Sources and Effects of Air Pollution
Air pollution is the presence in the atmosphere of substances in amounts that can harm people, other living things or the environment. The main pollutants include particulate matter, nitrogen oxides, sulfur dioxide, carbon monoxide, ground-level ozone and volatile organic compounds. In brief: most outdoor air pollution comes from burning fossil fuels in vehicles, power stations and industry, and it can cause lung disease, acid rain and smog.
Clean air is a mixture of gases in fixed proportions, and pollutants upset this balance. Some pollutants have natural sources, such as volcanoes and wildfires, but human activity has increased them. This article lists the main pollutants, their sources and effects, describes smog, acid rain and ozone depletion, works through a concentration calculation and discusses ways of reducing pollution.
What are the main air pollutants?
A pollutant can be a gas, liquid droplets or tiny solid particles. Pollutants released directly from a source are primary pollutants, for example soot and carbon monoxide. Secondary pollutants form in the atmosphere from reactions of primary ones. Ground-level ozone, for example, forms when nitrogen oxides and volatile organic compounds react in sunlight.
Sources and effects of air pollutants
| Pollutant | Main sources | Main effects |
|---|---|---|
| Carbon monoxide (CO) | Incomplete burning in engines and heaters | Reduces oxygen carried by blood; toxic |
| Sulfur dioxide (SO2) | Burning coal and oil, volcanoes | Acid rain, breathing problems |
| Nitrogen oxides (NO and NO2) | Vehicle engines, power stations | Smog, acid rain, lung irritation |
| Particulate matter (PM10, PM2.5) | Diesel engines, wood smoke, dust, industry | Heart and lung disease, reduced visibility |
| Ground-level ozone (O3) | Formed from NOx and VOCs in sunlight | Lung damage, crop damage |
| Volatile organic compounds (VOCs) | Solvents, paints, fuel vapors | Ozone formation, some are toxic |
| Lead | Historically leaded fuel, some industries | Damages the nervous system |
Carbon dioxide and methane are not usually classified as pollutants that are directly harmful to breathe, but they are greenhouse gases that cause climate change.
What is smog?
The word smog comes from “smoke” and “fog”. Two main types exist. Classical smog arises from burning coal and combines smoke, sulfur dioxide and fog, as in London in the 1950s. Photochemical smog forms in sunny, busy cities when nitrogen oxides and VOCs from vehicles react in sunlight to produce ozone and other irritants, and it often appears as a brownish haze. Cold air near the ground under a warm layer, called a temperature inversion, can trap smog over a city.
What causes acid rain?
Sulfur dioxide and nitrogen oxides react with water in the atmosphere and form sulfuric and nitric acids: overall, 2SO2 + O2 + 2H2O → 2H2SO4. Normal rain is slightly acidic, with a pH of about 5.6, because dissolved carbon dioxide forms carbonic acid. Acid rain has a pH below this value. It damages forests, makes lakes too acidic for fish and dissolves limestone and marble in buildings.
What about the ozone layer?
The ozone layer in the stratosphere absorbs most of the harmful ultraviolet radiation from the Sun. In the 1970s and 1980s scientists found that chlorofluorocarbons (CFCs), once used in refrigerators and aerosol sprays, release chlorine atoms that destroy ozone, and an “ozone hole” appeared over Antarctica. The Montreal Protocol of 1987 phased out CFCs, and the ozone layer is slowly recovering. Ozone high in the atmosphere is protective, but ozone at ground level is a pollutant.
Worked example: concentrations of pollutants
Concentrations of pollutants are often given in micrograms per cubic meter (µg/m3), and 1 g = 1,000,000 µg.
- Mass of particles in a room. A room of 60 m3 has an average PM2.5 level of 25 µg/m3. The mass of fine particles in the room is 25 × 60 = 1,500 µg = 1.5 mg.
- Comparison with a guideline. The 2021 World Health Organization guideline for the annual mean of PM2.5 is 5 µg/m3. The ratio is 25 ÷ 5 = 5, so the level is five times the guideline value.
- Percentage by volume. A carbon monoxide level of 9 ppm means 9 parts per million, which is 9 ÷ 1,000,000 × 100% = 0.0009% by volume.
- Breathing. An adult breathes about 12 m3 of air per day. At 25 µg/m3, the amount of PM2.5 in the air breathed is 25 × 12 = 300 µg per day.
How can air pollution be reduced?
- Cleaner energy: replace coal and oil with renewable sources and use scrubbers in power stations to remove sulfur dioxide.
- Catalytic converters: fitted to cars, they convert carbon monoxide, nitrogen oxides and unburnt fuel into carbon dioxide, nitrogen and water.
- Transport policies: public transport, electric vehicles and low-emission zones.
- Regulations: limits on emissions from industry, and fuel standards.
- Indoor measures: ventilation, clean stoves and avoiding smoking indoors.
How did air quality control begin?
The smoke of burning coal was a problem in cities for centuries, and regulations in medieval London tried to limit it. The Great Smog of December 1952 in London is estimated to have caused thousands of deaths, and it led to the Clean Air Act of 1956 in the United Kingdom. The U.S. Clean Air Act of 1970 set national standards, and international agreements, such as the Montreal Protocol, have addressed pollutants that cross borders.
Which mistakes are common?
Mistake 1: Assuming that clear air is clean
Many harmful gases, such as carbon monoxide and ozone, are invisible.
Mistake 2: Mixing up the ozone layer and ground-level ozone
Stratospheric ozone is protective. Ground-level ozone is harmful to breathe.
Mistake 3: Believing that air pollution is only an outdoor problem
Indoor air can contain smoke, gas from stoves, mold spores and chemicals from products.
Mistake 4: Confusing the greenhouse effect with the ozone hole
They are separate problems with different causes, although both involve changes in atmospheric gases.