The concentration of a material in air which, on the basis of laboratory tests, is expected to kill 50% of a group of test animals when administered as a single exposure (usually 1 or 4 hours). The LC50 is expressed as parts of material per million parts of air, by volume (ppm) for gases and vapors, or as micrograms of material per liter of air (micro-g/L) or milligrams of material per cubic meter of air (mg/m3) for dusts and mists, as well as for gases and vapors. Importance: Both are measures of the toxicity of a substance.
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