One-hour air MEGs were developed to consider three levels of health effects:
Minimal effects. Above this level, individuals could begin to experience mild, transient effects that should not impair performance.
Significant effects. Above this level, individuals could begin to experience irreversible or serious effects that might degrade performance and incapacitate a small portion of the people exposed.
Severe effects. Above this level, some within an exposed population could begin to experience life-threatening or lethal effects.
The hierarchy used to select source material was (1) acute exposure guideline levels (AEGLs), (2) emergency response planning guidelines (ERPGs), (3) temporary emergency exposure limits (TEELs), and (4) other.
AEGLs are developed by the U.S. Environmental Protection Agency (EPA) and reviewed by a National Advisory Committee and by the National Research Council (NRC). AEGLs are developed for three severity levels, and all of the values are intended to protect the general public, including sensitive and susceptible subpopulations. Above AEGL-1 concentrations, the general population could experience discomfort and irritation effects that are not disabling and are reversible upon cessation of exposure. Above AEGL-2 concentrations, the general population could experience irreversible or serious health effects or impaired ability to escape. Above AEGL-3, the general population could experience life-threatening health effects or death. These levels were developed for exposure durations of 10 minutes (min), 30 min, 1 hour, 4 hours, and 8 hours. All AEGL-1 and AEGL-2 exposures were reviewed by EPA to ensure that they do not pose an excess cancer risk greater than 1 × 10-4.
ERPGs are developed by the American Industrial Hygiene Association (AIHA) and are intended for emergency planning and response operations. They also have three levels of health effects that are quite similar to those of the AEGLs. They were created to target the general population, but not particularly susceptible individuals. TEELs are developed by the U.S. Department of Energy and are essentially interim ERPGs.