High School Earth Science/Effects of Air Pollution

People in developing countries often do not have laws to protect the air that they breathe. The World Health Organization estimates that 22 million people die each year from complications due to air pollution. Even in the United States, more than 120 million Americans live in areas where the air is considered unhealthy. This lesson looks at the human health and environmental problems caused by different types of air pollution.

Lesson Objectives

• Describe the damage that is being done by smog.
• Discuss how acid rain is formed and the damage it does.
• Discus how chlorofluorocarbons destroy the ozone layer.

Smog

All air pollutants cause some damage to living creatures and the environment. Different types of pollutants cause different types of harm. Particulates reduce visibility. For example, in the western United States, people can now ordinarily see only about 100 to 150 kilometers (60 to 90 miles), which is one-half to two-thirds the natural (pre-pollution) range on a clear day. In the East, visibility is worse. People can only see about 40 to 60 kilometers (25-35 miles), which is one-fifth the distance they could see without any air pollution.

Particulates reduce the amount of sunshine that reaches the ground. Since plants also receive less sunlight, there may be less photosynthesis. Particulates also form the nucleus for raindrops, snowflakes or other forms of precipitation. An increase in particles in the air seems to increase the number of raindrops, but often decreases their size. By reducing sunshine, particulates can also alter air temperature. In the three days after the terrorist attacks on September 11, 2001, jet airplanes did not fly over the United States. Without the gases from jet contrails blocking sunlight, air temperatures increased 1°C (1.8°F) across the U.S (Figure 22.6). Imagine how much all of the sources of particulates combine to reduce temperatures.

Figure 22.6: Jet contrails block sunlight.

Ozone damages some plants. Since ozone effects accumulate, plants that live a long time show the most damage. Some species of trees appear to be the most susceptible. If a forest contains ozone-sensitive trees, they may die out and be replaced by species that are not as easily harmed. This can change an entire ecosystem, since animals and plants may not be able to survive without the habitats created by the native trees.

Some crop plants show ozone damage. When exposed to ozone, spinach leaves become spotted. Soybeans and other crops have reduced productivity. In developing nations, where getting every last bit of food energy out of the agricultural system is critical, any loss is keenly felt. Many of these nations, like China and India, also have heavy air pollution. Some pollutants have a positive effect on plant growth. Increased CO₂ seems to lessen ozone damage to some plants and it may promote growth. Unfortunately, CO₂ and other greenhouse gases cause other problems that harm the ecosystem and reduce growth of some plants.

Other air pollutants damage the environment (Figure 22.7). NO₂ is a toxic, orange-brown colored gas that gives air a distinctive orange color and an unpleasant odor. Nitrogen and sulfur-oxides in the atmosphere create acids that fall as acid rain. Human health suffers in locations with high levels of air pollution. Lead is the most common toxic material for humans and is responsible for lead poisoning. Carbon monoxide is a toxic gas and can kill people in poorly ventilated spaces, like tunnels. Nitrogen and sulfur-oxides cause lung disease and increase rates of asthma, emphysema, and viral infections like flu. Ozone also damages the human respiratory system, causing lung disease. High ozone levels are also associated with increased heart disease and cancer. Particulates enter the lungs and cause heart or lung disease. When particulate levels are high, asthma attacks are more common. By some estimates, 30,000 deaths a year in the United States are caused by fine particle pollution.

Figure 22.7: Smog in New York City.

Although not all cases of asthma can be linked to air pollution, many can. During the 1996 Olympic Games, Atlanta, Georgia closed off their downtown to private vehicles. As a result, ozone levels decreased by 28%. At the same time, there were 40% fewer hospital visits for asthma.

Lung cancer among non-smokers is also increasing. One study showed that the risk of being afflicted with lung cancer increases directly with a person's exposure to air pollution. The study concluded that no level of air pollution should be considered safe. Exposure to smog also increased the risk of dying from any cause, including heart disease.

Children are more vulnerable to problems from breathing dirty air than adults because their lungs are still growing and developing. Children take in 50% more air for their body weight than adults. Children spend more time outside in unfiltered air and are more likely to breathe hard from playing or exercising. One study found that in the United States, children develop asthma at more than twice the rate of two decades ago and at four times the rate in Canada. Adults also suffer from air pollution-related illnesses that include lung disease, heart disease, lung cancer, and weakened immune systems. The asthma rate worldwide is rising 20% to 50% every decade.

Especially dangerous are pollutants that remain in an organism throughout its life, called bioaccumulation. In this process, an organism accumulates the entire amount of a toxic compound that it consumes over its lifetime. Not all substances bioaccumulate. A person who takes a daily dose of aspirin only has that day's worth of aspirin in her system, because aspirin does not stay within her system. When a compound bioaccumulates, the person has all of that compound she's ever eaten in her system. Compounds that bioaccumulate are usually stored in the organism's fat.

