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.
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.
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.