Pollutant

A pollutant is any substance introduced into the environment that adversely affects the usefulness of a natural resource or the health of humans, animals, or ecosystems. Pollutants can be artificial substances, such as pesticides and fossil fuel combustion products, or naturally occurring substances, such as radon (Rn) or carbon dioxide (CO2) that may occur in harmful concentrations in a given environment.

Three factors determine the severity of a pollutant: its chemical composition, concentration and persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the long term.

Air pollutants
There are a great many types of air pollutants. The most notable ones are:


 * Ambient air quality pollutants such as sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter, carbon monoxide (CO), ozone (O3) and lead (Pb).


 * Greenhouse gases such as water vapor (H2O), ozone, carbon dioxide and methane (CH4).

Air and water pollutants
There are also a great many pollutants which may contaminate either air or water:


 * Heavy metals such as arsenic (As), chromium (Cr), cadmium (Cd) or mercury (Hg) which are metallic elements with high atomic weights, can damage living things at low concentrations and tend to accumulate in the food chain.


 * Persistent organic pollutants (POPs) such as aldrin, chlordane, DDT, dieldrin, dioxins, endrin, furans, heptachlor, hexachlorobenzene, mirex, polychlorinated biphenyls (PCBs), and toxaphene.


 * Polycyclic aromatic hydrocarbons (PAHs) are a large group of chemical compounds consisting of fused aromatic rings such as benzo(a)pyrene, benzanthracene, benzo(b)fluoranthene, fluoranthene, naphthalene to name but a few. They are formed during the incomplete combustion of carbon-containing substances and they are also manufactured for use as dyes, plastics and pesticides.


 * Volatile organic compounds (VOCs) are defined: by the European Union as organic chemicals having a normal boiling point of 250 °C (482 °F) or less; by Canada as organic chemicals that have normal boiling points roughly in the range of 50 − 250 °C (122 - 482 °F) and by the United States as organic chemicals that participate in atmospheric photochemical reactions except those designated by U.S. Environmental Protection Agency as having negligible photochemical reactivity.

Other pollutants

 * Soil pollutants are chemicals released by spills or underground leakage. Among the most significant are hydrocarbon liquids, heavy metals, methyl tert-butyl ether (MTBE), pesticides, herbicides, and chlorinated hydrocarbons.
 * Radioactive pollutants resulting from activities in atomic physics, such as nuclear power generation and nuclear weapons research, manufacture and deployment.
 * Noise pollution encompasses unwanted noise such as roadway noise, aircraft noise, and industrial noise.
 * Light pollution such as unwanted light trespass or over-bright illumination.
 * Visual pollution refers to the presence of unsightly roadway billboards, power lines, scarred landforms (as from strip mining) and open storage of trash or municipal solid waste.
 * Thermal pollution by heat transmitted to natural waterways through the discharge of warm water from industrial facilities.