What are the major types and sources of
air pollution?
Air pollution is the presence of one or more chemicals
in the atmosphere in quantities and duration that cause harm to humans,
other forms of life, and materials. As clean air in the troposphere
moves across the earth's surface, it collects the products of natural
events (dust storms and volcanic eruptions) and human activities (emissions
from cars and smokestacks). Theses potential pollutants, called primary
pollutants, are mixed vertically and horizontally and are
dispersed and diluted by the churning air in the troposphere. While
in the troposphere, some of these primary pollutants may react with
another or with the basic components of air to form new pollutants,
called secondary pollutants.
Long-lived primary and secondary pollutants can travel great distances
before they return to the earth's surface as solid particles, droplets,
or chemicals dissolved in precipitation.
Most pollutants in urban areas enter the atmosphere from the burning
of fossil fuels in both power plants and factories (stationary sources)
and in motor vehicles (mobile sources). In car-clogged cities such
as Los Angeles, California, Sao Paulo, Brazil, Bangkok, Thailand,
Rome, Italy, and Mexico City, Mexico, motor vehicles are responsible
for 80-88% of the air pollution. According to the World Health Organization,
more than 1.1 billion people - one of every five - live in urban areas
where the air is unhealthy to breathe.
Because they contain large concentrations of cars and factories,
cities normally have higher air pollution levels than rural areas.
However, prevailing winds can spread long-lived primary and secondary
air pollutants from the emissions in urban and industrial areas to
the countryside and to other downwind urban areas.
What is photochemical smog? Brown-Air-Smog
Any chemical reaction activated by light is called a photochemical
reaction. Air pollution known as photochemical smog
is a mixture of primary and secondary pollutants formed under
the influence of sunlight. The resulting mixture of more than 100
chemicals is dominated by ozone, a highly reactive gas that harms
most living organisms.
Virtually all modern cities have photochemical smog, however, it
is much more common in cities with sunny; warm, dry climates and lots
of motor vehicles such as Los Angeles, California.
The hotter the day, the higher the levels of ozone and other components
of photochemical smog. As traffic increases in the morning, levels
of Nitric oxide (NO) and nitrogen dioxide (NO2) and unburned hydrocarbons
rise and begin reacting in the presence of sunlight to produce photochemical
smog. On a sunny day the photochemical smog builds up to peak levels
by early afternoon, irritating people's eyes and respiratory tracts.
What is Industrial Smog? Grey-Air-Smog
Thirty years ago cities such as London, England, and Chicago and
Pittsburgh in the United States burned amounts of coal and heavy oil
(which contain sulfur impurities) in power plants, factories and for
space heating. During Winter, people in such cities were exposed to
industrial smog consisting mostly of sulphur dioxide,
suspended droplets of sulfuric acid (formed from some of the sulfur
dioxide), and a variety of suspended solid particles and droplets
(called aerosols).
Urban industrial smog is rarely a problem today in most developed
countries because coal and heavy oil is burned only in large boilers
with reasonably good pollution control or with tall smokestacks. However,
industrial smog is a problem in industrialized urban areas of China,
India, Ukraine, and some eastern European countries, where large quantities
of coal are burned with inadequate pollution controls.
What factors influence the formation of photochemical and
industrial smog?
The frequency and severity of smog in an area depend on several things:
the local climate and topography, the population density, the amount
of industry, and the fuels used in industry, heating, and transportation.
In areas with high average annual precipitation, rain and snow help
cleanse the air of pollutants. Winds help sweep pollutants away to
down-wind areas.
Hills and mountains tend to reduce the flow of air in valleys below
them and follow pollutant levels to build up at ground level. Buildings
in cities generally slow wind speed, thereby reducing dilution and
removal of pollutants.
During the day the sun warms the air near the earth's surface. Normally
this heated air expands and rises, carrying low-lying pollutants higher
into the troposphere. Colder, denser air from surrounding high-pressure
areas then sinks into the low-pressure area created when the hot air
rises. This continual mixing of the air helps keep pollutants from
reaching dangerous concentrations near the ground.
