What are Acute Exposure Guideline Levels (AEGLs)?

Emergency response personnel may have never heard the acronym AEGL, but that doesn’t mean it does not play a major role in the decision making process at almost all emergency responses to hazardous materials.  AEGL’s and ERPG’s are the most widely used exposure guidelines in emergency responses to hazardous materials.  In fact, did you know the DOT Emergency Response Guidebook uses the AEGL’s as the first choice in establishing the “initial isolation” distance and “protective action” distances?  Here’s more on AEGLs…

The Development of Acute Exposure Guideline Levels (AEGLs) is a collaborative effort of the public and private sectors worldwide. AEGLs are intended to describe the risk to humans resulting from once-in-a-lifetime, or rare, exposure to airborne chemicals. The National Advisory Committee for the Development of Acute Exposure Guideline Levels for Hazardous Substances (AEGL Committee) is involved in developing these guidelines to help both national and local authorities, as well as private companies, deal with emergencies involving spills, or other catastrophic exposures.

In 1986, the National Advisory Committee for the Development of Acute Exposure Guideline Levels for Hazardous Substances (AEGL/NAC Committee or AEGL Committee) was formed to develop and recommend AEGLs for hazardous chemicals for use in chemical emergency programs. These levels can be used by federal, state and local agencies, the private sector and foreign organizations for emergency planning, prevention and response activities related to the accidental release of hazardous substances.

EPA has been actively involved since 1988 in a program leading to the development of short-term exposure guidelines for accidental air-borne chemical releases. In April, 1988, EPA’s Office of Pollution Prevention and Toxics (OPPT) organized and held a National Workshop to determine the status of short-term exposure guideline level development. Based on the findings of the workshop, the idea to establish a combined effort for the future development of short-term guideline levels was conceived and a program was implemented.

In September, 1989, OPPT and the Agency for Toxic Substances and Disease Registry (ATSDR) provided funding for a cooperative agreement with the National Academy of Sciences (NAS) to develop a methodology that could be used to develop short-term exposure guideline levels. The NAS commenced work on this task in October, 1990. From October, 1990 until February, 1993, OPPT staff continually met with the NAS Committee on Toxicology to articulate specific uses of, and requirements for, short-term exposure guideline levels. In June, 1993, the NAS published its report, entitled Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances.

With the methodology now available, OPPT initiated the next series of steps to develop the concept of a working committee and solicit support and participation from federal and state agencies and organizations in the private sector to establish a joint committee to develop the exposure guideline levels. The AEGL Committee was formed and first met in June, 1996. For 15 years, the AEGL committee discussed over 300 chemicals and developed AEGL values for at least 273 of the 329 chemicals on the AEGL priority chemical lists. The last meeting of the AEGL committee was in April 2010 and the charter of the FACA committee expired in October 2011. Although the work of the AEGL committee has ended, the AEGL program is still operational and works with the National Academies to publish final AEGLs.

During its operation, the AEGL/NAC committee was fortunate to have dedicated members that contributed to set the standard for a new and fruitful way to develop a single high quality scientific product required by many federal and state agencies as well as private sector groups.

Some of the key accomplishments of the AEGL committee are the following:

• Represented a major pooling of resources from the public sector (federal and state agencies) and the private sector (academic, non-profit organizations, and private industry). This provided greater scientific validity of the exposure guideline levels developed as a result of greater resources for more comprehensive data gathering and the utilization of a broader base of scientific knowledge and expertise.
• Represented a significant cross-section of scientists in toxicology and related fields from both the public and private sectors as well as the international community. This fostered consensus among the entire scientific community.
• Resulted in the establishment of national, standardized acute exposure guideline levels for all federal and state agencies in the public sector and all private companies and non-profit organizations in the private sector. This prevented individual federal and state agencies and private industry from developing their own individual exposure guideline levels, which would result in the publication and use of different values and cause major confusion and controversy among regulatory agencies, private industry and confusion and concern among the general public.
• Resulted in greater efficiency and greater productivity in the development of exposure guideline levels. This approach eliminated duplication of effort and, through the use of pooled financial and staff resources and the overlap of priority lists of chemicals from various agencies and organizations, substantially reduced the cost of developing the exposure guideline levels per chemical per agency or entity.
• Fostered the acceptance of the exposure guideline levels by all participating agencies and organizations in the public and private sectors as well as foreign countries, since they are both the creators and users of the exposure guideline levels. Also, the AEGL committee fostered confidence among the general public with the concurrence and publication of the values by NAS.

Acute Exposure Guideline Levels (AEGLs) are exposure guidelines designed to help responders deal with emergencies involving chemical spills or other catastrophic events where members of the general public are exposed to a hazardous airborne chemical. (Acute exposures are single, non-repetitive exposures that don’t exceed 8 hours.)

AEGLs estimate the concentrations at which most people—including sensitive individuals such as old, sick, or very young people—will begin to experience health effects if they are exposed to a hazardous chemical for a specific length of time (duration). For a given exposure duration, a chemical may have up to three AEGL values, each of which corresponds to a specific tier of health effects.

AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposure periods ranging from 10 minutes to 8 hours. AEGL-2 and AEGL-3, and AEGL-1 values as appropriate, will be developed for each of five exposure periods (10 and 30 minutes, 1 hour, 4 hours, and 8 hours) and will be distinguished by varying degrees of severity of toxic effects. It is believed that the recommended exposure levels are applicable to the general population including infants and children, and other individuals who may be susceptible. The three AEGLs have been defined as follows:

AEGL-1 is the airborne concentration, expressed as parts per million or milligrams per cubic meter (ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.

AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

AEGL-3 is the airborne concentration (expressed as ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death. 

Airborne concentrations below the AEGL-1 represent exposure levels that can produce mild and progressively increasing but transient and nondisabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity of effects described for each corresponding AEGL. Although the AEGL values represent threshold levels for the general public, including susceptible subpopulations, such as infants, children, the elderly, persons with asthma, and those with other illnesses, it is recognized that individuals, subject to unique or idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL.

All three tiers (AEGL-1, AEGL-2, and AEGL-3) are developed for five exposure periods: 10 minutes, 30 minutes, 60 minutes, 4 hours, and 8 hours. The table below shows how the chlorine AEGL values vary with exposure duration.

Typically, the AEGL values will be different for each exposure duration (such as the AEGL-3 values in the table above). This is because the physical effects are typically related to dose (that is, concentration over exposure duration). However, in some cases, the AEGL values will be the same for all durations. This situation usually occurs at the AEGL-1 level (as in the table above), because it is a threshold for non-disabling effects; some effects (for example, whether people will be able to smell the chemical) depend only on concentration—not on the length of time people are exposed.

The AEGL development process changed in November 2011; a brief summary of the process—present and historical—is included below. To find out more specific details, go to the AEGL Program website.

New AEGL Process (November 2011 to Present)
In November 2011, the National Advisory Committee for AEGLs was eliminated and the AEGL development process was modified. Future development work on the AEGLs will focus on finalizing interim AEGLs through the National Academy of Sciences.

Old AEGL Process (1996 to October 2011)
In the past, AEGLs were developed by the National Advisory Committee for AEGLs. The committee established detailed guidelines for developing uniform, meaningful emergency response standards for the general public.

Each AEGL was developed independently by a team of scientists who assigned priority to current data from human and animal studies. The process was exhaustive and the guidelines were thoroughly reviewed. As a result, AEGLs represent the best public exposure guidelines available to date.

As part of the development process, interim AEGL values were established—following review and consideration of public comments—by the National Advisory Committee for AEGLs. Interim AEGLs are available for use by organizations while awaiting peer review and publication of final AEGLs by the National Research Council of the National Academy of Sciences.

As of early 2012, about 70 substances have final AEGLs and nearly 200 substances have interim AEGLs. To see the current list of substances with AEGLs, go to the AEGL Program website. You can also search for a specific substance on the CAMEO Chemicals site to find response recommendations and chemical data (including AEGLs).

AEGLs should be used to help protect the public when there has been a chemical release that is short-term in duration. AEGLs estimate how the general public would react to a release of this nature, so they can be used to identify areas where a hazard exists if the concentration of hazardous gas is exceeded for the specified exposure duration. For example, in areas with concentrations just above the AEGL-1, most people would experience temporary, non-disabling effects. On the other hand, in areas with concentrations just above the AEGL-2, most people would experience significant—but not life-threatening—health effects.

Note: Each defined chemical has AEGL values for five exposure durations, and you should use the duration that is appropriate for your release.

AEGLs should NOT be used as:

• Guidelines for workers who are routinely exposed to chemicals for longer durations. In such cases, you should use workplace exposure limits(WELS, OSHA’s PELs, ACGIH TLV’s) because they contain safety factors specific to that type of exposure.
• Guidelines for members of the public who are exposed to background chemical releases for longer durations. In these types of air quality issues, values such as the National Ambient Air Quality Standards (NAAQS) should be used rather than emergency response guidelines.

In Areal Locations of Hazardous Atmospheres (ALOHA), you can choose AEGLs as your toxic Levels of Concern (LOCs) when modeling a toxic chemical release, if AEGLs have been defined for that chemical. ALOHA allows you to specify up to three toxic LOCs. So, you can choose the AEGL-1, AEGL-2, and AEGL-3 values to generate a threat zone estimate where yellow, orange, and red zones indicate areas where those values were predicted to be exceeded at some point after the chemical release began. (To determine how long the LOC was exceeded at a particular location, read the Ask Dr. ALOHA article on working with the concentration graph.)

If final or interim AEGLs are available for the chemical you are modeling, ALOHA will provide the AEGL values with a 60-minute exposure duration as the default toxic LOCs. Even though AEGLs are available for five exposure durations, only the 60-min AEGLs are provided in ALOHA (because it models the release for 60 minutes from your start time).

Note: If your release has a short duration and ALOHA’s concentration graphs show that the predicted exposure durations are short at all points in the threat zone, you may want to use the 10-min or 30-min AEGL values instead of the 60-min AEGLs included in ALOHA. Read the Ask Dr. ALOHA article on choosing toxic LOCs (to learn how to enter your own LOCs) and the article on working with the concentration graph (to learn how to use this graph to consider exposure durations). You can look up the AEGL values for all durations on the CAMEO Chemicals site.