Susceptible individuals, including those with heart or lung disease, should avoid outdoor activities until the advisory is cancelled. Currently, the federal Environmental Protection Agency is reviewing the adequacy of the federal health standard for ozone and is considering tightening it. Ozone, an invisible gas, is not emitted directly into the air, but forms when nitrogen oxides from fuel combustion and volatile organic gases from evaporated petroleum products react in the presence of sunshine. Ozone levels are highest during the warm months when there is strong sunshine, high temperatures and an inversion layer. Nitrogen oxides are produced when fossil fuels are burned in motor vehicles, power plants, furnaces and turbines.
Carbon monoxide is a by-product of combustion that comes almost entirely from motor vehicles.
Fine particulates, which are emitted directly as smoke and diesel soot and form in the air out of nitrogen oxides and sulfur oxides, obscure visibility and can be inhaled deep into the lungs. During the early years of World War II, Los Angeles residents began to realize the consequences of an increasingly industrialized area. Investigations began to determine the cause of resident's eye irritation, crop damage, severe reductions in visibility and the rapid deterioration of rubber products. In the mid-1950s, the state of California's Public Health division started to step up its efforts to define the problem of how and where smog forms, as well as address the health concerns associated with exposure to smog. Ozone levels were reaching peaks of .68 parts per million, more than six times the federal health standard.