Sometimes, however, a layer of dense, cool air can be trapped beneath
a layer of less dense, warm air in an urban basin valley, causing
a phenomenon known as a temperature inversion, or
a thermal inversion. The changing temperature in
the warm air above the pool of cool air prevents ascending air currents
(that would disperse and dilute pollutants) from developing. These
inversions usually last for several days, allowing air pollutants
at ground level to build up to harmful and even lethal concentrations.
The United States suffered its first major air pollution disaster
in Donora, Pennsylvania, a small industrial town southeast of Pittsburgh.
The town is surrounded by mountains and nestled in the Monongahela
Valley, which is subject to frequent thermal inversions. In 1948 Donora
experienced a thermal inversion that trapped pollutants from steel
mills, a zinc smelter, a sulfuric acid plant, and several other industrial
plants for 5 days. About 6,000 of the town's 14,000 inhabitants fell
ill, and 20 of them died.
A city with several million people and motor vehicles in an area
with a sunny climate, light winds, mountains on three sides, and the
ocean on the other has ideal conditions for photochemical smog worsened
by frequent thermal invasions. This describes California's Los Angeles
basin, which has 14 million people, 23 million motor vehicles, thousands
of factories, and thermal inversions at least half of the year. Despite
having the world's toughest air-pollution control program, Los Angeles
is the air pollution capital of the United States. Other cities with
frequent thermal inversions are Denver in the United States, Mexico
City, Mexico, Rio de Janeiro and Sao Paulo in Brazil, and Beijing
and Shenyang in China.
What is Acid Deposition?
To reduce local air pollution (and meet government standards without
having to add expensive air-pollution control devices), most coal-burning
power plants, ore smelters, and other industrial plants in developed
countries use tall smokestacks to emit sulfur dioxide, suspended particles,
and nitrogen oxides above the inversion layer. As this practice spread
in the 1960s and 1970s, pollution in downwind areas began to increase.
In addition to smokestack emissions , large quantities of nitrogen
oxides are also released by motor vehicles.
This 'dilution solution' reduces local air pollution. However, it
increases pollution downwind because what goes up must come down -
another example of connections or unintended consequences. As the
primary pollutants sulfur dioxide and nitrogen oxides are transported
as much as 600 miles by prevailing winds, they form secondary pollutants
such as nitric acid vapor, droplets of sulfuric acid, and particles
of acid-forming sulfate and nitrate salts.
These chemicals descend to the earth's surface in two forms: wet
(as acidic rain, snow, fog, and cloud vapor) and dry
(as acidic particles). The resulting mixture is called acid
deposition. Although this form of pollution is commonly called
acid rain, acid deposition is a better term because the acidity
can reach the earth's surface not only in rain but also as gases and
as solid particles.
Acidity in substances in water is commonly expressed in terms of
pH. Solutions with pH values less than 7 are acidic,
and those with pH values greater than 7 are alkaline or basic. Natural
precipitation is slightly acidic, with a pH of 5.0 - 5.6.
What areas are most affected by acid deposition?
Acid deposition occurs on a regional rather than global basis because
the acidic components remain in the atmosphere only for a few days.
However, acid deposition is a serious regional problem in many areas
downwind from coal burning power plants, smelters, factories, and
large urban areas. However seriously vegetation and aquatic life in
nearby lakes are affected by an urban area receiving acid deposition
depends mostly on whether its soils are acidic or basic.
In most areas soils are basic enough to neutralize or buffer some
inputs of acids. The ecosystems most harmed by acid deposition are
those containing thin, acidic soils without such natural buffering,
and those where the buffering capacity of soils has been depleted
because of decades of exposure to acid deposition.
Many acid-producing chemicals generated by power plants, factories,
smelters, and cars in one country may be exported to others by prevailing
winds. For example, more than three-forth's of the acid deposition
in Norway, Switzerland, Austria, Sweden, Netherlands, and Finland
is blown to those countries from industrialized areas of western Europe
(especially the United Kingdom and Germany) and eastern Europe.
