Particulates include small bits of soot and ashes that emanate from incinerators and smokestacks. They are produced mainly by industrial plants that burn fuel containing sulfur as an impurity. So named because they contain both hydrogen and carbon, these compounds come from the incomplete burning of fuel, mainly gasoline. Although hydrocarbons are not very harmful themselves, they can react with sunlight to form smog, which is irritative. In industrial areas most hydrocarbons are from automobiles.
Ozone is a specific photochemical oxidant that can be irritative to the lungs and eyes.
A host of other pollutants, including lead, arsenic, asbestos, mercury, beryllium, plutonium, cadmium fluoride, and organic pesticides, may enter the atmosphere from various sources. Airborne lead, mainly from leaded gasoline, has decreased considerably since the conversion by many cars to unleaded gasoline. This combination, the original smog, used to be common in London when coal was widely used to heat homes. The term smog has since been adopted to describe the haze that occurs when sunlight reacts with the hydrocarbons and nitrogen dioxide that are part of automobile exhaust. The reaction of sunlight with hydrocarbons and NO 2 results in a variety of chemical products.