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There’s nothing quite like opening the door and breathing fresh, clean, air—but how clean is the air you’re breathing right now? Unless you’re a scientist with a chemistry lab at your fingertips, there’s no real way of knowing. The gases you’re sucking up through your nose could be slowly killing you: according to the World Health Organization, around two million people die prematurely from the effects of polluted air every single year. Air pollution is a huge problem—and not just for people living in smog-choked cities: through such things as global warming and damage to the ozone layer, it has the potential to affect us all. So what exactly causes this major environmental issue and what can we do about it? Let’s take a closer look!

Air pollution is obvious when it pours from a smokestack (chimney), but it’s not always so easy to spot. This smoke comes from a coal-fired power plant and its pollutants include sulfur dioxide and the “greenhouse gas” carbon dioxide. Photo by courtesy of US Department of Energy/National Renewable Energy Laboratory.

What is air pollution?

Air lets our living planet breathe—it’s the mixture of gases that fills the atmosphere, giving life to the plants and animals that make Earth such a vibrant place. Broadly speaking, air is almost entirely made up of two gases (78 percent nitrogen and 21 percent oxygen), with a few other gases (such as carbon dioxide and argon) present in absolutely minute quantities. We can breathe ordinary air all day long with no ill effects, so let’s use that simple fact to define air pollution, something like this:

Air pollution is a gas (or a liquid or solid dispersed through ordinary air) released in a big enough quantity to harm the health of people or other animals, kill plants or stop them growing properly, damage or disrupt some other aspect of the environment (such as making buildings crumble), or cause some other kind of nuisance (reduced visibility, perhaps, or an unpleasant odor).
As with water pollution and land contamination, it’s the quantity (or concentration) of a chemical in the air that makes the difference between “harmless” and “pollution.” Carbon dioxide (CO2), for example, is present in the air around you at a typical concentration of less than 0.05 percent and breathing it in usually does no harm (you breathe it out all day long); but air with an extremely high concentration of carbon dioxide (say, 5–10 percent) is toxic and could kill you in a matter of minutes. Since Earth’s atmosphere is very turbulent—many of us live in windy countries—air pollution will often disperse relatively quickly. In less enlightened times, factory operators thought that if they built really high smokestacks, the wind would simply blow their smoke away, diluting and dispersing it so it wouldn’t be a problem. The only trouble was, Earth is a much smaller place than we think and pollution doesn’t always disappear so conveniently.

Natural air pollution

Burning forest releasing smoke.

Photo: Forest fires are one completely natural cause of air pollution. We’ll never be able to prevent them breaking out or stop the pollution they cause; our best hope is to manage forests, where we can, so fires don’t spread. Photo by courtesy of US Fish and Wildlife Service.

When we think of pollution, we tend to think it’s a problem that humans cause through ignorance or stupidity—and that’s certainly true, some of the time. However, it’s important to remember that some kinds of air pollution are produced naturally. Forest fires, erupting volcanoes, and gases released from radioactive decay of rocks inside Earth are just three examples of natural air pollution that can have hugely disruptive effects on people and the planet.

Forest fires (which often start naturally) can produce huge swathes of smoke that drift for miles over neighboring cities, countries, or continents. Giant volcanic eruptions can spew so much dust into the atmosphere that they block out significant amounts of sunlight and cause the entire planet to cool down for a year or more. Radioactive rocks can release a gas called radon when they decay, which can build up in the basements of buildings with serious effects on people’s health.

All these things are examples of serious air pollution that happen without any help from humans; although we can adapt to natural air pollution, and try to reduce the disruption it causes, we can never stop it happening completely. For the rest of this article, we’ll consider only the “unnatural” types of pollution: the problems that people cause—and the ones we can solve.

Anything people do that involves burning things (combustion), using household or industrial chemicals (substances that cause chemical reactions and may release toxic gases in the process), or producing large amounts of dust has the potential to cause air pollution. Step back a century or two and the cause of most air pollution was easy to identify: filthy factories, powering the Industrial Revolution. Today, tighter air pollution laws, greater environmental awareness, and determined campaigns mounted by local communities make it far harder—though by no means impossible—for factories to pollute in post-industrial nations such as the United States and Britain.

