Chapter 3
Occurrence of Toxicants
3.1 INTRODUCTION
A large number of pollutants are present in the environment, often in very
large quantities. They arise from many sources and exposure to these
pollutants may occur through a range of routes. For example, the ambient
air in urban areas may contain sulfur dioxide (SO 2 ), carbon monoxide (CO),
and nitrogen oxides (NO x ), as well as smoke and suspended particles
containing metals and hydrocarbons produced mainly from coal or heavy-oil
combustion by industries, power plants, and some households. Several
pollutants are also found in the indoor environment. Some examples include
CO arising from incomplete combustion of fossil fuels and tobacco smoke, lead
(Pb) from paint used in old houses, and formaldehyde from insulation and
wood preservatives and adhesives.
This chapter will focus on where and how certain pollutants may occur in
the environment. This is followed by a brief review of major pollution episodes
and disasters that have occurred in recent decades.
3.2 VISIBLE SMOKE OR SMOG
The presence of visible smoke or smog is a manifestation of air pollution.
Smoke is composed of the gaseous products of burning carbonaceous materials
made visible by the presence of small particles of carbon. The brownish to
blackish materials emitted from the stack of an inadequately controlled coalburning industrial plant, or from the chimney of a wood-burning home, are
examples. Wood burning has become a common practice in many American
homes, especially in winter. Burning wood in a well-insulated home, however,
can lead to discomfort associated with indoor pollution. The problem
associated with indoor air pollution is particularly serious in many villages
in southern China, where indoor combustion of coal for cooking meals or
drying vegetables is common.
Smog, on the other hand, is a natural fog made heavier and darker by
smoke and chemical fumes. Smog is formed mainly as a result of photochemical reactions. In the presence of UV rays in sunlight, nitrogen dioxide
(NO 2 ) is broken down into nitric oxide (NO) and atomic oxygen. Atomic
oxygen can then react with molecular oxygen in the air to form ozone (O 3 ). A
large number of chemical reactions may also occur among hydrocarbons or
between hydrocarbons and NO, NO 2 , O 3 , or other chemical species in the
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-003.3d]
Ref: 4365 MING-HO YU Chap-003 Page: 31 31-44
31
Occurrence of Toxicants
3.1 INTRODUCTION
A large number of pollutants are present in the environment, often in very
large quantities. They arise from many sources and exposure to these
pollutants may occur through a range of routes. For example, the ambient
air in urban areas may contain sulfur dioxide (SO 2 ), carbon monoxide (CO),
and nitrogen oxides (NO x ), as well as smoke and suspended particles
containing metals and hydrocarbons produced mainly from coal or heavy-oil
combustion by industries, power plants, and some households. Several
pollutants are also found in the indoor environment. Some examples include
CO arising from incomplete combustion of fossil fuels and tobacco smoke, lead
(Pb) from paint used in old houses, and formaldehyde from insulation and
wood preservatives and adhesives.
This chapter will focus on where and how certain pollutants may occur in
the environment. This is followed by a brief review of major pollution episodes
and disasters that have occurred in recent decades.
3.2 VISIBLE SMOKE OR SMOG
The presence of visible smoke or smog is a manifestation of air pollution.
Smoke is composed of the gaseous products of burning carbonaceous materials
made visible by the presence of small particles of carbon. The brownish to
blackish materials emitted from the stack of an inadequately controlled coalburning industrial plant, or from the chimney of a wood-burning home, are
examples. Wood burning has become a common practice in many American
homes, especially in winter. Burning wood in a well-insulated home, however,
can lead to discomfort associated with indoor pollution. The problem
associated with indoor air pollution is particularly serious in many villages
in southern China, where indoor combustion of coal for cooking meals or
drying vegetables is common.
Smog, on the other hand, is a natural fog made heavier and darker by
smoke and chemical fumes. Smog is formed mainly as a result of photochemical reactions. In the presence of UV rays in sunlight, nitrogen dioxide
(NO 2 ) is broken down into nitric oxide (NO) and atomic oxygen. Atomic
oxygen can then react with molecular oxygen in the air to form ozone (O 3 ). A
large number of chemical reactions may also occur among hydrocarbons or
between hydrocarbons and NO, NO 2 , O 3 , or other chemical species in the
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-003.3d]
Ref: 4365 MING-HO YU Chap-003 Page: 31 31-44
31