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Major Air Pollutants

Discussion and list of the 6 major air pollutants (a.k.a. criteria air pollutants) and sources.

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Criteria Air Pollutants — Six Major Air Pollutants

We usually don't think much about major air pollutants and how they affect our breathing and overall health—unless maybe we're trapped behind a truck belching black smoke or we're attending the annual Bean & Vinegar Lovers conference.

Many people in the US also have the sense that air pollution has gotten better since the Clean Air Act was passed in the 1970s, picture of deer standing on hill; city with bad haze is in background and it is true in general that air quality continues to improve overall. But not nearly fast enough—some major urban areas are still smogged-out, and nearly half of US residents live in counties that have unhealthy levels of major air pollutants. So, the battle for clean air continues.

Not all air pollutants are created equal. The importance of any given air pollutant to our overall air quality depends on the severity of the pollutant's health and environmental effects as well as how much of it we're pumping into the air and what happens to it once it's out there. The original US Clean Air Act identified a "Dirty Half-Dozen"—six of the most common, most important air pollutants:

  • Ground-Level Ozone (smog)
  • Particulate Matter
  • Sulfur Dioxide
  • Nitrogen Oxides
  • Lead
  • Carbon Monoxide

These six pollutants became known as "criteria air pollutants" because the act requires EPA to regulate them by developing human health-based and/or environmentally-based criteria for setting permissible emission levels. That may sound a bit dry and unexciting at first, but the characteristics, sources, and effects of these major air pollutants are actually rather interesting, and they can affect your life in a variety of unexpected ways—none of them good.

After we've dished the dirt, we'll offer a few ways you personally can help clear the air of these major air pollutants. Breathe on....

Major Air Pollutants -- #1


Ozone high up in the atmosphere is essential to life on the planet—it keeps the sun's high-energy radiation from frying everything. But down here at surface level, ozone is a bad thing—it's the primary constituent of smog.

Ground-level ozone is created when oxides of nitrogen (NOx) react with volatile organic compounds (VOCs) in the presence of sunlight. Typical sources of these ozone precursors are:

  • motor vehicle exhaust and gasoline vapors;
  • power plants and industrial emissions;
  • chemical solvents and processes; and
  • natural sources.

Sunlight and hot weather are catalysts in the NOx/VOC reactions that cause ground-level ozone to form; hence, smog is typically a summertime problem. Areas with the highest concentrations of motor vehicles and industrial emissions tend to have the worst ground-level ozone problems. Sometimes unique geographical features can cause smog to get trapped in our breathing zones—such as happens in Los Angeles—which makes the smog situation worse than it would be based solely on the levels of air pollutants.


Is your city/urban area among the worst in the nation for ozone pollution? Here's the Top 10 Worst list from the American Lung Association.

1. Los Angeles/Long Beach/Riverside, CA
2. Bakersfield, CA
3. Visalia/Porterville, CA
4. Fresno/Madera, CA
5. Houston/Baytown/Huntsville, TX
6. Merced, CA
7. Dallas/Fort Worth, TX
8. Sacramento/Arden/Arcade/Truckee, CA/NV
9. Baton Rouge/Pierre Part, LA
10. New York/Newark/Bridgeport, NY/NJ/CT

If you live in some other city, don't start breathing easy yet—get the full list of cities with the dirtiest (and cleanest) air.

In humans, breathing ozone can trigger or exacerbate a variety of respiratory problems, including bronchitis, emphysema, asthma, chest pain, coughing, throat and lung irritation, congestion, and general reduction of lung function. Ground-level ozone can have similar effects on animals, and it also damages natural vegetation and crops.

Major Air Pollutants -- #2


Particulate matter—"PM" for short—is an air pollutant category that comprises very, very small solid particles and liquid droplets. These include soot, smoke, nitrates, sulfates, metals, dust particles, and organic chemicals.

Particles that are in the size range of 2.5 to 10 micrometers are sometimes called "inhalable coarse particles" but are more commonly referred to as PM-10. Direct-emission sources of PM-10 include fires and smokestacks, as well as dust kicked up from unpaved roads, construction sites, farm fields, and other areas of exposed earth.

