Protective Action Criteria - Emergency Exposure Limits

Most of us are aware that there are exposure limits for chemical exposures in the workplace, so called occupational exposure limits (OELs). In the US, the legal exposure limits are in the form of OSHA Permissible exposure limits (PELs) available at 29 CFGR1910.1000 Tbl Z-1. There are several other OELs that are based on scientific data including the NIOSH recommended exposure limits (RELs), the ACGIH threshold limit values (TLVs) and the AIHA’s workplace environmental exposure levels (WEELs).

In addition to OELs, there are also emergency exposure levels.  The main difference between OELs and PAC levels are that OELs are intended for the workplace where one may be safely exposed to this concentration or concentration over time, for eight hours, 5 days a week for a full work life without adverse effects. Emergency exposure levels on the other hand are intended for emergency responders as a tool to assess the risk to people who have had a one time exposure to a chemical.

There are the three main emergency exposure levels in common usage. The EPA has promulgated acute exposure guidelines (AEGLs) and the AIHA has issued Emergency Response planning guidelines (ERPLs). The AEGLs and ERPLs are subdivided based on the impact of an exposure.The AEGL levels are 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.

 The ERPLs are similarly divided, with similar definitions:

• ·ERPG-1 is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hr without experiencing other than mild transient adverse health effects or perceiving a clearly defined, objectionable odor.

• ERPG-2 is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hr without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action.

• ERPG-3 is the maximum airborne concentration below which it is believed that nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects.

The main difference between the AEGLs and the ERPGs is the ERPGs are based on a one hour exposure, whereas the AEGLs are calculated for exposures from 10 minutes to 8 hours.

The AEGL and ERPG values can be compared with the NIOSH immediately dangerous to life and health levels (IDLH).

Some values for representative compounds are shown in the table below:

Compound	AEGL 1	AEGL 2	ERPG 1	ERPG 2	IDLH	ACGIH TLV
Ethylene oxide	n/a	45 ppm	n/a	50 ppm	800 ppm	1 ppm 8 Hr TWA
Formaldehyde	1.7 ppm	5.0 ppm	1 ppm	10 ppm	20 ppm	0.3 ppm ceiling
Glutaraldehyde	n/a	n/a	0.2 ppm	1 ppm	n/a	0.05 ppm ceiling
Hydrogen Peroxide	n/a	n/a	10 ppm	50 ppm	75 ppm	1 ppm 8 Hr TWA
Peracetic Acid	0.17 ppm	0.51 ppm	n/a	n/a	n/a	n/a*

Notes: AEGLs based on 60 minute exposure.
        *The ACGIH has proposed a 15 minute STEL for peracetic acid of 0.4 ppm.

As may be expected, the AEGLs and ERPGs are significantly higher than the ACGIH TLVs, since the former apply to a one time exposure in an emergency situation and the latter to occupational exposure, 40 hours a week.

Both the AEGLs and the ERPGs are based on scientific studies; however for many chemicals systematic exposure studies have not been conducted. The last member of the PAC family are the DOE Office of Emergency Operations’ temporary emergency exposure limits (TEELs) developed by SCAPA.

The TEELs are estimates made based on OELs, chemicalproperties, chemical structures, health hazard warnings etc.  The goal behind the TEEL program is to estimate an emergency exposure level based on the information available. The development of a TEEL is not a rigorous as an AEGL or ERPG, but in an emergency situation it is much better to have TEEL guidance than nothing at all. The TEELs are subject to periodic update and TEEL compounds may also rise to the top of the pile and become and AEGL or ERPG.  The TEELs are also divided similarly to the AEGLs and ERPGs into TEEL-1, TEEL-2 and TEEL-3. 

Today there are over 3000 TEELs  compared with about 140 final AEGLs and about 145 ERPGs.

The Emergency Management Special Interest Group (sponsored by the Department of Energy)  combines these emergency exposure limits into Protection Action Criteria (PACs)  to give PAC-1, PAC-2 and PAC-3 as with the AEGLs and the ERPGs.  For any particular chemical, the PAC is based on the following hierarchy of values:

1) Use AEGLs (including final or interim values) if they are available.

2) If AEGLs are not available, use ERPGs.

3) If neither AEGLs or ERPGs are available, use TEELs.

The PAC values are designed for use by first responders and other people dealing with potential chemical exposures in an emergency. The PAC values are available from an on-line database and tabulated in pdf and Excel formats.