Acute Exposure Guideline Levels (AEGLs) set levels of chemical concentration that pose a defined level of risk to humans (the general population, including susceptible individuals). [1] [2] [3] These levels are used in preventing and responding to disasters. These guidelines are ascertained for one, short exposure (with a maximum of eight hours) by the air. The AEGL values are determined for varying times of exposure, such as ten minutes, thirty minutes, one hour, four hours and eight hours.
The AEGL values describe the expected effects of inhalation exposure to certain compounds (airborne concentrations in ppm or mg/m3). Each AEGL is determined by different levels of a compound's toxicological effects, based on the 4 Ds: detection, discomfort, disability and death. There are three levels of AEGL-values: AEGL-1, AEGL-2 and AEGL-3. [2] AEGL-1 is the airborne concentration above which notable discomfort or irritation could be experienced. However, the effects are not disabling and reversible once exposure stops. AEGL-2 is the airborne concentration above which irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape could be experienced. AEGL-3 is the airborne concentration above which life-threatening health effects or death could be experienced. An example is shown below for chlorine gas AEGLs:
| Description | 10 min | 30 min | 60 min | 4 hours | 8 hours | |
|---|---|---|---|---|---|---|
| AEGL-1 | discomfort, irritation no life threatening effects | 0.50 | 0.50 | 0.50 | 0.50 | 0.50 |
| AEGL-2 | long lasting health issue | 2.8 | 2.8 | 2.0 | 1.0 | 0.71 |
| AEGL-3 | life threatening effects or death | 50 | 28 | 20 | 10 | 7.1 |
In this example, several trends can be observed. First, as one moves from AEGL-1 to AEGL-3, the concentrations increase, based on the dose predicted to produce the respective effects (discomfort/irritation vs disability vs death). Second, as one moves from shorter exposures to longer (left to right in the same row), the overall concentration allowed decreases due to the effects of cumulative dose. This is not always the case for all values, however, not all toxicological effects are dose-dependent and sometimes only depend on air concentration. [3]
Over 2,000 people died as a result of the 1984 methyl isocyanate leak in Bhopal, India. While numerous others had permanent lung and eye damage. One methyl isocyanate canister was the source of the leak. [1] The necessity for governments to detect dangerous compounds and assist local communities in dealing with them was brought to the attention of the world by this and previous unintentional or deliberate chemical releases of airborne compounds, the AEGL program was created to offer advice for disaster preparedness programs and emergency responder. [5]
The EPA advises using AEGLs for inhalation exposures to pollutants lasting less than 24 hours because the shortest exposure duration for PAL is 24 hours. PALs are a tiered risk system that forecast the chance of injury as exposure time and dose increase. By expanding the AEGL architecture to incorporate longer inhalation exposure periods and the oral exposure route, PALs offer more context for risk evaluation. Based on the severity of anticipated health effects, risk levels (PAL 1, PAL 2, and PAL 3) are determined for each of the 24-hour, 30-day and 90-day PALs. [6]