOP) document.102, 103 The development process includes one of the highest levels of peer-review and public participation.
The SOP document states that AEGLs “represent threshold exposure limits for the general public and are applicable to emergency exposures ranging from 10 min to 8 h.”104 The intended application of AEGL values is “for conducting various risk assessments to aid in the development of emergency preparedness and prevention plans, as well as real-time emergency response actions, for accidental chemical releases at fixed facilities and from transport carriers.”105 The SOP document lays out the purpose and objectives of AEGLs by stating that “the primary purpose of the AEGL program and the AEGL Committee is to develop guideline levels for once-in-a-lifetime, short-term exposures to airborne concentrations of acutely toxic, high-priority chemicals.”106 Three health effect levels are developed for 10- and 30-minute and 1-, 4-, and 8-hour exposures, resulting in as many as 15 different AEGL concentration values for a specific chemical. These values are intended to protect the general public and include consideration of sensitive and susceptible persons, including sensitive subpopulations, but not hyper-sensitive or hyper-susceptible persons. The three AEGL health effect levels are defined below.
- AEGL-1: The airborne concentration of a substance above which it is predicted that the general population, including susceptible persons, could experience notable discomfort, irritation, or certain asymptomatic, non-sensory effects. However, the effects are non-disabling and are transient and reversible upon cessation of exposure.
- AEGL-2: The airborne concentration of a substance above which it is predicted that the general population, including susceptible persons, could experience irreversible or other serious, long-lasting health effects or impaired ability to escape.
- AEGL-3: The airborne concentration of a substance above which it is predicted that the general population, including susceptible persons could experience life-threatening health effects or death.
The AEGLs are based primarily on acute toxicology data for vapor exposures, not subchronic or chronic exposure data. The AEGL values include uncertainty factors to account for variability in biological response in the human population. For carcinogens, the chemical-specific Technical Support Document (TSD) includes an evaluation of the degree of excess cancer risks anticipated for one-time exposure at the various AEGL levels (typically less than 1 in 1,000). The guidance does not consider or evaluate the effects that could result from repeated exposures.
AEGLs are not regulatory values, and the AEGL Committee does not provide specific guidance on their implementation or use. Instead, choices made on how and/or which AEGL value to use for various response decisions, such as evacuating or sheltering-in-place, are typically left up to the FSLTT officials responding to the incident. However, it is highly recommended that the expert scientific judgment of qualified toxicologists and/or hazard assessors be sought to help inform chemical- and site-specific decisions.
For each set of AEGLs for a chemical, an associated TSD describes the toxicological derivation of the values. Because the AEGL TSD contains a comprehensive review of all identified acute toxicology data on the subject chemical and the basis for the development of the AEGL values, these documents may also have general use as toxicological references in situations involving an acute exposure scenario that goes beyond the intended purpose of the AEGLs. Planners and risk managers should seek the advice of qualified scientific expertise (toxicologists and/or risk assessors) who are familiar with the TSDs for specific chemicals in order to understand the basis for the AEGL values prior to using these values outside of their stated purpose.
3.3.2. EMERGENCY RESPONSE PLANNING GUIDELINES (ERPGS)—AMERICAN INDUSTRIAL HYGIENE ASSOCIATION
The ERPGs are developed by the American Industrial Hygiene Association and are intended for emergency planning and response operations (similar to AEGLs), but ERPGs are only based on a 1-hour exposure duration.107 ERPGs are intended to protect most persons in the general population, but not particularly sensitive persons. They are reviewed at regular intervals as new information becomes available.
Definitions of the three levels of ERPG values are as follows:
- ERPG-1: The maximum airborne concentration below which it is believed nearly all persons could be exposed for up to 1 hour without experiencing more than mild, transient adverse health effects or without perceiving a clearly defined objectionable odor.
- ERPG-2: The maximum airborne concentration below which it is believed nearly all persons could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms that could impair a person’s ability to take protective action.
- ERPG-3: The maximum airborne concentration below which it is believed nearly all persons could be exposed for up to 1 hour without experiencing or developing life-threatening health effects.
