Asphalt is a sticky, highly viscous liquid or semi-solid form of petroleum that is composed of carbon, hydrogen, sulfur, oxygen, and nitrogen as well as trace amounts of iron, nickel and vanadium. It is found in natural deposits and can also be obtained by the refining of less viscous forms of petroleum. The world's largest natural deposit of asphalt, estimated to contain roughly ten million metric tons, is Pitch Lake in La Brea in southwest Trinidad. The largest application by far is a binder for gravel in road construction. Other major uses are pavements for airport runways and in waterproofing products, especially roofing tiles and sealants for flat roofs.
Asphalt can harm the environment and human health in several ways. One is the processes employed to extract the natural asphalt or other petrochemicals used to make the asphalt and the various refining processes for such petrochemicals, including the energy consumption for such processes and their toxic emissions.
But perhaps the greatest concern is with regard to the large volume of ultra-fine particles of asphalt that are created by abrasion by the tires on automobiles, trucks, buses and other road vehicles. These particles (along with the particles from the abrasion of tires and brakes) can linger in the air for days and travel great distances. The smallest of them can penetrate deep into the lungs and then enter the bloodstream and other organs, which can ultimately promote or cause heart disease and cancer. Their harmful effects could be due both to the physical irritation and resultant inflammation that these particles cause and to their toxic chemical properties.
Asphalt particles also contaminate the soil and water bodies, both from their settling from the air and from the runoff from roads and parking lots due to rain. Unlike the air, in which the particles gradually disperse, the total buildup of asphalt (along with tire and brake) particles in the environment is cumulative and such particles do not biodegrade easily, if at all. The particles in water can be easily ingested by fish and other marine organisms and possibly harm them. Moreover, they subsequently can be ingested by humans who drink the water and consume the marine organisms that contain them.
It is claimed by the asphalt industry in the United States that most of the asphalt removed from roads during repaving and widening is recycled and reused for new pavements, roadbeds, etc., and that asphalt is the most recycled material in that country. Although this is superficially very impressive, it is highly misleading because it does not include the vast – and ever-growing – quantity of asphalt fine particles that are dispersed into the environment and can never be recovered.