The amount of pollution in our environment continues to pose a challenge for industry, business, and decisionmakers. Some of the major chemicals of concern are those that harm the environment on a large scale, such as chlorofluorocarbons, which destroy the atmospheric ozone layer. Other chemicals that are harmful to human health are lead in water, cyanide (a deadly poison) leaching from landfills into water supplies, ammonia from car exhaust, and a long list of carbon-based chemicals that are byproducts of industrial processes.

Pollutants may start out as a solid, liquid, or gas, but eventually are counted as individual molecules within another substance. For instance, if a contaminant contains many toxic chemicals, each one will be measured separately. Some chemicals are more toxic to living organisms than others, so they are measured independently to discover their presence in the environment.

A toxic substance is usually...