Air Pollution FAQ

Frequently Asked Questions about Air Pollution
What is Air Pollution?
The air we breathe is made up of a mixture of gases and small particles. Pollutants in the air are chemicals or substances that are harmful to humans, other species, or ecosystems as a whole. These such pollutants can come from human (anthropogenic) sources, or from natural sources such as volcanos or dust storms. For more information visit the Weather Underground's main air pollution page.
What are the major classifications of Air Pollutants?
There are two basic types of pollutants: gases and aerosols. Aerosols consist of either solid materials or liquid droplets such as sulfuric acid. Most air pollutant gases are invisible to the naked eye, with the exception of nitrogen dioxide, which has a brownish color. Even air that appears to be clean and clear contains a multitude of small solid particles. One cubic foot of air can contain millions of air pollution particles! Scientists have observed that cities can have hundreds to thousands of times more particles than rural regions.

What is ozone pollution?
Ozone forms in both the upper and the lower atmosphere. Ozone is helpful in the upper atmosphere, called the stratosphere, because it absorbs most of the harmful ultraviolet light coming from the sun. Ozone found in the lower atmosphere, called the troposphere, is harmful. Ozone found here is the prime ingredient for the formation of photochemical smog. This pollutant can irritate the eyes and throat, and damage crops. Visit the Weather Underground's ozone pollution page, or our ozone action page for more information.

What is particulate matter pollution?
Particulate matter (PM) pollution is composed of very small solid or liquid particles suspended in the air. Many man-made and natural sources emit PM directly or emit other pollutants that react in the atmosphere to form PM. PM is the most noticeable pollutant because it dramatically reduces visibility in urban areas. For more information, or to learn about the health effects from particulate matter, visit the Weather Underground's particulate matter pollution page.

What is carbon monoxide pollution?
Carbon Monoxide (CO) is also a major urban air pollutant. It is a colorless, odorless, poisonous gas that forms during the incomplete combustion of carbon-containing fuels. Scientists estimate that in the U.S. alone, over 60 metric tons of CO enter the air annually. The Weather Underground has compiled a page that deals specifically with the topic of carbon monoxide pollution, visit this page for more information.

What is sulfur dioxide pollution?
Sulfur Dioxide (SO2) pollution is produced when sulfur containing fuels are burned. High concentrations of SO2 can aggravate respiratory problems, such as asthma, bronchitis, and emphysema. In high quantities, SO2 can harm plants and cause rain to become acidic. Visit the Weather Underground's sulfur dioxide pollution page for more information.

What is nitrogen dioxide pollution?
Nitrogen Dioxide (NO2) pollution forms when nitrogen in the air reacts with oxygen during the high temperature combustion of fuel. High concentrations of this pollutant can lead to heart and lung problems, as well as lowering a body's natural immune system. NO2 is a key component in producing photochemical smog and ozone pollution. The Weather Underground's nitrogen dioxide pollution page has all the information that you need to further your knowledge on this topic.

What is lead pollution?
Lead has long been known as a harmful environmental pollutant, and has been called the most harmful pollutant to small children. People are easily exposed to lead pollution through the ingestion of contaminated water, food, air, soil, deteriorating paint and dust. Lead pollution is formed and emitted during the processing of metals. The highest concentration of this pollutant can be found in the vicinity of nonferrous and ferrous smelters, battery manufacturers, and other sources of stationary lead emissions. According to the American Lung Association, exposure to lead pollution can cause neurological impairments such as seizures, mental retardation, and/or behavioral disorders. Even in low amounts, exposure to lead is can cause damage to the nervous systems of fetuses and young children. The National Ambient Air Quality Standard for lead is 1.5 micrograms per cubic meter of air, averaged over a three-month period. In 1998, the EPA monitored these 5 US counties to be in violation of this standard: Philadelphia Co, PA (1.64g/m3), Shelby Co, TN (2.02g/m3), King Co, WA (2.03g/m3), Madison Co, IL (2.59g/m3), and Jefferson Co, MO (11.54g/m3).

