Volatile organic compounds (VOCs) are small organic molecules that take part in photochemical reactions in the atmosphere, resulting in smog. They have low boiling points and vaporize easily. When present in the atmosphere, VOCs, such as benzene and ethylbenzene, are not removed by passing the air through a filter. The atmosphere also contains nonvolatile organic compounds and semivolatile species such as anthracene and nicotine. The latter separate partly on a filter and partly in the gas phase, depending on temperature. VOCs (isoprene and pinene) are emitted by living trees and decomposition of vegetation. The process of refining crude oil to various fuels and the use, spillage, and incomplete combustion of those fuels in vehicles is another major source of VOCs. When mixed with nitric oxide emissions, mainly from combustion sources, and allowed to stagnate in intense sunlight, this mix forms ozone (a colorless gas) and oxidizes much of the VOCs to involatile particulate matter that scatters and absorbs light. This combination is termed photochemical smog.

Environmental destruction as a tool of war is not new. In 146 B.C.E., at the end of the Third Punic War, Roman soldiers reportedly plowed salt into the fields of Carthage, leaving them infertile and ensuring that the North African city would never again be a challenge to the Roman Empire.

Internet Resource

U.S. Environmental Protection Agency. "Organic Gases (Volatile Organic Compounds—VOCs)." Available from http://www.epa.gov/iaq.