Everyone likes clean air, but not everyone agrees on how hard the federal government should push for - or enforce - regulations to keep it clean. Environmentalists, it goes without saying, lean toward strict rules and strict enforcement. Those who argue for laxer rules say a lot can be achieved by giving industry leaders flexibility to clean up their act and incentives to find new innovations. We don't go into the pros and cons of clear air regs here – see our issue guide on the Clear Skies Act – we instead offer a picture of the current state of air pollution, how far we've come in cleaning it up, and where some say we need to go.

Air pollution comes in many varieties: mercury emissions, acid rain, emissions that damage the ozone, and plain old smog. We thought we'd make your – and yes, our - life easier by starting off with the six pollutants the EPA puts at the top of their list.

After the Six Principal Pollutants, we offer more background info on some of the problems they've been known to cause - including smog, acid rain - as well as the basics on the Ozone Layer.

There isn't much dispute among environmentalists that the six principal pollutants are the baddies – but those worried about global warming would also add carbon dioxide to the list of emissions to be controlled. Mercury is another air pollutant that's the focus of frequent debate, but since it does its harm to the water, we list it under that fact page.

The EPA's Six Principal Pollutants

a glance at where they're from, what they do and how much they've been cleaned up ( EPA)

Cleaning the air – the six pollutants since 1970

Where they come from and why environmentalists don't like them

  • Nitrogen dioxide (NO2) comes from cars, electric utilities, and residential sources EPA . It's a main ingredient in ground-level ozone (see below) EPA

  • Sulfur dioxide (SO2) mostly comes from burning coal. It can add to lung and heart problems and is also a primary part of acid rain.

  • Ozone (O3) comes in both a natural “good” form – up in the ozone layer - and manmade “pollutant” form when oxides of nitrogen and volatile organic compounds (VOC) react closer to earth. The bad kind, also known as smog, is responsible for a variety of health problems including lung damage EPA .

  • Particulate matter (PM): particles found in the air from cars, factories, construction sites, etc EPA . They cause respiratory problems such as asthma and bronchitis. PM2.5 are any particles less than 2.5 micrometers wide. EPA (The EPA also tracks larger particles - PM10 – but they are not included in the six pollutants.)

  • Carbon monoxide (CO): colorless, odorless gas that mostly comes from cars EPA . It causes vision problems, decreased dexterity, slower learning and – if you have heart disease - respiratory problems.

  • Lead (Pb): metal found naturally but also emitted from cars and the processing of metals EPA . It causes damage to the kidneys, liver, brain and nerves, and can result in seizures, mental retardation, high blood pressure, and other ailments EPA . Note: because eliminating lead in the air has pretty much been a success story, we don't say more about it below.

Good Ozone – the Ozone Layer:


What is it: The ozone we like occurs naturally in the Earth's upper atmosphere (10 to 30 miles above the Earth's surface) and forms a protective layer that shields us from the sun's hurtful ultraviolet rays (which can cause cancer and other health problems) EPA

What's eatin' it: Chemicals that contain chlorine compounds (ex: Chlorofluorocarbons (CFCs)) and bromine compounds (ex: halons) break down ozone in the ozone layer.

The “ozone hole:” Some parts of the ozone layer are more heavily hit than others. The infamous “ozone hole” usually refers to area over Antarctica where ozone levels can drop to 70% lower than normal during the Spring. Parts of North America can have less leaky holes – with 5-10% ozone depletion. EPA

  • On average, global ozone has decreased by about 3% from 1997-2001. In recent years, the rate of ozone depletion has gone down EPA .

  • (For a detailed look at ozone depletion, the Montreal Protocol, and rates of skin cancer, visit the EPA's guide to ozone depletion )

Bad Ozone - AKA smog:


What is it?: a harmful air pollutant formed at ground level.

Where it comes from: NOx and VOCs are emitted by cars, power plants, industrial boilers, refineries, chemical plants.