Mercury is a good example of a substance that bioaccumulates. Bacteria and plankton store all of the mercury from all of the seawater they ingest. A small fish that eats bacteria and plankton accumulates all of the mercury from all of the tiny creatures it eats over its lifetime. A big fish accumulates all of the mercury from all of the small fish it eats over its lifetime. The organisms that accumulate the most mercury are the large predators that eat high on the food chain. Tuna pose a health hazard to anything that eats them because their bodies are so high in mercury. This is why the government recommends limits on the amount of tuna that people eat. These limits are especially important for children and pregnant women, since mercury particularly affects young people. If the mercury just stayed in fat, it would not be harmful, but that fat is used when a woman is pregnant or nursing a baby, or when she burns the fat while losing weight. Methyl mercury poisoning can cause nervous system or brain damage, especially in infants and children. Children may experience brain damage or developmental delays. Like mercury, other metals and VOCs can bioaccumulate, causing harm to animals and people high on the food chain.

Acid Rain

Acid rain is caused by sulfur and nitrogen oxides. These pollutants are emitted into the atmosphere from power plants or metal refineries. The oxides come out of smokestacks that have been built tall so that pollutants don't sit over cities. The high smokestacks allow the emissions to rise high into the atmosphere and travel up to 1000 km (600 miles) downwind. As they move, these pollutants combine with water vapor to form sulfuric and nitric acids. The acid droplets form acid fog, rain, snow, or they may be deposited dry. Most typical is acid rain (Figure 22.8).

Figure 22.8: How acid rain is formed. Anthropogenic pollutants are those that are humanmade. Deposition of a pollutant occurs when it is placed on a surface. Rain can bring wet deposition or a pollutant can be blown onto the ground for dry deposition.

Acid rain water is more acidic than normal rain water. To be called acid rain, it must have a pH of less than 5.0. Acidity is measured on the pH scale, which goes from 1 to 14. A value of 7 is neutral. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline). The strongest acids are at the low end of the scale and the strongest bases are at the high end. Natural rain is somewhat acidic with a pH of 5.6. The acid comes from carbonic acid that forms when CO₂ combines with water in the atmosphere. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic.

Regions that have a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet bowl cleaner. In arid climates, like in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are 5 to 10 times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need.

Acid rain takes a toll on ecosystems (Figure 22.9). Plants that are exposed to acids become weak and are more likely to be damaged by bad weather, insect pests, or disease. Snails die in acid soils, so songbirds do not have as much food to eat. Young birds and mammals do not build bones as well and may not be as strong. Eggshells may also be weak and break more easily.

Figure 22.9: Acid rain has killed trees in this forest in the Czech Republic.

The nitrates found in acid rain cause some plants to grow better. These nitrate-lovers can drive out other plants, which may cause the ecosystem to change. Nitrates also fertilize the oceans, which makes more algae grow. The algae use up all the oxygen in the water, which can bring about disastrous ecological changes, including the deaths of many fish. As lakes become acidic, organisms die off. If the pH drops below 4.5, all the fish die. Organic material cannot decay, and mosses take over the lake. Wildlife that depend on the lake for drinking water suffer population declines. Crops are damaged by acid rain. This is most noticeable in poor nations where people can't afford to fix the problems with fertilizers or other technology. Buildings and monuments are damaged by acid precipitation (Figure 22.10). These include the U.S. Capitol and many buildings in Europe, such as Westminster Abbey.

Figure 22.10: Acid rain damages cultural monuments like buildings and statues.

Carbonate rocks can neutralize acids and so some regions do not suffer the effects of acid rain nearly as much. The Midwestern United States is protected by the limestone rocks throughout the area, which are made up of calcium carbonate. One reason that the northeastern United States is so vulnerable to acid rain damage is that the rocks are not carbonates.

Because pollutants can travel so far, much of the acid rain that falls hurts states or nations other than ones where the pollutants were released. All the rain that falls in Sweden is acidic and fish in lakes all over the country are dying. The pollutants come from the United Kingdom and Western Europe, which are now working to decrease their emissions. Canada also suffers from acid rain that originates in the United States, a problem that is also improving. Southeast Asia is experiencing more acid rain between nations as the region industrializes.

Ozone Depletion

At this point you might be asking yourself, "Is ozone bad or is ozone good"? There is no simple answer to that question: It depends on where the ozone is located. In the troposphere, ozone is a pollutant. Higher up, in the stratosphere, ozone screens out high energy ultraviolet radiation and thus makes Earth habitable. This protective ozone is found in the ozone layer.

The ozone layer is being attacked by human-made chemicals that break ozone molecules apart in the stratosphere. The most common of these chemicals are chlorofluorocarbons (CFCs), but includes others such as halons, methyl bromide, carbon tetrachloride, and methyl chloroform. CFCs were once widely used because they are cheap, nontoxic, nonflammable, and non-reactive. They were used as spray-can propellants, refrigerants, and in many other products.