What cities in the world have the worst pollution?
Beijing, Shanghai, Tehran and Calcutta follow Mexico City on the list of cities whose air poses greatest risk to children. Respiratory disease is now the leading cause of death for children worldwide, according to http://www.overpopulation. org/catholic.html.

What are America's 25 most OZONE polluted cities?
The American Lung Association's State of the Air 2001 has listed the following 25 cities as the worst ozone polluted:
    1. Los Angeles-Riverside-Orange County, CA
    2. Bakersfield, CA
    3. Fresno, CA
    4. Visalia-Tulare-Porterville, CA
    5. Houston-Galveston-Brazoria, TX
    6. Atlanta, GA
    7. Washington-Baltimore, DC-MD-VA-WV
    8. Charlotte-Gastoria-Rock Hill, NC-SC
    9. Knoxville, TN
    10. Philadelphia-Wilmington-Atlantic City, PA-NJ-DE-MD
    10. Raleigh-Durham-Chapel Hill, NC
    12. Sacramento-Yolo, CA
    13. Merced, CA
    14. Dallas-Fort Worth, TX
    15. New York- Northern New Jersey-Long Island, CT-NJ-NY
    16. Nashville, TN
    17. Phoenix-Mesa, AZ
    17. San Diego, CA
    19. Pittsburgh, PA
    20. Lancaster, PA
    21. Redding, CA
    22. Memphis, TN-AR-MS
    23. Richmond-Petersburgh, VA
    24. Baton Rouge, LA
    25. Louisville, KY-IN
    25. Greensboro-Winston-Salem-High Point, NC
    25. Chattanooga, TN-GA
What are America's Environmentally Kid-Friendly Cities?
In depth analysis by U.S. House of Representatives, Rep. David Wu, city expert Bruce Adams of the Worldwatch Institute, and researcher Molly O'Meara-Sheehan have graded many major cities as "Environmentally - Kid Friendly". A "Bad Air" day is defined as when pollution from ozone reaches a level that violates federal standards.
City Name Grade Number of
Bad Air Days
San Francisco, CA A+ 0
Minneapolis, MN A+ 1
Seattle, WA A 3
San Jose, CA A 8
Miami, FL A 8
Denver, CO A- 9
Boston, MA A- 9
Chicago, IL A- 10
Tampa, FL A- 11
Detroit, MI B+ 17
Fort Worth, TX B+ 17
New York, NY B 21
Cleveland, OH B 21
St. Lois, MO B 24
Phoenix, AZ C+ 33
San Diego, CA C+ 35
Dallas, TX C+ 36
Philadelphia, PA C+ 37
Pittsburgh, PA C 39
Houston, TX C 40
Los Angeles, CA C- 46
Washington, DC C- 47
Baltimore, MD C- 51
Atlanta, GA C- 60
What are primary and secondary pollutants?
Primary pollutants enter the atmosphere directly, such as from smoke stacks or vehicle exhausts. On the other hand, secondary pollutants are formed in the atmosphere when a chemical reaction takes place between a primary air pollutant and some other component of the air. For example, ozone is a secondary pollutant.

What is indoor air pollution?
The air we breathe inside our homes can be 5 to 100 times more polluted than the air outside! The EPA identifies sources of indoor air pollution as second-hand cigarette smoke, building materials, pressed wood products, home cleaning products, pesticides, adhesives, personal care products and others. Without proper ventilation to outside air, you and your family may be at risk for the development of short term as well as long term health effects. Some short term effects are headaches, dizziness, shortness of breath, irritation of the eyes, nose, and throat. Most of the time these symptoms are easily treated by removing yourself from the source of pollution. Long term effects may show up years after the exposure or after several periods of exposure. Heart disease, cancer, and some respiratory diseases are a few of the possible health risks from exposure to indoor air pollution.

Sadly, more than one half of all air pollution deaths worldwide are due to indoor air pollution in developing countries. The American Lung Association estimates that ~2 million people per year die from this type of pollution. For more information visit the EPA's Indoor Air Pollution site.

