Environmental pollution occurs when humans contaminate the air, water, or land. Pollution can be categorized into two types: primary and secondary. Primary pollution involves direct contamination of the earth by substances such as mercury, sulfur, and carbon dioxide. Secondary pollution occurs when a primary pollutant reacts with another primary pollutant, sunlight, or water to create a different pollutant. An example is acid rain: sulfur dioxide, a primary pollutant, reacts with precipitation to form acid rain, a secondary pollutant. A significant issue with pollution is that those responsible for it often do not suffer its consequences; instead, the affected parties are typically downwind from the pollution source.

14.5.1 Air Pollution

The atmosphere is composed primarily of 78% nitrogen, 21% oxygen, and small percentages of trace molecules such as ozone, carbon dioxide, water vapor, and aerosols. Air pollution occurs when human activities introduce unnatural substances into the atmosphere. Industrial air pollution is predominantly caused by coal combustion, while automobiles emit substantial amounts of ozone, carbon dioxide, and sulfur. However, the Clean Air Act, implemented by the United States in the 1970s, has significantly improved air quality nationwide. For more information, refer to this National Geographic video on global air quality.

Industrialization in eastern North America and eastern Europe has produced substantial amounts of pollutants, notably sulfur oxides and nitrogen oxides, through the combustion of fossil fuels. These pollutants react with water vapor to form acid precipitation, which poses significant environmental threats. Acid precipitation can severely damage aquatic ecosystems and forested areas by acidifying water bodies and soil, leading to weakened and dying vegetation. In forests, this can result in increased vulnerability to diseases and pest infestations. Furthermore, acid precipitation can deteriorate buildings and monuments, particularly those constructed from marble, such as tombstones.

14.5.2 Ozone Hole

In the 1920s, humans developed chlorofluorocarbons (CFCs) for use in refrigeration and air conditioning. However, in the 1970s, two American scientists discovered that CFCs were depleting the ozone layer. They found that when CFCs reach the ozone layer, ultraviolet radiation from the sun breaks off chlorine atoms, which can destroy over 100,000 ozone molecules and persist in the upper atmosphere for over 100 years. After much debate, the global community signed the Montreal Protocol in 1987 to phase out CFCs. Today, most industrialized countries have eliminated CFC use, but the ozone layer is not expected to fully recover for another 50-100 years. For more information, visit NASA’s Ozone Hole Watch.

14.5.3 Water Pollution

Water is the most valuable resource on the planet, but humans keep polluting it in various ways. Manufactures use water to create and process food. Farmers pollute vast amounts of water through fertilizer and waste from pigs and cows in unhealthy feedlots. Water is used by coal powerplants to extract and wash coal, along with cooling the steam used to make electricity. All of these processes, along with residential use, have negative impacts on water quality.

Water pollution can significantly harm aquatic life in rivers, lakes, and the ocean. Many of the fertilizers in farmers and the cleaners we use can create algae blooms in our local rivers. When the algae die, it can also remove the oxygen from the water, which can kill fish and other aquatic life. These are called dead zones, and one of the biggest in the world is forming in the Gulf of Mexico because of the pollution in the Mississippi River. Just like our air, the nation’s water has dramatically improved since the 1970s because of the Clean Water Act.

14.5.4  Plastics