In origin. Primary pollutants are released in

In United States, principal
pollutants are identified and regulated by Clean Air Act of 1970. The
pollutants can divide into two types, point sources and non-point sources. A
point source is a smokestack or other concentrated pollution origin. Primary pollutants
are released in harmful form. Secondary pollutants became hazardous after
reactions in air. The examples of secondary pollutants are atmospheric acids
and photochemical oxidants. For non-point source, it does not go through a
smokestack. Examples of non-point source are dust from soil erosion, crushing,
destruction and building construction. Conventional pollutants are a group of 7
major pollutants which are sulphur dioxide, carbon monoxide, particulates,
volatile organic compounds, nitrogen oxides, oxon and lead. They contribute
largest volume of air quality degradation and considered as the most serious
threat of air pollutants to human health and welfare. Sulphur dioxide is
colourless and corrosive gas that damages plants and animals. It further
oxidized to sulphur trioxide. Sulphur trioxide will reacts with water to form
sulphuric acid which is a major content of acid rain. Nitrogen oxide is highly
reactive gas when combustion initiates reactions between nitrogen and oxygen.
The initial product, nitric oxide further oxidized to nitrogen dioxide.
Nitrogen oxide combines with water to form nitric acid which is major component
of acid decomposition. Excess nitrogen in water cause eutrophication. Carbon
monoxide is colourless, odorless, highly toxic gas produced by incomplete
combustion of fuel. Carbon monoxide binds with haemoglobin in blood and
inhibits respiration in animals. Carbon monoxide emissions create by internal
combustion engines of transportation, land-clearing fires and cooking fire.
Particulate material includes dust, soot, smoke, spores and other suspended
material. They leave dirty deposits on painted surfaces, window or textiles.
Breathable particles which are smaller than 2.5µm can damage lung tissues. Volatile
organic compounds are organic gases. The largest sources of volatile organic
compounds are plants, bogs and termites. For lead and other toxic elements,
toxic metals and halogens are chemical elements which are toxic when
concentrated and released to the environment. Besides, transportation and power
plants are dominant sources of most criteria pollutants. Unconventional
pollutants are compound that are produced in less volume than conventional
pollutants but toxic or hazardous. The examples of unconventional pollutants
are benzene, berrylium, mercury and vincyl chloride.