Within a few decades, NOx and SO2 emissions from developing countries,
leading to greatly increased damage from acid deposition over a much
wider area, especially where soils are sensitive to acidification.
What are the effects of acid deposition?
Risk analysis experts rate acid deposition as a medium-risk ecological
problem and a high risk to human health. Acid deposition has many
harmful ecological effects, especially when the pH falls below 5.1
for terrestrial systems and below 5.5 for aquatic systems. It also
contributes to human respiratory diseases such as bronchitis and asthma
(which can cause premature death), and it damages statues, buildings,
metals, and car finishes.
Acid deposition and other air pollutants such as ozone (O3) can damage
tree foliage directly, but the most serious effect is weakening trees
so they become more susceptible to other types of damage. The areas
hardest hit by acid deposition are mountaintop forests, which tend
to have thin soils without much buffering capacity. Trees on mountaintops,
especially conifers such as red spruce that keep their leaves year-round,
are bathed almost continuously in very acidic fog and clouds.
A combination of acid deposition and other air pollutants (especially
ozone) can make trees more susceptible to stresses such as cold temperatures,
diseases, insects, drought, and fungi (which thrive under acidic conditions),
and to a drop in the net primary productivity from loss of soil plant
nutrients. Although the final cause of tree damage or death may be
mosses, insect attacks, diseases, and lack of plant nutrients, the
underlying cause is often years of exposure to an atmospheric cocktail
of air pollutants and soil overloaded with acids. Drops in forest
productivity because of depletion of soil nutrients and acid-buffering
chemicals can reduce biodiversity and have significant economic implications
for timber companies.
Because it can take decades to hundreds of years for soil to replenish
nutrients leached out by acid deposition, losses in plant productivity
in damaged areas could continue for decades even if emissions of sulfur
dioxide and nitro oxides are reduced by air pollution control programs.
Until recently scientists expected some of the lost forest productivity
to be offset by increased productivity from the larger input of nitric
acid and nitrate salts from acid deposition in areas where this nutrient
is the limiting factor. However, recent research revealed that much
of the nitrate raining down on the forest areas in parts of Germany
and Norway damaged by air pollution is not being taken up by the trees
or by nitrogen-using microbes in the soil.
Acid deposition can also reach aluminium ions (Al3+) attached to
soil minerals. Once released from soil particles, these water-soluble
ions can damage tree roots. When washed into lakes, aluminium ions
can also kill many kinds of fish by stimulating excessive mucus formation,
which asphyxiates the fish by clogging their gills.
Excess acidity can contaminate fish in some lakes with highly toxic
methyl mercury. Increased acidity of lakes apparently converts moderately
toxic inorganic mercury compounds in lake-bottom sediments into highly
toxic methyl mercury, which is more soluble in the fatty tissue of
animals and can be bio magnified to higher concentrations in aquatic
food chains and webs. However, acid runoff into lakes and streams
is rated by risk analysis experts as a low-risk ecological problem.
Solutions: What can be done to reduce acid deposition?
According to scientists studying acid deposition, the best solutions
are prevention approaches. They include:
Reducing
energy use and thus air pollution by improving energy efficiency;
Switching
from coal to cleaner-burning natural gas and renewable energy resources;
Removing
Sulfur from coal before it is burned;
Burning
low-sulfur coal;
Removing
SO2, particulates, and nitrogen oxides from smokestack gases; and
Removing
nitrogen oxides from motor vehicle exhaust.
Reducing coal use, the major culprit, will be economically and politically
difficult, For example, China (the world's largest user of coal) and
India (the fourth largest user) are using their own coal reserves
to fuel rapid industrial growth and so far have not put much money
into pollution control.
Some cleanup approaches can be used, but they are
expensive and merely mask some of the symptoms temporarily without
treating underlying causes. Acidified lakes can be neutralized by
treating them or the surrounding soil with large amounts of limestone
or lime. However, such liming is an expensive and temporary remedy
that usually must be repeated on a yearly basis. Liming could also
kill some types of plankton and aquatic plants and can harm wetland
plants that need acidic water. It is also difficult to know how much
of the lime to put where (in the water or selected places on the ground).