Particles that are less than 2.5 micrometers in diameter are called "fine particles" and are collectively referred to as PM-2.5. These particles can be emitted directly from diesel exhaust or forest fires or can form indirectly when gases emitted from power plants, industrial operations, and automobiles react with air. Sulfur dioxide and nitrogen oxides emitted from these latter sources form sulfates and nitrates and are the cause of most of the fine-particle pollution in the US.

graphic map of U S

The State of the Air

site has an easy way for you to see how your area rates on air pollutants. Look for the yellow box that says "Report Card" and put in your zip code.

Particulate matter is breathed in and finds its way deep into the lungs. PM-2.5 particles can even enter the bloodstream. Typically, the smaller the size of the particles, the greater their potential for causing health problems. Particle pollution exposure is linked to a variety of health problems, including:

  • transitory respiratory symptoms such as irritation of the airways, coughing, or difficulty breathing;
  • serious respiratory symptoms such as decreased lung function, aggravation of asthma, and development of chronic bronchitis;
  • increased cardiovascular risk, especially for people with diabetes;
  • higher general risk of heart attacks, strokes, hypertension and angina pectoris;
  • premature death in people with heart or lung disease.

Environmentally, particulate matter pollution can ...

  • acidify and change the nutrient balance in lakes, streams, rivers, watersheds, and estuaries;
  • deplete soil nutrients, alter soil acidity, and degrade or suppress soil biological activity;
  • damage farm crops, forests, and ecosystems;
  • cause reduced visibility (haze);
  • stain or damage buildings and monuments.

Major Air Pollutants -- #3


Some air-pollution impacts of sulfur dioxide (SO2) were already covered in the particulate matter section, but there is more to talk about with SO2, one of our worst air pollutants.

Sulfur dioxide is a gas. It can be produced by volcanoes, but the greater cause for concern is the SO2 that comes from manmade sources.

Sulfur is prevalent in raw fossil fuels, notably coal and crude oil. SOx gases form when the coal and heavy oil are burned or when gasoline is refined from crude oil. Sulfur also contaminates ores used to make common metals like aluminum, copper, zinc, lead, and iron. SOx gases form when the metals are extracted from ore.


In the US, much progress has been made on cleaning up sulfur dioxide pollution from power plants and industrial sources using a cap-and-trade system for emissions. But many old, dirty coal-fired power plants still remain online, spewing SO2, mercury, and other pollutants into the air. The US EPA notes that over 65% of SO2 released into the air today—more than 13 million tons per year—still comes from electric utilities, especially those that burn coal. Plans are in place to further restrict such emissions—in a decade or so. Gah!

Historically, diesel fuel has also been a big source of sulfur and soot pollution. Europe was the first to embrace better refining techniques for cleaner, low-sulfur diesel. It took the US until 2006 to begin doing the same. The levels of sulfur in the new clean diesel are 97% lower than in the old fuels.

Plans for improving engines and cleaning up the diesel fuels used in locomotives, ships, and boats are in the works, but trains, ships, and pleasure craft continue to be a major source of sulfurous air pollution.

SO2 readily dissolves in water vapor and leads to the formation of airborne sulfuric acid and other acidic sulfur compounds; and it interacts with other gases and particles in the air to form sulfates and other polluting compounds. These and other forms of SO2 pollution harm people, plants, animals, and ecosystems.

Because SO2 pollution is usually released simultaneously with particulate matter and other related air pollutants, direct association of just SO2 with disease has been tricky to document. Generally, though, sulfur dioxide pollution is thought to promote wheezing, bronchial constriction, shortness of breath, and exacerbation of asthma and heart disease, and even premature death.

Environmentally, SO2 contributes to acid rain (acid deposition)—see sidebar—and, as mentioned under the Particulate Matter section, sulfate particles cause visibility-reducing haze.

Major Air Pollutants -- #4


Nitrogen oxides, nicknamed "NOx," are highly reactive gases that contain nitrogen and oxygen in varying molecular combinations. Nitrogen oxides form when fuel is burned at high temperatures, such as in an automobile engine, coal-fired power plant, or a process that burns fuel. Nitrogen oxide emissions from transportation sources account for slightly more than half of all NOx emissions in the US.


Both sulfur dioxide and nitrogen oxides contribute to acid rain. Just a few decades ago, SO2 and NOx pollution had gotten so bad that acid rain had damaged countless buildings, monuments, and car finishes; had "killed" some areas of forest; and had made some freshwater bodies too acidic to support normal aquatic life. Things have gotten better since then, at least in terms of the amount of SO2 being emitted into the atmosphere. Progress on NOx pollution has been much slower.

