Public exposure guidelines are intended to predict how members of the general public would be affected (that is, the severity of the hazard) if they are exposed to a particular hazardous chemical in an emergency response situation. The most common public exposure guidelines are:

• AEGLs (Acute Exposure Guideline Levels)
• ERPGs (Emergency Response Planning Guidelines)
• TEELs (Temporary Emergency Exposure Limits)

Each of these guidelines has three tiers of exposure values (e.g., AEGL-1, AEGL-2, and AEGL-3) for each covered chemical. There are some key differences between the exposure guidelines; however, at a very general level, the tiers are similar:

• The first tier (e.g., ERPG-1) is a temporary, non-disabling effects threshold.
• The second tier is a disabling (escape impairment) threshold.
• The third tier is a life-threatening effects threshold.

A particular chemical may have values in any—or all—of these systems. As a result, you may want to select your values based on a hierarchy of available public exposure guidelines (for instance, using AEGLs preferentially over TEELs if both systems define values for a chemical). The PACs dataset combines all three public exposure guidelines and implements a hierarchy-based system for you.

When modeling a toxic chemical release in ALOHA, you can use any of these three-tiered guidelines as toxic Levels of Concern (LOCs). ALOHA uses your LOCs and other scenario information to generate a threat zone estimate where red, orange, and yellow zones indicate areas where the LOCs were exceeded at some point after the chemical release began. The red zone typically indicates the most hazardous LOC chosen, such as the AEGL-3. ALOHA offers you default LOC options based on this hierarchy:

• AEGLs are used preferentially, because they are the best public exposure LOCs to date. The development process and guidelines are thoroughly reviewed. Additionally, AEGLs are designed for nearly all members of the general public, including sensitive individuals (such as very young people). About 175 substances have final AEGLs, and there are interim AEGLs defined for about 80 additional substances. (ALOHA only includes the AEGL values for a 60-minute exposure duration; go to CAMEO Chemicals to see the AEGL values for other exposure durations.)
• ERPGs are used next. They are developed from experimental data like the AEGLs—but ERPG values are only available for a 60-minute exposure duration and they are not designed as guidelines for sensitive individuals. ERPGs have been defined for about 150 chemicals.
• PACs are used next. This dataset combines all three common public exposure guideline systems (AEGLs, ERPGs, and TEELs) and implements a hierarchy-based system for you. (AEGLs are used preferentially, followed by ERPGs, and then TEELs.) If ALOHA is defaulting to the PAC values, it means that there are no AEGL or ERPG values in the ALOHA chemical library for that substance. In this case, the PAC values will be the TEEL values. TEELs are derived using existing LOCs and by manipulating current data. This process is less intensive than the AEGL or ERPG process, and TEELs have been defined for more than 3,000 chemicals.
• IDLH limits are used when no public exposure guidelines are defined for a given chemical. An IDLH limit is a workplace exposure limit that is used primarily for making decisions regarding respirator use. In the 1980s, before public exposure guidelines were available for most common chemicals, the IDLH limit was used in public exposure situations. For example, the Technical Guidance for Hazards Analysis (which was developed in 1987 by the Environmental Protection Agency and other federal agencies to provide guidance for hazard planning) primarily used 1/10th of the IDLH limit. However, unlike the three-tiered public exposure guidelines, only a single IDLH value is defined for applicable chemicals.

Some chemicals are defined under multiple hazard classification systems. In these cases, ALOHA will provide the default value according to the above hierarchy, but it will also provide others as LOC options. Additionally, you can also specify your own LOCs (see the Ask Dr. ALOHA article on choosing toxic LOCs).