Air pollutants are chemicals emitted into the atmosphere, as a result of human activities, which have negative consequences for humans or the environment. According to the United States Environmental Protection Agency (EPA), the six main pollutants are ozone, particulate matter, carbon monoxide, nitrogen oxides, sulfur dioxide and lead. To this list one could add a number of other pollutants, including volatile organic compounds and chlorofluorocarbons. In Western countries, most of the main air pollutants have been regulated and reduced over the past 40 years, but others remain serious problems.
Nitrogen oxides (NOx)
According to the Department of Oceanography at Texas A&M University, nitrous oxides are gases released mainly by motor vehicle engines, but to a lesser extent by power plants and other fuel-burning activities. They react with water and oxygen in the atmosphere to form acid rain. The problem of acid raid was one of the main concerns of the environmental movement in the 1980s. In addition, the EPA states that nitrogen dioxide can trigger symptoms of asthma.
Nitrogen oxide regulations were first introduced in the 1970s. In addition, catalytic converters in most modern vehicles convert some of the nitrogen oxides in the exhaust into harmless nitrogen. The EPA says that nitrogen oxide levels have been cut in half since 1980.
Sulfur dioxide (SO2)
Sulfur dioxide is released through fossil fuel combustion, mainly from coal- and natural gas-fired power plants. Sulfur dioxide worsens asthma and, along with nitrogen oxides, is one of the main causes of acid rain.
A variety of measures exists to reduce sulfur dioxide emissions. As a result, the EPA reports that sulfur dioxide concentrations in the atmosphere have declined by over 80 percent in the United States since 1980. However, the same is not true in rapidly developing countries with lax environmental regimes. China is now the world’s largest emitter of sulfur dioxide, and emissions there are still growing. As a direct result, several Chinese regions face severe problems caused by acid rain.
Particulate matter (PM)
Particulate matter refers to a wide range of chemicals, metal dusts and soil blown into the air. Particulate matter consists of dust and particles small enough that it can enter the body through the lungs, but large enough to cause potential health problems. The EPA currently divides particulate matter into “inhalable coarse particles,” which are larger and found mainly by roads and dusty industrial areas, and “fine particles,” which are smaller and found mainly in smoke and haze from power plants, industrial processes, fires and motor vehicles.
Particulate matter is regulated under a variety of national and international standards. However, there has been much less progress made in reducing particulate matter in most countries than in reducing sulfur dioxide or nitrogen oxides. The EPA claims that it has achieved a 27 percent decrease in American particulate matter levels.
Carbon oxides (CO)
Carbon monoxide is a highly poisonous gas emitted by burning fuels. Common sources include motor vehicle engines. Because the human body lacks the ability to distinguish oxygen from carbon monoxide, exposure to high levels of carbon monoxide can cause severe health problems or even death. However, this level of carbon monoxide poisoning is normally only a risk in closed indoor environments, where carbon monoxide released by a fire can quickly reach toxic levels. The EPA states that carbon monoxide levels have fallen significantly in the United States since 1980.
In recent years, there has been some discussion of listing carbon dioxide as a main air pollutant as well. Although carbon dioxide can be a health threat in high concentrations, it is a naturally occurring gas, and the amounts released into the air by burning fossil fuels do not pose a direct chemical threat to human health. Instead, carbon dioxide emissions are controversial because they are believed to be the main contributor to harmful climate change.
Ozone
Ozone is a naturally occurring compound of oxygen which occurs at higher concentrations in upper layers of the atmosphere, where it plays a vital role in reducing ultraviolet radiation. However, when it occurs at elevated levels at ground level, it is normally classified as a pollutant. (In this case, ozone is referred to as “ground-level ozone.”) Ozone is one of the major contributors to urban smog, and significantly worsens symptoms of asthma.
There has been some improvement in ozone levels in urban areas, but countries still struggle to make more needed progress. According to the EPA, ground-level ozone concentration has declined since the 1980s, but is still above the national standard which was established following the passage of the Clean Air Act.
Chlorofluorocarbons (CFCs)
According to James Elkins of the National Oceanic and Atmospheric Administration, CFCs are organic chemicals which contain a mix of carbon, chlorine and fluorine atoms. Until the 1990s, CFCs were frequently used in a number of settings, but especially in refrigerators and air conditioners. Manufacturers liked the gas because it was an efficient, non-toxic refrigerant. However, in the 1980s, scientists linked CFC emissions to the collapse of the ozone layer, a level of the atmosphere which blocks potentially harmful ultraviolet light from reaching the surface.
Under the Montreal Protocol, most countries agreed to restrict use of CFCs. However, scientists have yet to chart a full recovery of the ozone layer. There is currently some controversy over the scientific assessments of environmental problems caused by CFCs.
Lead
Lead is an extremely toxic metal which can cause mental deficits and nerve problems in children, anemia and, in severe cases, death. According to the National Institutes of Health, lead is a particularly problematic toxin because lead is stored in the body, allowing trace amounts to build up over time.
Lead was formerly a serious pollution problem because of its widespread use as an additive in gasoline. However, according to the EPA, the United States and many other countries began to restrict the use of lead in gasoline in the 1970s and 1980s. As of 2010, the EPA says that lead levels in the American air had fallen from 3 micrograms per cubic metre to less than 0.5. Other countries have had similarly effective results.
In addition to these main air pollutants, there is a very large number of other chemicals which are emitted by industrial processes and which have known or suspected consequences for human or environmental health. These include trace radioactive emissions from power plants, volatile organic compounds and ammonia from agricultural processes. For the most part, however, these pollutants are emitted at much lower levels and therefore currently pose much less risk.
