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Public Exposure Guidelines
Public exposure guidelines are intended to predict how members of the general public would be affected if exposed to a particular hazardous chemical. Typically, these guidelines are used to assess the severity of a hazard to the general public when a hazardous chemical release occurs.
The most common public exposure guidelines are:
- AEGLs (Acute Exposure Guideline Levels)
- ERPGs (Emergency Response Planning Guidelines)
- TEELs (Temporary Emergency Exposure Limits)
How does ALOHA use public exposure guidelines?
When modeling a toxic chemical release in ALOHA, you can use each of these three-tiered guidelines as Levels of Concern (LOCs). (A toxic LOC is the value above which the toxic gas concentration might be high enough to harm people.) ALOHA uses your LOCs and other scenario information to generate a threat zone plot 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 LOCs available 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). Relatively few AEGLs have been finalized (about 40 as of mid-2008), although nearly 200 interim AEGLs are defined. (ALOHA only uses final 60-minute AEGLs.)
- ERPGs are used next. They are developed from experimental data like the AEGLs, but ERPG values do not account for how sensitive individuals would react to exposure. ERPGs have been defined for more than 100 chemicals.
- TEELs are used next. They 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 is used when no public exposure guidelines are defined. The Immediately Dangerous to Life and Health (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.
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.
The table below compares available public exposure guidelines for several chemicals as of mid-2008. Note that once a chemical has been assigned an ERPG value (such as nitrogen trifluoride), the TEEL value will be changed to match the ERPG. However, once a final or an interim AEGL value is established, the TEEL value will be changed to match the AEGL. For instance, the TEEL-3 value for methanol matches the interim AEGL-3 (60 minute) value and the TEEL-3 for hydrogen chloride matches the final AEGL-3 (60 minute) value. However, ALOHA doesn't use interim AEGL values; so, in the case of methanol, the ERPG values will be used by default until final AEGLs are developed.
A comparison of public exposure guideline values (in parts per million)
| AEGL-3 (60 min)* | ERPG-3 | TEEL-3 |
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| Acetylene | Not available | Not available | 6,000 |
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| Chlorine | 20 | 20 | 20 |
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| Hydrogen Chloride | 100 | 150 | 100 |
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| Methanol | Not available | 5,000 | 7,200 |
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| Nitrogen Trifluoride | Not available | 800 | 800 |
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*Only final AEGL values for a 60-minute exposure duration are shown.
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