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What is the Acceptable Ambient Level?
At the outset of its air toxics program, North Carolina decided that the philosophy of the program would be based upon protection of public health. It established airborne concentration of chemicals "above which the substance may be considered to have an adverse effect on human health." The chemicals became known as toxic air pollutants or TAPs and the concentrations became known as acceptable ambient levels or AALs. AALs are expressed in weight per unit volume, most often as milligrams per cubic meter of air (mg/m3). North Carolina has developed AALs for 97 toxic air pollutants. By their nature, AALs are intrinsically different from measured air concentrations, and an understanding of this distinction is necessary to prevent misunderstanding and misapplication of AALs.
For more information see: http://daq.state.nc.us/toxics/aaldisc.pdf
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