Source:

https://response.restoration.noaa.gov/oil-and-chemical-spills/chemical-spills/resources/levels-concern.html


In ALOHA, a Level of Concern (LOC) is a threshold value of a hazard (toxicity, flammability, thermal radiation, or overpressure).


ALOHA includes the following LOCs to model different hazards:

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:


  1. The first tier (e.g., AEGL-1) is a temporary, non-disabling effects threshold.
  2. The second tier (e.g., AEGL-2) is a disabling (escape impairment) threshold.
  3. The third tier (e.g., AEGL-3) is a life-threatening effects threshold.


ALOHA determines its default toxic LOCs based on the following hierarchy:


  1. AEGLs are used preferentially, because they are the best public exposure LOCs to date. Additionally, AEGLs are designed for nearly all members of the general public. About 175 substances have final AEGLs as of mid-2016, and there are interim AEGLs defined for about 80 additional substances.


  1. 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.


  1. PACs (Protective Action Criteria for Chemicals) are used next. 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.


  1. IDLH (Immediately Dangerous to Life and Health) 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. 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).