1. a weather phenomenon where a warm layer of air is on top of a cooler layer of air; traps pollutants in the lower atmosphere air pollution from automobile exhaust that is formed by sunlight causing chemical reactions with nitrogen oxides and hydrocarbons; it forms a brownish haze these chemicals vaporize into the air; examples include gasoline, paint solvents, many cleaning agents, and are a major factor in the formation of photochemical smog • a natural radioactive gas produced by the decay of rock material; this has been a major concern because it may seep into buildings and cause cancer to occupants when the majority of a building’s occupants experience certain symptoms that vary with the amount of time spent in the building maximum permissible concentration of a pollutant in the air around us. Contrast with emission standards (maximum amount of pollutant permitted to be released from a given source).