In spite of the general progress on emissions that cause acid deposition, scientists have discovered that acid-rain wounds to the environment heal very slowly—on the order of years or decades for a reasonable level of health to return, and much longer (possibly centuries) for full biological recovery. A majority of previously acidified bodies of water are still damaged, and while the remaining acidified waters have improved, they are still hypersensitive to further acid deposition.

We still have quite a ways to go on fixing the acid rain problem.

Many of the nitrogen oxides are colorless, but nitrogen dioxide (NO2) combined with particles in the air can cause a reddish-brown haze. Like SOx pollution, NOx and the pollutants formed from it can be transported by wind over long distances. Thus, these types of pollution are not confined to areas where they are being emitted, and controlling them is best done using regional and national plans.

A number of NOx-related problems were mentioned in previous sections:

  • ground-level ozone (causing respiratory effects);
  • nitrate particles and acid aerosols (causing respiratory, coronary, and environmental problems);
  • acid rain (see sidebar);
  • haze and visibility impairment.

A couple of other important effects are worth mentioning:

Eutrophication of Waterways — Excess nutrients cause coastal dead zones. Even though most of the nutrient pollution comes from fertilizers and animal wastes that run off of farm fields and suburban lawns, deposition of nitrogen from fossil-fuel-based air pollution adds significantly to the problem. It's estimated that 1/3 of the excess nitrogen in the Chesapeake Bay originates from NOx pollution that is precipitated out of the air and carried to the bay by rain, runoff, and watershed drainage.

Global Warming — One member of the NOx family, nitrous oxide (N2O), is a potent greenhouse gas. Because of nitrous oxide's persistence in the atmosphere, it has 200 times more global warming impact than the same mass of carbon dioxide molecules. There is much more CO2 being pumped into the atmosphere than any other greenhouse gas, which is why CO2 gets the most press. But N2O comes in at #3 in terms of total impact on global climate change (with methane being #2), and control of nitrous oxide is a significant part of the effort to fight global warming.

Major Air Pollutants -- #5


Lead is a metal and a primary element—"Pb" on the periodic table. It serves a number of essential purposes; for instance, lead-acid batteries to start our vehicles and the lead in our computer monitors that shields us from the CRTs' harmful radiation.

For many years, lead was used in gasoline as a performance enhancer. But that meant many tons of lead pollution was being spewed into the air, and once the serious effects of lead contamination became apparent, the world began phasing out leaded gasoline. In the US, the use of leaded gasoline in highway vehicles has been completely banned since 1995. In the two decades since the phase-in of the ban began, airborne lead levels have dropped more than 90%.

Unfortunately, there are still some sources of airborne lead pollution in the US—primarily industrial activities like lead smelting, metals processing, and lead-acid battery manufacturing, plus some contribution from combustion of airplane fuel, waste incineration, and power generation. Worldwide, airborne lead comes from similar sources, and in some developing countries, leaded gasoline is still used.

Lead is tough stuff, healthwise. Inhaling or ingesting lead can ...

  • damage the kidneys, liver, and other organs;
  • damage the brain and peripheral nerves, and cause seizures, retardation, behavioral disorders, and memory problems;
  • cause high blood pressure, heart disease, and anemia;
  • lead to osteoporosis;
  • cause reproductive disorders.

In the "nature vs. nurture" debate, score another one for the side that thinks we're mostly products of our environments. A study by researcher Rick Nevin found a strong statistical correlation between violent crime in the United States and exposure to lead early in life. A similar correlation was seen internationally.

Environmentally, similar effects are observed in animals. Lead pollution can even impair vegetation.

For additional information on lead, see our two-part article, Lead Facts/Lead Poisoning.

Major Air Pollutants -- #6


Carbon monoxide—"CO" to chemists—is a colorless, odorless gas. Most CO air pollution is formed when fossil fuels are burned incompletely. Cars and trucks, boats, off-road vehicles, generators, and other off-road engines are responsible for a whopping 78% of carbon monoxide emissions in the US. Other sources include metals processing and chemical manufacturing; forest fires and wood burning for heat; and natural gas combustion.

Areas with high levels of CO pollution usually have heavy traffic and congestion. While vehicular pollution controls have reduced the amount of carbon monoxide produced per vehicle mile, today there are many more vehicles on the road compared to a few decades ago, each driving more miles than before. This large increase in total annual vehicle miles has offset a large portion of the technological progress on reducing CO pollution.

