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Air pollution
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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 4.2 million people die prematurely from the effects of polluted, outdoor 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  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. its pollutants include sulfur dioxide and the greenhouse gas carbon dioxide. Thanks to tougher pollution controls, modern power plants produce only a fraction as much pollution.
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 pland , 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

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 (each year, around 21,000 people die of lung cancer, due to radon gas in the United States).
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.
Top-ten kinds of air pollution

courtesy of US Department of Energy.
Any gas could qualify as pollution if it reached a high enough concentration to do harm. Theoretically, that means there are dozens of different pollution gases. In practice, about ten different substances cause most concern:
1.    Sulfur dioxide: Coal, petroleum, and other fuels are often impure and contain sulfur as well as organic (carbon-based) compounds. When sulfur (spelled "sulphur" in some countries) burns with oxygen from the air, sulfur dioxide (SO2) is produced. Coal-fired power plants are the world's biggest source of sulfur-dioxide air pollution, which contributes to smog, acid rain, and health problems that include lung disease.
2.    Carbon monoxide: This highly dangerous gas forms when fuels have too little oxygen to burn completely. It spews out in car exhausts and it can also build up to dangerous levels inside your home if you have a poorly maintained gas boiler, stove, or fuel-burning appliance. (Always fit a carbon monoxide detector if you burn fuels indoors.)
3.    Carbon dioxide: This gas is central to everyday life and isn't normally considered a pollutant: we all produce it when we breathe out and plants such as crops and trees need to "breathe" it in to grow. However, carbon dioxide is also a greenhouse gas released by engines and power plants. Since the beginning of the Industrial Revolution, it's been building up in Earth's atmosphere and contributing to the problem of global warming and climate change.
4.    Nitrogen oxides: Nitrogen dioxide (NO2) and nitrogen oxide (NO) are pollutants produced as an indirect result of combustion, when nitrogen and oxygen from the air react together. Nitrogen oxide pollution comes from vehicle engines and power plants, and plays an important role in the formation of acid rain, ozone and smog. Nitrogen oxides are also "indirect greenhouse gases" (they contribute to global warming by producing ozone, which is a greenhouse gas).
5.    Volatile organic compounds (VOCs): These carbon-based (organic) chemicals evaporate easily at ordinary temperatures and pressures, so they readily become gases. That's precisely why they're used as solvents in many different household chemicals such as paints, waxes, and varnishes. Unfortunately, they're also a form of air pollution: they're believed to have long-term (chronic) effects on people's health and they also play a role in the formation of ozone and smog.
6.    Particulates: These are the sooty deposits in air pollution that blacken buildings and cause breathing difficulties. Particulates of different sizes are often referred to by the letters PM followed by a number, so PM10 means soot particles of less than 10 microns (10 millionths of a meter or 10µm in diameter, roughly 10 times thinner than a thick human hair). The smaller ("finer") the particulates, the deeper they travel into our lungs and the more dangerous they are. PM2.5 particulates are much more dangerous (they're less than 2.5 millionths of a meter or about 40 times thinner than a typical hair). In cities, most particulates come from traffic fumes.
7.    Ozone: Also called trioxygen, this is a type of oxygen gas whose molecules are made from three oxygen atoms joined together (so it has the chemical formula O3), instead of just the two atoms in conventional oxygen (O2). In the stratosphere (upper atmosphere), a band of ozone ("the ozone layer") protects us by screening out harmful ultraviolet radiation (high-energy blue light) beaming down from the Sun. At ground level, it's a toxic pollutant that can damage health. It forms when sunlight strikes a cocktail of other pollution and is a key ingredient of smog (see box below).
8.    Chlorofluorocarbons (CFCs): Once thought to be harmless, these gases were widely used in refrigerators and aerosol cans until it was discovered that they damaged Earth's ozone layer. We discuss this in more detail down below.
9.    Unburned hydrocarbons: Petroleum and other fuels are made of organic compounds based on chains of carbon and hydrogen atoms. When they burn properly, they're completely converted into harmless carbon dioxide and water; when they burn incompletely, they can release carbon monoxide or float into the air in their unburned form, contributing to smog.
10.    Lead and heavy metals: Lead and other toxic "heavy metals" can be spread into the air either as toxic compounds or as aerosols(when solids or liquids are dispersed through gases and carried through the air by them) in such things as exhaust fumes and the fly ash (contaminated waste dust) from incinerator smokestacks.
What are the causes of air pollution?
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.
Traffic
There are over one 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.
Vehicles don't release pollution only from their tailpipes. Brake and tire wear and tear, the slow rubbing away of the road surface as tires rumble over it, and stirring up of the dust and debris on top of it also release significant amounts of PM10 and PM2.5 particulates into the air.
Smog

