Smoke, composed of carbon and other products of incomplete combustion, is the most obvious form of particulate pollution. Open fires, incinerators, petroleum refining, and fuel burning in vehicles and aircraft all produce these highly visible particulates. Industrial processes such as those used in refining crude oil and in manufacturing chemicals also contribute to particulate formation. Liquid aerosols and solid particles form photochemically in the atmosphere when sunlight reacts with waste gases. When metals are melted, the heated material emits fumes which may condense to form metallic oxides in the atmosphere.
Industrial dust is formed by grinding or pulverizing materials, as in cement production.
Earth-moving operations, especially farming and construction, also cause large amounts of dust to enter into the air. Some particulate emissions are considered more toxic than others. Highly toxic substances such as cadmium, beryllium, and asbestos are associated with specific industries and are of greatest concern in the immediate vicinity of the source. Organic Compounds Organic gases, or hydrocarbons, are released when fuels or organic waste materials are burned. These materials are the result of incomplete combustion and range in complexity from methane, a simple organic gas, to much more complex molecules containing carbon, hydrogen and oxygen in varying proportions.