Where, then, does modern air pollution come from? By far the biggest culprit today is traffic, though power plants and factories continue to make an important contribution. Before we start laying the blame for air pollution, let’s remember one very important thing: most of us drive (or travel in) cars, use electricity, and buy goods made in factories. If we’re pointing fingers, ultimately we’re going to have to point them at ourselves.

Now let’s look a bit more closely at the three key sources of air pollution.


There are something like a half billion cars on the road today—one for every two people in rich countries such as the United States. Virtually all of them are powered by gasoline and diesel engines that burn petroleum to release energy. Petroleum is made up of hydrocarbons (large molecules built from hydrogen and carbon) and, in theory, burning them fully with enough oxygen should produce nothing worse than carbon dioxide and water. In practice, fuels aren’t pure hydrocarbons and engines don’t burn them cleanly. As a result, exhausts from engines contain all kinds of pollution, notably particulates (soot of various sizes), carbon monoxide (CO, a poisonous gas), nitrogen oxides (NOx), volatile organic compounds (VOCs), and lead—and indirectly produce ozone. Mix this noxious cocktail together and energize it with sunlight and you get the sometimes brownish, sometimes blueish fog of pollution we call smog, which can hang over cities for days on end.


Power plants

Renewable energy sources such as solar panels and wind turbines are helping us generate a bigger proportion of our power every year, but the overwhelming majority of electricity (around 70 percent in the United States, for example) is still produced by burning fossil fuels such as coal, gas, and oil, mostly in conventional power plants. Just like car engines, power plants should theoretically produce nothing worse than carbon dioxide and water; in practice, fuels are dirty and they don’t burn cleanly, so power plants produce a range of air pollutants, notably sulfur dioxide, nitrogen oxides, and particulates. (They also release huge amounts of carbon dioxide, a key cause of global warming and climate change when it rises and accumulates in the atmosphere. We discuss this a bit more down below.)

Industrial plants and factories

Plants that produce the goods we all rely on often release small but significant quantities of pollution into the air. Industrial plants that produce metals such as aluminum and steel, refine petroleum, produce cement, synthesize plastic, or make other chemicals are among those that can produce harmful air pollution. Most plants that pollute release small amounts of pollution continually over a long period of time, though the effects can be cumulative (gradually building up). Sometimes industrial plants release huge of amounts of air pollution accidentally in a very short space of time. One notable case happened in Bhopal, India in December 1984, when a large chemical plant run by the Union Carbide company released a poisonous gas (methyl isocyanate) that hung over the local area, killing around 3000 people and injuring thousands more. (Wikipedia’s article on the Bhopal Disaster gives a comprehensive account of what happened.)

Other causes of air pollution

Although traffic, power plants, and industrial and chemical plants produce the majority of Earth’s manmade air pollution, many other factors contribute to the problem. In some parts of the world, people still rely on burning woodfuel for their cooking and heating, and that produces indoor air pollution that can seriously harm their health (solar cookers are one solution to that problem). In some areas, garbage is incinerated instead of being recycled or landfilled and that can also produce significant air pollution unless the incinerators are properly designed to operate at a high enough temperature (even then, there is a toxic residue left behind that must be disposed of somehow).


Raising awareness and changing behavior

Clean technologies can tackle dirty technologies, and laws can make polluters clean up their act—but none of this would happen without people being aware of pollution and its damaging effects. Sometimes it takes horrific tragedies (like the 1952 smog episode in London or the Chernobyl catastrophe) to prompt action. Often, we pollute the environment without even realizing it: how many people know that taking a shower or ironing a shirt can release indoor air pollution from hot water that they immediately breathe in, for example? Helping people to understand the causes and effects of pollution and what they can do to tackle the issue is very important—that’s why I’m writing these words now and probably why you’re reading them. Air pollution isn’t someone else’s problem: all of us help to cause it and we can all help to clean it up. Starting now!