3.3.3. PROVISIONAL ADVISORY LEVELS (PALS) – ENVIRONMENTAL PROTECTION AGENCY
The EPA Office of Research and Development’s HSRP developed health-based provisional advisory levels (PALs) for priority toxic industrial chemicals, chemical warfare agents, and biotoxins in air and drinking water.108 It is the intent of the PALs Program to provide exposure values for these agents as a means of assisting emergency response and decision-making, and to serve as criteria for determining reuse and temporary re-entry into affected areas. Situations that may necessitate the use of PALs include, but are not limited to, transport/storage accidents, natural disasters, and subversive activities. In the case of a nationally significant or large-scale chemical release, EPA can provide PALs to appropriate end-users and stakeholders to evaluate the severity of the situation, identify potential human health outcomes, and determine the most appropriate course of action.
PALs represent a tiered risk system that predicts the likelihood of harm with increasing dose and duration of exposure. PALs provide additional context for risk characterization by extending the AEGL construct to include longer durations of inhalation exposure and the oral exposure route. PALs are developed for 24- hour, 30-day, and 90-day durations, with risk tiers (PAL 1, PAL 2, PAL 3) identified for each based on the severity of expected health outcomes. The health effect for a specific PAL is the biological response identified by a specific study or data set for which an exposure concentration-response or dose-effect relationship has been defined. Although PALs are developed with considerable attention to sensitive populations (e.g., asthmatics, persons with age-dependent sensitivities), PALs are not intended to protect hypersensitive populations.
- PAL 1: Represents the assumed continuous exposure concentration of a chemical in air or water above which changes from baseline of specific biomarkers or mild physiological responses may occur in the general population. Concentrations at or below the PAL 1 values are not expected to be associated with adverse health effects. Increasingly greater concentrations above the PAL 1 value could cause progressively harmful effects in the general population, including all ages and sensitive subpopulations.
- PAL 2: Represents the assumed continuous exposure concentration of a chemical in air or water above which serious, irreversible, or escape-impairing effects could result. Increasingly greater concentrations above the PAL 2 value could cause progressively harmful effects in the general population, including all ages and sensitive subpopulations.
- PAL 3: Represents the assumed continuous exposure concentration of a chemical in air or water above which lethality in the general population, including all ages and sensitive subpopulations, could occur.
It is important to take the exposure duration into account when selecting an appropriate PAL. Since the shortest exposure duration for PALs is 24 hours, EPA recommends use of AEGLs for inhalation exposures to chemicals of less than 24 hours (See Section 3.3.1 above). The 30-day PAL is applicable to exposure durations of greater than 1 day, up to 30 days. The 90-day PAL is applicable to durations of greater than 30 days, up to 90 days.
3.3.4. TEMPORARY EMERGENCY EXPOSURE LIMITS (TEELS) – DEPARTMENT OF ENERGY
The Department of Energy (DOE) has published TEELs for about 1,200 chemicals.109 TEELs adopt AEGLs and then ERPGs as their primary hierarchy for publication of values, but they also present values obtained by other methods for use when AEGLs or ERPGs are not available. Values derived by these other methods are not peer reviewed. In the absence of AEGL and ERPG values, TEELs are based on the correlation between acute data (e.g., lethal concentration, lowest lethal concentration, median lethal concentration) and existing values (e.g., IDLH, STEL, TLVs, and various levels of existing ERPGs). DOE thus provides a methodology for combining hierarchy- and toxicity-based TEELs into procedure-derived TEELs to facilitate its use by anyone requiring concentration limits for chemicals. TEEL values, like the ERPGs, are based on a 1-hour exposure duration. The various TEEL definitions are as follows.
- TEEL-0: The threshold concentration below which most persons will experience no appreciable risk of health effects.
- TEEL-1: The maximum concentration in air below which it is believed nearly all persons could be exposed without experiencing other than mild transient adverse health effects or perceiving a clearly defined objectionable odor.
- TEEL-2: The maximum concentration in air below which it is believed nearly all persons could be exposed without experiencing or developing irreversible or other serious health effects or symptoms that could impair their abilities to take protective action.
- TEEL-3: The maximum concentration in air below which it is believed nearly all persons could be exposed without experiencing or developing life-threatening health effects.