  • 55% of NO2 emissions come from cars EPA

  • NOx and VOC emissions are decreasing in some regions, while staying constant in other areas (see the EPA's explanation of regions and concentrations in their ozone report )

Health Effects: Irritates airways in the lungs and causes inflammation (like a sunburn), resulting in permanent lung damage similar to UV rays on our skin. Causes aggravated asthma, reduced lung capacity, and increased vulnerability to respiratory illnesses like pneumonia and bronchitis. EPA

Particulate matter (PM)


What is it? : Microscopic particles (one-thirtieth the width of a human hair) found in the air, including dust, dirt, soot, smoke, and liquid droplets. Major components of PM are carbon, sulfate and nitrate compounds, as well as materials such as soil and ash.

Where it comes from:

  • Some PM is directly emitted into the air. It come from: Cars, trucks, buses, factories, construction sites, tilled fields, unpaved roads, stone crushing, and burning of wood.
  • Other PM is formed in the air from the chemical reactions of NOx and SOx compounds. It come from: Fuel combustion in motor vehicles, power plants, and other industrial processes.

How many of us are breathing it? : 62 million people in 97 U.S. counties lived with particle pollution levels higher than the PM2.5 air quality standards, the PM10 standards, or both (2003).

Health Effects of PM exposure:

  • Cardiac arrhythmias, heart attacks, respiratory problems, asthma attacks, and bronchitis. Premature death from heart or lung disease can also arise.

  • According to the EPA, PM accounts for 5,000 premature deaths each year in nine cities: Boston, Detroit, Los Angeles, Philadelphia, Phoenix, Pittsburgh, St. Louis, San Jose and Seattle .

  • Number of Americans who live in areas with dangerous levels of small particle (dust/soot) pollution: 1 out of 3 ( EPA , according to WP )

Acid Rain:

What it is? : “Acid rain" is a broad term used to describe ways acids fall out of the atmosphere, including rain, fog, and snow. Pure, neutral water has a pH value of 7 (on a scale from 1 to 14). Most acidic rain in the U.S. has a pH value of 4.3 EPA

Where it comes from: The primary causes of acid rain are sulfur dioxide (SO2) and nitrogen oxides (NOx), which come mostly from electric utilities and fuel combustion EPA

Harms of Acid Rain:

  • Fish can't hatch in acidic water, so species are threatened and biodiversity is decreased. EPA

  • Causes slower growth, injury, and death of forests, and degrades soil. EPA

  • Speeds up the decay of materials and paints, including buildings, statues, etc. EPA

  • What it doesn't harm: us. Acid rain looks, feels, and tastes just like clean rain. For humans, walking in acid rain or swimming in an acid rain lake is no more harmful than exposure to clean water. EPA

Note: not all acidity comes from acid rain. Acid rain causes acidity in 75 percent of the lakes and 50 percent of the streams that the EPA found to be acidic. EPA

Carbon Monoxide (CO)


What is it? : Colorless and odorless gas.

Where it comes from:

  • Nationwide: 60% comes from motor vehicle exhaust.

  • In cities with condensed traffic: 95% comes from motor vehicle exhaust.

  • The rest comes from industrial processes such as processing metals and chemical manufacturing, residential wood burning, and natural sources such as forest fires. EPA

Higher emissions:

  • From 1992-2001, there's been 6% increase in total CO emissions

  • But when compared to increases in driving, things look better: There has been a 35% increase in vehicle miles traveled during that time

    • And concentrations are down:

      • 62% lower concentrations than 1982 (2001); lowest in last 20 years.

      • 38% decreased concentrations than 1992 (2001)

Heath Effects:

  • Reduces oxygen delivery to body's organs/tissues.

  • Can be poisonous in high levels of exposure

  • Causes visual impairment, reduced dexterity, reduced learning/task performance ability.

Facts pulled together with Dylan Selterman, Summer 2005.

Did we miss something, let some slant slip in, lose a link - or do you just have something to say? Drop a line below! In the spirit of open dialogue, cJ asks you keep it civil, keep it real and keep it focused on the message, not the messenger. See our policy page for more on what that all means.

Posted In


The 'NOTE' at the end of the section on acid rain contains a typo: Acis raing causes acidity

david skolnik (not verified) | December 31, 2007 - 8:43am
talker's picture

thanks david

Wish we could blame acid rain for lazy typing...

talker | January 2, 2008 - 10:41am