atmosphere, leading to the formation of numerous chemical species. Both NO
and NO 2 are called primary air pollutants, as they are formed at the source of
combustion or emission. Compounds produced from chemical reactions that
occur after the primary pollutants are emitted into the atmosphere are called
secondary pollutants. Examples of secondary pollutants include O 3 , peroxyacyl
nitrate (PAN), and some aldehydes and ketones (NO 2 can also be included as a
secondary pollutant – see Chapter 8). Smog is composed of both primary and
secondary air pollutants; it contains NO 2 , O 3 , and other photochemical
oxidants and a large number of other chemical species.
Both smoke and smog cause reduction in visibility because light is scattered
by the surfaces of airborne particles. They can both cause adverse effects on
vegetation, animals, and humans.
Although Los Angeles is widely known for its smog, many large cities are
suffering increasingly from similar problems. This is particularly true in some
less-industrialized countries that have experienced unprecedented growth in
recent decades. This growth has led to the emergence of a number of
megacities, with populations of 10 million or more people. Globally, many
rapidly growing cities are also known to be among the most polluted in the
world. Residents in those cities are overwhelmed by environmental problems,
especially those related to air pollution. Examples of such countries include
China, India, Mexico, and Thailand. The megacities in these countries are
experiencing concentrations of a number of air pollutants well above the levels
recommended by the World Health Organization (WHO). For example,
Mexico City, with an estimated population of more than 20 million, has been
experiencing serious air pollution problems.
Shen-Chen, a rapidly growing city in southern China, is another example
with air pollution problems, even though its population is only about one
million. In the morning, visibility is often good: it is possible to see the green
mountain to the southwest of the city, but in the afternoon smog often
develops, resulting in poor visibility. Figure 3.1a and Figure 3.1b show
contrasting views of the city.
3.3 OFFENSIVE ODORS
Malodors are often the first manifestation of air pollution. They are present in
natural air, households, farms, sewage treatment plants, solid waste disposal
sites, and in many industrial areas. Natural air may contain odors arising from
a variety of sources. Decomposition of protein-containing organic matter
derived from vegetation and animals can contribute to odors in the air.
Odors from cooking foods, such as fish, meat, and poultry, can contribute
greatly to the odors sensed in a household. Fresh paints, fresh carpets,
furniture polish, cleaning fluid, wood-burning fireplaces, and deodorants are
some other examples. Cigarette smoking can also be an important cause of
odors in public places, restaurants or households.
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Environmental Toxicology
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Ref: 4365 MING-HO YU Chap-003 Page: 32 31-44
and NO 2 are called primary air pollutants, as they are formed at the source of
combustion or emission. Compounds produced from chemical reactions that
occur after the primary pollutants are emitted into the atmosphere are called
secondary pollutants. Examples of secondary pollutants include O 3 , peroxyacyl
nitrate (PAN), and some aldehydes and ketones (NO 2 can also be included as a
secondary pollutant – see Chapter 8). Smog is composed of both primary and
secondary air pollutants; it contains NO 2 , O 3 , and other photochemical
oxidants and a large number of other chemical species.
Both smoke and smog cause reduction in visibility because light is scattered
by the surfaces of airborne particles. They can both cause adverse effects on
vegetation, animals, and humans.
Although Los Angeles is widely known for its smog, many large cities are
suffering increasingly from similar problems. This is particularly true in some
less-industrialized countries that have experienced unprecedented growth in
recent decades. This growth has led to the emergence of a number of
megacities, with populations of 10 million or more people. Globally, many
rapidly growing cities are also known to be among the most polluted in the
world. Residents in those cities are overwhelmed by environmental problems,
especially those related to air pollution. Examples of such countries include
China, India, Mexico, and Thailand. The megacities in these countries are
experiencing concentrations of a number of air pollutants well above the levels
recommended by the World Health Organization (WHO). For example,
Mexico City, with an estimated population of more than 20 million, has been
experiencing serious air pollution problems.
Shen-Chen, a rapidly growing city in southern China, is another example
with air pollution problems, even though its population is only about one
million. In the morning, visibility is often good: it is possible to see the green
mountain to the southwest of the city, but in the afternoon smog often
develops, resulting in poor visibility. Figure 3.1a and Figure 3.1b show
contrasting views of the city.
3.3 OFFENSIVE ODORS
Malodors are often the first manifestation of air pollution. They are present in
natural air, households, farms, sewage treatment plants, solid waste disposal
sites, and in many industrial areas. Natural air may contain odors arising from
a variety of sources. Decomposition of protein-containing organic matter
derived from vegetation and animals can contribute to odors in the air.
Odors from cooking foods, such as fish, meat, and poultry, can contribute
greatly to the odors sensed in a household. Fresh paints, fresh carpets,
furniture polish, cleaning fluid, wood-burning fireplaces, and deodorants are
some other examples. Cigarette smoking can also be an important cause of
odors in public places, restaurants or households.
32
Environmental Toxicology
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-003.3d]
Ref: 4365 MING-HO YU Chap-003 Page: 32 31-44