Once they are released into the air, CFCs float up to the stratosphere. Air currents move them toward the poles. In the winter, they freeze onto nitric acid molecules in polar stratospheric clouds (PSC). PSCs form only where the stratosphere is coldest, and are most common above Antarctica in the wintertime. In the spring, the sun's warmth starts the air moving, and ultraviolet light breaks the CFCs apart. The chlorine atom floats away and attaches to one of the oxygen atoms on an ozone molecule. The chlorine pulls the oxygen atom away, leaving behind an O₂ molecule, which provides no UV protection. The chlorine then releases the oxygen atom and moves on to destroy another ozone molecule. One CFC molecule can destroy as many as 100,000 ozone molecules.

Ozone destruction creates the ozone hole where the layer is dangerously thin (Figure 22.11). As air circulates over Antarctica in the spring, the ozone hole expands northward over the southern continents, including Australia, New Zealand, southern South America, and southern Africa. UV levels may rise as much as 20% beneath the ozone hole. The hole was first measured in 1981 when it was 2 million square km (900,000 square miles)). The 2006 hole was the largest ever observed at 28 million square km (11.4 million square miles). It had the lowest ozone levels ever recorded and also lasted the longest. The difference in the size of the ozone hole each year depends on many factors, including whether conditions are right for the formation of polar stratospheric clouds.

Figure 22.11: The September 2006 ozone hole, the largest ever observed. Blue and purple colors show particularly low levels of ozone.

Ozone loss also occurs over the north polar region, but it is not enough for scientists to call it a hole. The region of low ozone levels is small because the atmosphere is not as cold and PSCs do not form as readily. Still, springtime ozone levels are relatively low. This low moves south over some of the world's most populated areas in Europe, North America, and Asia. At 40°N, the latitude of New York City, UV-B has increased about 4% per decade since 1978. At 55°N, the approximate latitude of Moscow and Copenhagen, the increase has been 6.8% per decade since 1978.

Ozone losses in population centers increase sunburns, cataracts (clouding of the lens of the eye), and skin cancers. A loss of ozone of only 1% is estimated to increase skin cancer cases by 5 to 6%. People may also suffer from decreases in their immune system's ability to fight off infectious diseases. Ozone loss may reduce crop yields, since many plants are sensitive to ultraviolet light. Excess UV appears to be decreasing the productivity of plankton in the oceans. A decrease of 6 to 12% has been measured around Antarctica, which may be at least partly related to the ozone hole. The effects of excess UV on other organisms is not known. When the problem with ozone depletion was recognized, world leaders took action. CFCs were banned in spray cans in some nations in 1978. The greatest production of CFCs was in 1986, but has declined since then. This will be discussed more in the next lesson.

Lesson Summary

• Air pollutants damage human health and the environment. Particulates reduce visibility, alter the weather, and cause lung problems like asthma attacks.
• Ozone damages plants and can also cause lung disease. Acid rain damages forests, crops, buildings, and statues.
• The ozone hole, caused by ozone-destroying chemicals, allows more UV radiation to strike the Earth.
• This can cause plankton populations to decline and skin cancers in humans to increase, along with other effects.

Review Questions

1. Why is visibility so reduced in the United States?
2. Why do health recommendations suggest that people limit the amount of tuna they eat?
3. Why might ozone pollution or acid rain change an entire ecosystem?
4. Why does air pollution cause problems in developing nations more than in developed ones?
5. Why are children more vulnerable to the effects of air pollutants than adults?
6. Describe bioaccumulation.
7. How does pollution indirectly kill or harm plants?
8. What do you think the effect is of jet airplanes on global warming?
9. Why is air pollution a local, regional and global problem?
10. How do CFCs deplete the ozone layer?

Vocabulary

acid rain

Rain that has a pH of less than 5.0.

alkaline

Also called basic. Substances that have a pH of greater than 7.0.

bioaccumulation

The accumulation of toxic substances within organisms so that the concentrations increase up the food web.

ozone hole

A region around Antarctica in which ozone levels are reduced in springtime, due to the action of ozone-destroying chemicals.

pH scale

A scale that measures the acidity of a solution. A pH of 7 is neutral. Smaller numbers are more acidic and larger numbers are more alkaline.

polar stratospheric clouds (PSC)

Clouds that form in the stratosphere when it is especially cold; PSCs are necessary for the breakup of CFCs.

Points to Consider

• Since mercury bioaccumulates and coal-fired power plants continue to emit mercury into the atmosphere, what will be the consequence for people who like to eat tuna and other large predatory fish?
• What are the possible causes of rising asthma rates in children?
• A ban has been imposed on CFCs and some other ozone-depleting substances. How will the ozone hole change in response to this ban?