Energy/National Renewable Energy Laboratory.
Smog isn't the stuff that pumps from a car's tailpipe or drifts from a factory smokestack—it's the nasty brown or blue haze that builds up over a city as a result.
Smog (a combination of the words "smoke" and "fog") forms when sunlight acts on a cocktail of pollutant gases such as nitrogen and sulfur oxides, unburned hydrocarbons, and carbon monoxide; that's why it's sometimes called photochemical smog (the energy in light causes the chemical reaction that makes smog). One of the most harmful constituents of smog is a toxic form of oxygen called ozone, which can cause serious breathing difficulties and even, sometimes, death. When smog is rich in ozone, it tends to be a blueish color, otherwise it's more likely to be brown.
Although smog can happen in any busy city, it's a particular problem in places such as Los Angeles where the local climate (influenced by the ocean and neighboring mountains) regularly causes what's known as a temperature inversion. Normally, air gets colder the higher up you go but in a temperature inversion the opposite happens: a layer of warm air traps a layer of cold air nearer the ground. This acts like a lid over a cloud of smog and stops it from rising and drifting away. Largely because of their traffic levels, smog afflicts many of the world's busiest cities, including Athens, Beijing, Delhi, Madrid, Mexico City, Milan, Paris, and Tokyo.

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 aluminumand 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).
What effects does air pollution have?
Air pollution can harm the health of people and animals, damage crops or stop them growing properly, and make our world unpleasant and unattractive in a variety of other ways.
Human health
We know air pollution is a bad thing without even thinking about it. Have you ever coughed when a truck drove past belching out its sooty exhaust? Instinctively, you cough to clear your lungs and protect your body and you might even cover your face with your handkerchief or sleeve to filter the air until it feels safe to breathe deeply again. You don't have to be told that pollution like this might harm your health to want to steer clear of it: your body takes action automatically. The only trouble is, we can't always see or smell air pollution, tell when it's affecting us, or know how it might harm us days, months, or even years in the future.

Sometimes the connection between air pollution and human health is obvious, as in the Bhopal Disaster. Another notable incident happened in London, England in 1952 when thick, deadly pollution known as the Great Smog, caused by people burning coal in home fires and coal-fired power plants, killed an estimated 4000 people. Other times, it's much more difficult to make the link. Some estimates suggest perhaps 10–20 percent of cancers are caused by air pollution of one kind or another, but cancers can take a long time to develop and many other things can cause them too. Proving a direct link with a particular kind of air pollution (say, a garbage incinerator in your community or a neighbor who persistently burns plastic on garden bonfires) is very difficult.
"In 2016, 91% of the world population was living in places where the WHO air quality guidelines levels were not met."
World Health Organization, 2018
According to the World Health Organization (WHO), air pollution is one of the world's biggest killers: it causes around four million people to die prematurely each year. Many of these deaths happen in developing countries (over half a million in India alone), but wealthier industrial nations suffer too: in the United States, for example, around 41,000 people a year are estimated to die early because of air pollution. Imagine how much media coverage there would be if several million people (that's roughly the population of Houston, Texas or the West Midlands conurbation in England) were killed in a terrorist incident or an earthquake. Because air pollution kills quietly and relentlessly, and its finger is hard to detect on the trigger, people barely seem to notice—or care.
Deaths aren't the only human consequence of air pollution. For every person who dies, hundreds or thousands more suffer breathing problems such as asthma and bronchitis. Workers exposed to high levels of dust sometimes suffer years of misery before dying from illnesses such as silicosis.
Agricultural effects
Farming is as much of an art as a science; crops can thrive—or fail—for all sorts of reasons. One of the things that characterized the 20th century was the huge growth inindustrial agriculture—using fertilizers, pesticides, and so on to increase crop yields and feed the world's ever-growing population. These aren't the only chemicals that crops are exposed to, however. We know that air pollution (in common with water pollution) can seriously affect the growth of plants. At one end of the spectrum, it's easy to find chemical residues (everything from toxic heavy metals such as lead to cocktails of brake fluids and other chemicals) in plants that grow alongside highways. At the opposite extreme, the huge increase in atmospheric carbon dioxide now causing global warming and climate change is expected to have a major impact on the world's agriculture (reducing crop yields in some places but potentially increasing yields elsewhere).

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