Encyclopedia of Environmental Science and Engineering, Volume I and II

AIR POLLUTION SOURCES 71

and eventually settle out great distances from the source. The

very smallest particulates exhibit certain gaseous characteris-

tics, remaining suspended in the atmosphere for long periods

of time and are readily transported by wind currents.

Classification According to Chemical Composition

Pollutants may also be classified according to their chemi-

cal structure. The basic classifications are (1) organic and

(2) inorganic.

Organic Pollutants Organic compounds may be defined

as those which contain carbon, hydrogen, and may contain

other elements. By this definition we exclude the very simple

carbon monoxide and carbon dioxide. These contain carbon,

but no hydrogen.

Inorganic Pollutants Inorganic pollutants may be defined as

compounds which do not contain compounds of carbon, with

the exception of carbon oxides, like CO and CO 2 , and carbon

disulfide. Many of the most commonly encountered pollut-

ants are inorganic. You might be asking yourself why CO 2

is considered a pollutant. Isn’t CO 2 beneficial in the mainte-

nance of the earth’s ecological system by providing a source

of energy for manufacturing plants? The answer is yes, but the

earth’s ecosystem can utilize only so much carbon dioxide.

The surplus of CO 2 in the atmosphere is believed to be one of

the contributors to the “Greenhouse Effect.” Excesses of this

gas are believed to cause the global heating that is now being

experienced. The long-term outlook for this phenomenon is

the melting of the polar icecaps resulting in the oceans’ levels

rising and threatening population areas that are located at the

coastline.

Classification According to the Nature of the Problem

or Health Threat Posed by the Pollutant

Under the Clean Air Act, the Congress of the United States

established a classification system which recognized two dis-

tinct categories of air pollutants: those air pollutants which

because of their universal nature or ubiquity, presented a threat

to public health and welfare (called criteria pollutants); and

those pollutants, while not widespread, contribute to higher

mortality rates in humans (called hazardous pollutants).

Criteria Pollutants These are air pollutants for which a

national ambient air quality standard has been established.

In the selection of these standards, certain criteria are estab-

lished using observed levels of air pollution and the associated

impacts on human health, vegetation and materials relating

air quality level to health and welfare effects. Six specific

TABLE 1

Classification of Pollutants

Major Classes Sub-classes Typical Members of Sub-classes

Organic Alkanes Ethane

Gases Alkenes Ethylene

(Hydrocarbons) Alkynes Acetylene

Alkyl Halides Ethylenedichloride

Aldehydes Formaldehyde

Ketones Acetone

Amines Methyl Amine

Alcohols Ethanol

Aromatics Benzene

Inorganic Photochemical Oxidants Ozone

Gases Oxides of Nitrogen Nitrogen Dioxide, Nitric Oxide

Oxides of Sulfur Sulfur Dioxide, Sulfur Trioxide

Oxides of Carbon Carbon Monoxide, Carbon Dioxide

Halides Chlorine, Flourine

Miscellaneous Ammonia, Hydrogen Sulfides

Particulates Solid Particulates Dust, Smoke

Liquid Particulates Mist, Spray

Heavy Metals

Other Pollutants Include:

—Radioactive Substances

—Pesticides

—Aeroallergens

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