
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. [4] 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 Photo: Flying molecules—if you could see air pollution close up, this is what it would look like. Image 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. It's important to note that not all the things we think of as pollution are gases: some are aerosols (liquids or solids dispersed through gases). In practice, about ten different substances cause most concern: 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 (SO 2 ) 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. [5] Large amounts of sulfur dioxide are also produced by ships, which use dirtier diesel fuel than cars and trucks. [6] 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.) [7] 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. [8] Nitrogen oxides : Nitrogen dioxide (NO 2 ) 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). [9] 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 play a role in the formation of ozone and smog. VOCs are also released by tobacco smoke and wildfires. [10] Particulates : There are many different kinds of particulates, from black soot in diesel exhaust to dust and organic matter from the desert. Airborne liquid droplets from farm pollution also count as particulates. Particulates of different sizes are often referred to by the letters PM followed by a number, so PM 10 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. PM 2.5 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. [11] 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 O 3 ), instead of just the two atoms in conventional oxygen (O 2 ). 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). [12] 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. [13] 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. 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. [14]

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 and power plants, driving the Industrial Revolution. Today, better technology, 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. But this type of "old-school" pollution remains a major problem in rapidly industrializing nations such as China and India. Where, then, does modern air pollution come from? In developed countries such as the United States and the UK, 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 these 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) and there will be two billion by 2040. Despite the fast growing interest in electric cars, virtually all of the world's vehicles are currently 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 (they contain extra chemicals called "additives") 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), 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. [15]

Smog—a closer look Photo: Brown smog lingers over Denver, Colorado. Photo by Warren Gretz courtesy of US Department of 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. Chart: Most of the world's major cities routinely exceed World Health Organization (WHO) air pollution guidelines. This chart compares annual mean PM2.5 levels in 12 representative cities around the world with the WHO guideline value of 10μg per cubic meter (dotted line). PM2.5 particulates are those smaller than 2.5 microns and believed to be most closely linked with adverse health effects. Chart drawn using data from Ambient (outdoor) air pollution in cities database 2016 courtesy of World Health Organization, which was the newest available data at the time this article was last checked and updated (March 2020).

Indoor air pollution Photo: Air freshener—or air polluter? If you live in a city, you might think your home is the cleanest place you can be—but you're probably wrong. Outside, though the air may seem polluted, it's constantly moving and (in theory at least) pollutants are continually being diluted and dispersed. Inside, your home is packed with all kinds of chemicals that generate pollution every time you use them. And, unless you open the windows regularly, those pollutants aren't going anywhere fast: Detergents and household cleaners, aerosol sprays, shoe polish, hair wax, paints, and glues are just a few of the everyday chemicals that can release air pollution into your home.

If you have a gas or oil-fired boiler or a coal- or wood- fired stove and it's not properly ventilated, it will generate dangerous and toxic (but colorless and odorless) carbon monoxide gas.

Surprisingly, even the water that pipes into our homes can be a source of air pollution. Every time you heat water (on a stove, in a kettle, in a shower, or even when you're steam ironing clothes), you can evaporate VOC chemicals trapped inside and release them into the air.

pollution. Every time you heat water (on a stove, in a kettle, in a shower, or even when you're steam ironing clothes), you can evaporate VOC chemicals trapped inside and release them into the air. Even your shiny new shower curtain could be releasing VOCs if it's made from a type of plastic called PVC.

Maybe your building has air conditioning? Chances are, the air it blows through has already circulated through other rooms in the same building or even other people's offices or apartments.

Perhaps your building is located somewhere near a source of natural radioactivity so radon gas is slowly accumulating inside? None of these things are meant to scare you—and nor should they. Just remember that there's pollution inside your home as well as outside and keep the building well ventilated. (If you're worried about wasting energy by opening windows on cold days, there are systems that can let air into a building without letting the heat escape, known as heat-recovery ventilation.) Further reading Indoor air quality: Comprehensive information from the US Environmental Protection Agency (EPA).

Household air poillution and health from the World Health Organization (WHO).

Neighborhood air pollution How clean your air is depends on where you live: air is generally far cleaner in rural than in urban areas, for example, where factories, chemical plants, and power plants are more likely to be located and traffic levels are much higher. Exactly how clean your neighborhood is can also depend critically on the weather, especially if you live somewhere prone to temperature inversions and smog. Neighborhood air pollution problems are often best tackled through local community campaigns. Regional air pollution Tall smokestacks designed to disperse pollution don't always have that effect. If the wind generally blows in the same direction, the pollution can be systematically deposited on another city, region, or country downwind. Sometimes air pollution is carried back down to Earth as contaminated rain or snow, which dissolves in watercourses or oceans causing what's known as atmospheric deposition. In other words, the air pollution becomes water pollution. According to the US Environmental Protection Agency (EPA): "The air is a major source of chemicals affecting the health of the Great Lakes. Certain persistent air toxics may contribute not only to atmospheric pollution but to water pollution. These toxics can bioaccumulate in the food web [and] may endanger the environment, affecting the health of humans and wildlife." Acid rain (see box below) is the best known example of atmospheric deposition. It's often said that pollution knows no boundaries—and that's particularly true of air pollution, which can easily blow from one country or continent where it's produced and cause a problem for someone else. Air pollution that travels like this, from country to country, is called transboundary pollution; acid rain is also an example of this and so is radioactive fallout (the contaminated dust that falls to Earth after a nuclear explosion). When the Chernobyl nuclear power plant exploded in the Ukraine in 1986, wind dispersed the air pollution it produced relatively quickly—but only by blowing a cloud of toxic radioactive gas over much of Europe and causing long-lasting problems in a number of other countries (70 percent of the fallout landed on neighboring Belarus).