In addition, some research suggests that liming can increase populations
of microbes that deplete carbon stored in the slowly decaying soil
matter (humus) and thus reduce forest productivity. Recently, however,
researchers in England found that adding a small amount of phosphate
fertilizer can neutralize excess acidity in a lake.
Indoor Air Pollution
What are the types and sources of indoor pollution?
According to the Environmental Protection Agency (EPA) and public
health officials, cigarette smoke, formaldehyde, asbestos, and radioactive
radon-222 gas are the four most dangerous indoor air pollutants. A
number of research studies on laboratory animals have also identified
tiny fibres if fiberglass as a widespread and potentially potent carcinogen
in indoor air.
The chemical that causes most people difficulty s formaldehyde, an
extremely irritating gas. As many as 20 million Americans suffer from
chronic breathing problems, dizziness, rash, headaches, sore throat,
sinus and eye irritation, and nausea caused by daily exposure to low
levels of formaldehyde emitted from common building materials, furniture,
drapes, upholstery, and adhesives in carpeting and wallpaper. The
EPA estimates that as many as 1 out of every 5,000 people who live
in manufactured homes for more than 10 years will develop cancer from
formaldehyde exposure.
In developing countries, the burning of wood, dung, and crop residues
in open fires or in unvented or poorly vented stoves for cooking and
heating exposes exposes inhabitants, especially women and young children,
to very high levels of particulate air pollution. Partly as a result,
respiratory illnesses are a major cause of death and illness among
the poor in most developing countries.
Effects of air pollution on living organisms
and materials
How is human health harmed by air pollution?
Your respiratory system has a number of mechanisms that helps protect
you from air pollution. Hairs in your nose filter out large particles.
Sticky mucus in the lining of your upper respiratory tract captures
smaller particles and dissolves some gaseous pollutants. Sneezing
and coughing expel contaminated air and mucus when your respiratory
system is irritated by pollutants. The cells of your upper respiratory
tract are also lined with hundreds of thousands of tiny, mucus-coated
hairlike structures called cilia that continually wave back
and forth, transporting mucus and the pollutants they trap to your
throat (where they are either swallowed or expelled).
Years of smoking and exposure to air pollutants can overload or break
down these natural defenses, causing or contributing to respiratory
diseases. Examples are:
lung
cancer
asthma
chronic
bronchitis
Emphysema
About 90% of the carbon monoxide (CO) - a colour-less,
odorless, poisonous gas - in the troposphere comes from natural sources.
Most of this is produced by reaction in the upper troposphere between
methane and oxygen. Because this CO is diluted by the turbulent air
flows in the troposphere, it does not build up to harmful levels.
However, the remaining 10% of the CO added to the atmosphere comes
from the incomplete burning of carbon-containing chemicals (primarily
fossil fuels). Cigarette smoking is responsible for the largest human
exposure to CO, but this gas is also released by motor vehicles, kerosene
heaters, wood stoves, fireplaces, and faulty heating systems.
CO reacts with hemoglobin in red blood cells and thus reduces the
ability of blood to carry oxygen. This impairs the perception and
thinking, slows reflexes, and causes headaches, drowsiness, dizziness,
and nausea. CO can also trigger heart attacks and angina attacks in
people with heart disease. It can damage the development of fetuses
and young children and aggravate chronic bronchitis, emphysema, and
anemia. Exposure to high levels of CO causes collapse, coma, irreversible
damage to brain cells, and even death.
Inhaling suspended particulate matter aggravates
bronchitis and asthma, and long-term exposure can contribute to development
of chronic respiratory disease and cancer. Invisible fine particles
1/28 as thin as a human a hair, emitted by incinerators, motor vehicles,
radial tires, wind erosion, wood-burning fireplaces, and power and
industrial plants are especially hazardous.
