HUMAN BIOLOGY

476 Chapter 25

acid rain Rain or snow that
contains large amounts of
acid, usually sulfuric and
nitric acids from industrial
emissions.

smog Haze that develops
when dust, smoke, acids,
and other air pollutants
accumulate in the lower
atmosphere.

Where winters are cold and wet, industrial smog forms

as a gray haze over industrialized cities that burn coal

and other fossil fuels for manufacturing, heating, and

generating electric power. The burning releases airborne

dust, smoke, ashes, soot, asbestos, oil, bits of lead and other

heavy metals, and sulfur oxides. Most industrial smog

forms in cities of developing countries, including China

and India, as well as in eastern Europe.

In warm climates, photochemical smog forms as a

brown, smelly haze over large cities. The key culprit is

nitric oxide. After it is released from vehicles, nitric oxide

reacts with oxygen in the air to form nitrogen dioxide.

When exposed to sunlight, nitrogen dioxide can react with

hydrocarbons (such as partly burned gasoline) to form

photochemical oxidants. Some of those in smog resemble

tear gas; even traces can sting the eyes, irritate lungs, and

damage crops.

Oxides of sulfur and nitrogen are among the worst air

pollutants. These substances come mainly from power

plants and factories fueled by coal, oil, and gas, as well

as from motor vehicles. Dissolved in atmospheric water,

they form weak sulfuric and nitric acids that winds may

disperse over great distances. If they fall to Earth in rain

and snow, they form acid rain. Acid rain can be much

more acidic than normal rainwater, sometimes as acidic as

lemon juice (pH 2.3). The acids eat away at marble, metals,

even nylon. They also seriously damage the chemistry of

ecosystems (Figure 25.7).

n    A variety of human activities are polluting the air, with

major consequences for ecosystems and human health.

Air pollutants include carbon diox­

ide, oxides of nitrogen and sulfur,

and chloro fluorocarbons (CFCs).

Others are photochemical oxidants

formed as the sun’s rays interact with

certain chemicals. The United States

releases more than 700,000 metric

tons of air pollutants every day.

In some weather conditions, a

layer of cool, dense air gets trapped

under a warmer air layer (Figure 25.6). As a result of this

thermal inversion, the atmospheric condition called smog

develops. Some of the worst air pollution disasters have

been due to thermal inversions.

25.4 Assaults on Our Air

Figure 25.6 Thermal inversions set the stage for smog. A How

air normally would circulate in smog-forming regions. B How

air pollutants become trapped under a thermal inversion layer.

C Boy wearing an air-filtering mask in Beijing, China, where

air pollution is severe. (A–B: © Cengage Learning; C: © Hung Chung Chih/

Shutterstock.com)

cool air

warm inversion layer

cool air

B

cooler air

cool air

warm air

A

Figure 25.7 Animated! Part of a forest in Great Smoky

Mountains National Park, where nitrogen oxides and other

forms of air pollution have killed trees. (Frederica Georgia/Science Source)

C

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