Acid rain—a closer look When rain falls through polluted air, it can pick up some of the pollution and turn more acidic—producing what's known as acid rain. Simply speaking, the air pollution converts the rain into a weak acid. Photo: Acid rain can turn lakes so acidic that fish no longer survive. Picture courtesy of U.S. Fish and Wildlife Service Division of Public Affairs. Why does that matter? Pure water is neither acidic nor alkaline but completely neutral (we say it has an acidity level or pH of 7.0). Ordinary rainwater is a little bit more acidic than this with about the same acidity as bananas (roughly pH 5.5), but if rain falls through sulfur dioxide pollution it can turn much more acidic (with a pH of 4.5 or lower, which is the same acidity as orange or lemon juice). When acid rain accumulates in lakes or rivers, it gradually turns the entire water more acidic. That's a real problem because fish thrive only in water that is neutral or slightly acidic (typically with a pH of 6.5–7.0). Once the acidity drops below about pH 6.0, fish soon start to die—and if the pH drops to about 4.0 or less, all the fish will be killed. Acid rain has caused major problems in lakes throughout North America and Europe. It also causes the death of forests, reduces the fertility of soil, and damages buildings by eating away stonework (the marble on the US Capitol in Washington, DC has been eroded by acid-rain, for example). One of the biggest difficulties in tackling acid rain is that it can happen over very long distances. In one notable case, sulfur dioxide air pollution produced by power plants in the UK was blamed for causing acid rain that fell on Scandinavian countries such as Norway, producing widespread damage to forests and the deaths of thousands of fish in acidified lakes. The British government refused to acknowledge the problem and that was partly why the UK became known as the "dirty man of Europe" in the 1980s and 1990s. [18] Acid rain was a particular problem in the last 30–40 years of the 20th century. Thanks to the decline in coal-fired power plants, and the sulfur dioxide they spewed out, it's less of a problem for western countries today. But it's still a big issue in places like India, where coal remains a major source of energy.

What can you do to help reduce air pollution? Photo: Buying organic food reduces the use of sprayed pesticides and other chemicals, so it helps to reduce air (as well as water) pollution. So now you know the problems, but what's the solution? Here are ten simple things you can do that will make a difference (however small) to the problem of air pollution. Save energy : Making electricity in conventional power plants generates pollution, so anything you can do to save energy will help to reduce pollution (and global warming as well). Switch to low-energy lamps, use a laptop computer instead of a desktop, dry your clothes outdoors, and heat insulate your home. Sounds too worthy? Just remember this: every bit of energy you save also saves you money you can spend on something better! If you're not sure how you're wasting energy, use an electricity monitor to help identify your most inefficient appliances. Save water when you can : Producing cool, clean water needs huge amounts of energy so cutting water waste is another good way to save energy and pollution. Cut the car : Sometimes we have to use cars, but often we can get a bus or a train or (for shorter distances) walk or cycle. Road vehicles are now the biggest source of air pollution in most urban areas, so traveling some other way through a town or city helps to keep the air clean. When you have to use your car, drive efficiently to save fuel and money, and cut pollution. It's particularly important to avoid car use when smog is bad in your city. Cut out garden bonfires : Did you know that a garden bonfire can contain up to 350 times as much of the cancer-causing chemical benzo[a]pyrene as cigarette smoke? Well you do now! Having a bonfire is one of the most selfish things you can do in your local neighborhood. Compost your garden refuse, bury it, or take it to a municipal waste dump. Never burn household waste : If you burn plastic, you release horrible toxic chemicals into the local environment, some of which will be sucked up your own nose! Recycle your trash instead. Garden organically : Would you spray pesticides on your dinner? So why spray them on your garden? You can tackle virtually all garden pests and diseases in more environmentally friendly organic ways. Buying organic food is a good option if you can't grow your own. Cut the chemicals : Do you really need to spray an air freshener to make your home feel nice? Yes, you fill your room with perfume, but you're also choking it with chemical pollution. Why not just open a window instead? How many of the chemicals you buy do you really need to use? Why not try cleaning with microfiber cloths instead of using detergents? Use water-based paints and glues : Avoid the nasty solvents in paints, varnishes, and wood preservatives. Remember that if anyone's going to suffer from the air pollution they create, you're first in line. Get in the habit of reading the labels on the DIY products you buy. If you live in Europe, you'll find a little badge on them somewhere indicating what sort of VOC emissions they make. Reduce, reuse, and recycle : Buying new stuff is fun, but reusing old things can be just as good. Don't smoke : Cigarettes contain an addictive chemical called nicotine that makes you want to go on smoking them. The smoke itself contains about 7000 other chemicals and at least 70 of them cause cancer. Cigarettes also cause very localized air pollution for other people but, if you're the smoker, sucking on the toxic tube, you're first in line for their ill effects.

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