Such tiny particles are not effectively captured by modern air-pollution
control equipment, and they are small enough to penetrate the respiratory
system's natural defenses against air pollution. they can also bring
with them droplets or other particles of toxic or cancer-causing pollutants
that become attached to their surfaces. Once lodged deep inside the
lungs, these fine particles can cause chronic irritation that can
trigger asthma attacks, aggravate other ling diseases, cause lung
cancer, and interfere with the bloods ability to take in oxygen and
release CO2. This strains the heart, increasing the risk of death
from heart disease.
Sulfur dioxide causes some constriction of the air
ways in healthy people and severe restriction in people with asthma.
Chronic exposure causes a condition similar to bronchitis. Sulfur
and suspended particles react to form far more hazardous acid sulphate
particles, which are inhaled more deeply into the lungs than SO2 and
remain there for long periods.
Nitrogen oxides (especially NO2) can irritate the
lungs, aggravate asthma or chronic bronchitis, cause conditions similar
to bronchitis and emphysema, and increase susceptibility to respiratory
infections such as the flu and common colds.
Research indicates that many volatile organic compounds and
toxic particulates can cause mutations, reproductive
problems, or cancer.
Evidence also shows that inhaling ozone, a component
of photochemical smog, causes coughing, chest pain, shortness of breath,
and eye, nose, and throat irritation. It also aggravates chronic diseases
such as asthma, bronchitis, emphysema, and heart trouble and reduces
s resistance to colds and pneumonia.
How are plants damaged by air pollutants?
Some gaseous pollutants (especially ozone) damage leaves of crop
plants and trees directly when they enter leaf pores. Chronic exposure
of leaves and needles to air pollutants can break down the waxy coating
that helps prevent excessive water loss and damage from diseases,
pests, drought, and frost. Such exposure also interferes with photosynthesis
and plant growth, reduces nutrient uptake, and causes leaves or needles
to turn yellow or brown and drop off. Spruce, fir, and other conifers,
especially at high elevation, are most vulnerable to air pollution
because of their long life spans and the year-round exposure of their
needles to a mixture of air pollutants.
Prolonged exposure to high levels of air pollutants from smelters
and coal-burning power plants and from cars can damage and kill trees.
However, the damage may not become visible for several decades, when
large numbers of trees suddenly begin dying off because of depletion
of soil nutrients and increased susceptibility to pests, diseases,
fungi, and drought. This phenomenon, known as waldsterben
(forest death), has turned whole forests of spruce, fir, and beech
into stump-studded meadows and mountainsides.
Trees at high elevations in mountain biomes, with their thin and
easily erodable soils and almost year-round exposure to air pollutants,
have suffered the most damage. The diebacks of trees in such areas
lead to extensive soil erosion, which in turn can lead to increased
flooding and avalanches.
How can air pollution damage aquatic
life?
High acidity (low pH) can severely harm the aquatic life in fresh
water lakes, both where the surrounding soils have little acid-neutralizing
capacity and in the northern hemisphere, where there is significant
winter snowfall. Much of the damage to aquatic life in such areas
is a result of acid shock caused by the sudden runoff
of large amounts of high acidic water and aluminium ions into lakes
and streams, when snow melts in the spring or after unusually heavy
rains. The aluminium ions leached from the soil and lake sediment
by this sudden input of acid can kill fish and inhibit their reproduction.
As the acidity of the lake increases and its food chain is disrupted,
there is a decline in net primary productivity. This can turn a moderately
eutrophic lake into a clear blue oliutrophic lake.
What are the harmful effects of pollution
on materials?
Each year air pollutants cause billions of pounds in damage to various
materials we use. The fallout of soot and grit on buildings, cars,
and clothing requires costly cleaning. Air pollution breaks down exterior
paint on cars and houses, and they deteriorate roofing materials.
Irreplaceable marble statues, historic buildings, and stained glass
windows around the world have been pitted, gouged, and discoloured
by air pollutants.
Solutions: Preventing and Reducing
Pollution
