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.
Airborne concentrations below the AEGL-1 represent exposure concentrations that could produce mild and progressively increasing but transient and non-disabling odor, taste, and sensory irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity of effects described for each corresponding AEGL. Although the AEGL values represent threshold levels for the general public, including susceptible subpopulations, such as infants, children, the elderly, persons with asthma, and those with other illnesses, it is recognized that individuals, subject to idiosyncratic responses, could experience the effects described at concentrations below the corresponding AEGL.
Nitrogen oxide compounds occur from both natural and anthropogenic sources. Nitrogen dioxide (NO2) is the most ubiquitous of the oxides of nitrogen and has the greatest impact on human health. Nitrogen tetroxide (N2O4) is a component of rocket fuels. Very few inhalation toxicity data are available on N2O4. Nitric oxide (NO) is an endogenous molecule that mediates the biologic action of endothelium-derived relaxing factor. The toxicity of NO is associated with methemoglobin formation and oxidation to NO2. NO is also a component of air pollution and is generally measured as part of the total oxides of nitrogen (NO + NO2).
The reactions of the oxides of nitrogen consist of a family of reaction paths that is temperature dependent and generally favors NO2 production. A significant fraction of N2O4 and NO will be converted to NO2. Since NO2 is the most ubiquitous and the most toxic of the oxides of nitrogen, AEGL values derived from NO2 toxicity data are considered applicable to all oxides of nitrogen. NO2 exists as an equilibrium mixture of NO2 and N2O4, but the dimer is not important at ambient concentrations (EPA 1993). When N2O4 is released, it vaporizes and dissociates into NO2, making it nearly impossible to generate a significant concentration of N2O4 at atmospheric pressure and ambient temperatures without generating a vastly higher concentration of NO2. Almost no inhalation toxicity data are available on N2O4 because of this effect, and no information was found on the interactions of nitrogen trioxide (N2O3).