Carbon monoxide is poisonous to us oxygen breathers, at least at high enough levels. It can cause harmful health effects by reducing oxygen delivery to the body's tissues and organs (most notably the heart and brain). That's why people with natural-gas heat or appliances are supposed to have carbon monoxide detectors in their homes.

Here are a few other CO-related health problems:

  • Chronic CO exposure may cause cardiovascular problems in healthy people.
  • Individuals who already suffer from heart problems may experience chest pain and reduced physical function upon even brief exposure to CO.
  • Breathing high levels of CO can cause vision problems and physical or mental impairment.

Major Air Pollutants -- Solutions


Government regulations are an important way society works to make the air cleaner. But individuals can act on their own, too, to help reduce their contributions to the six major air pollutants. Here are some ideas.

Curtail the Tailpipe — Vehicle emissions are a major source of multiple criteria air pollutants, including nitrogen oxides and carbon monoxide.

Clean-air action ideas:

  • Keep your vehicle well maintained. A car that gets regular oil changes, tune-ups, tire-pressure checks, and overall mechanical inspections emits fewer air pollutants.
  • Combine trips. Maintain a to-do list, so when it comes time for your Grand Day Out, you can get it all done with the least possible miles driven. This will also save you time!
  • Walk or bike when the distance is short. It's great exercise too!
  • Carpool, vanpool, or take mass transit to work. It's less convenient, yes, but you can save money and free up a little time for relaxing, reading the morning paper, or just zoning out.
  • Support community plans for sidewalks, bike trails, mass transit systems, and elimination of car-centric tax breaks like free job-related parking.

Skip the Weekend at Burnies — The burning of firewood and trash is one cause of particulate pollution. In many parts of the country, it's a primary source of such pollution.

Clean-air action ideas:

  • If you must burn wood to keep warm, use a high-efficiency catalytic wood stove or fireplace insert. Even better, convert from wood to natural gas, which emits far fewer pollutants.
  • Don't burn trash, especially plastic. Compost and recycle as much as possible and dispose of other waste using a trash service or the local dump.
  • Support efforts in your community to ban outdoor burning of household trash, yard waste, and construction trash. (Whhaaat? You burn your leaves every fall? Egad! You're burning gardening gold! Check out You Bet Your Garden's page on fall leaves.

Squeeze Down Your Juice Use — Electric power plants—especially coal-fired power plants, which account for about half of all the electricity generated in the US—are still a major source of air pollution, including sulfur dioxide and particle pollution. Using less electricity means less air pollution from power plants.

Clean-air action ideas:

  • Buy energy-efficient appliances and gadgets.  (Related article: Vampire Power)
  • Change all of your light bulbs to compact fluorescents.
  • Consider installing a geothermal heat pump—they're four times as efficient as a normal heat pump!
  • Improve your home's insulation; install high-efficiency windows; seal cracks around windows and doors. This will reduce the energy you have to use to heat and cool your home.
  • Implement a low-tech system of "passive solar" heating and cooling by installing double-celled window shades. During the winter, keep the shades on the east, south, and west sides up when the sun is shining to collect the free solar heat. In the summer, keep those same windows closed and their shades down to block the sun and keep the cool air in the house.
  • Let your lawmakers know you support tax incentives and regulations that promote energy-efficient appliances, building designs, and heating/cooling systems.

Don't Skirt the Smog Alert — When smog alerts come out in your area, officials' requests to avoid filling up your gas tanks, making unnecessary trips, or cutting the lawn on those days may seem silly. But these activities all add more ozone precursors to the air, so deferring the tasks to a day when weather conditions are less favorable to smog formation helps keep the quality of the air you breathe from getting into the danger zone.

Clean-air action ideas:

  • Do your best to reschedule your tasks. Come on—aren't you looking for an excuse to just watch TV for a while?

Major Air Pollutants


Let's all apply Pink Floyd's Dark Side of the Moon advice to air-pollution actions:

Breathe... breathe in the air;
Don't be afraid to care ...

This is part one of a two-part series. The next part will cover the remaining categories of air pollutants and is scheduled for publication later in 2007. GP offers free email reminders.

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  1. The Six Major Air Pollutants

  2. Overview of All Air Pollutants

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