ABSTRACT
The air around us has never been completely pure. It has always contained some natural pollution: windblown dust, smoke from forest fires, salt particles from the oceans, gases generated by the decay of plant and animal life, and occasional torrents of gases and dust particles from volcanic eruptions. The first air pollution control ordinance in the United States was enacted in Pittsburgh around 1815. Despite scattered successes in smoke and soot cleanup after the war, air pollution continued to increase and its nature began to change. Particulates are solid particles or liquid droplets small enough to remain suspended in air. Sulfur dioxide is a corrosive and poisonous gas produced mainly from the burning of sulfur-containing fuel and from certain industrial processes. Ozone is the principal constituent of modern smog and serves as an indicator of all the other photochemical oxidants—peroxyacetal nitrates, formaldehydes, and other organic compounds of nitrogen.