How much pollution do airplanes emit?
Every airplane is equipped with several engines that are burning fuels and emitting exhaust fumes. Most of the exhaust is water vapor, which quickly turns to ice crystals in the upper atmosphere. This exhaust can be seen as contrails or line clouds formed behind the airplane as it flies overhead. Aircraft engines emit carbon particles, sulfur dioxides, and oxides of nitrogen. Carbon monoxide emitted from planes accounts for ~3% of the planet-wide total of this pollutant. The quantity of pollutants emitted changes each time the plane is flown, since aircraft performance changes with pressure and temperature, therefore measurements of pollution aren't easily made.
Are boats, gas-powered lawnmowever, and other small engines significant sources of air pollution?
Yes! The EPA classifies "non-road", "off-road", or "off-highway" engines as power equipment, recreation vehicles, farm and construction machinery, lawn and garden equipment, marine vessels, locomotives, aircraft, and many others. Until amendments were made to the Clean Air Act in 1990, these engines had no federal law regulating their emissions. The new amendments stated that the EPA now had to research the air quality degradation ensued when these different machines were operated. They found that non-road engines emitted large amounts of oxides of nitrogen, hydrocarbons, carbon monoxide, and particulate matter. The EPA also found that the particulate matter emitted from most of these engines was worse than that emitted from heavy-duty highway trucks! This led to more stringent regulations for these types of engines. Check out the EPA's Nonroad Engine website for a thorough explanation of these laws.

What is Acid Rain?
Acid rain is any precipitation, rain or snow, that is more acidic than "normal" rain or snow. Acidity of substances is measured on the pH scale. Normal levels for rain and snow have a pH level of 5.5. Anything less than this is considered acidic. In 2000, the lowest pH recorded in the United States was 4.3! When sulfur dioxide and nitrogen dioxide are released into the air from cars or factories, they cause the air to become more acidic. These gases combine with the water vapor to form weak acids that become part of the precipitation.
What is the Clean Air Act?
The Clean Air Act is a Federal law that sets air quality standards for emissions from area, stationary, and mobile sources. This law states that the U.S. Environmental Protection Agency is authorized to set National Ambient Air Quality Standards (NAAQS) which protect human health and the environment. The Act was was amended in 1977 and then again in 1990. Amendments were made in order to address problems such as acid rain, ground-level ozone, upper level ozone depletion, and air toxics. The Clean Air Act in full can be found on the EPA's website.

What are the U.S. Federal air pollution standards?
The Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards for six pollutants, called "criteria pollutants", considered harmful to public health and the environment. These pollutants include: carbon monoxide, nitrogen dioxide, particulate matter, ozone, sulfur dioxide, and lead. The EPA maintains a database of air pollution forecasts generated for over 150 locations for these six pollutants by the local or state agencies responsible. For a detailed table of these standards, visit the Weather Underground's main air pollution page.
How much pollution do cars emit?
According to the EPA, cars are getting cleaner and cleaner. New cars today are capable of emitting 90% less air pollution on a per-mile basis than the unregulated models of 1970, and every year they are getting "cleaner". The EPA has found that each year, the average car causes over 600 pounds of air pollution. Transportation sources (including highway and off-highway vehicles) now account for 77 percent of national total carbon monoxide emissions. Motor vehicle exhaust contributes about 60 percent of all carbon monoxide emissions nationwide. Approximately 3.8 grams of volatile organic compounds are emitted by every car every day, even when they are not driven.
Whate are the international standards?
The World Health Organization has compiled the following international guildelines for airborne pollutants.

World Health Organization Air Pollution Guidelines


Guideline Value[m g/m3]

Averaging Time

Carbon Monoxide

100 000

15 minutes

60 000

30 minutes

30 000

1 hour

10 000

8 hours



1 year

Nitrogen Dioxide


1 hour


1 year



8 hours

Sulphur Dioxide


10 minutes


24 hours


1 year

What was the London black smog event of 1952?
In December 1952, the people of London experienced a killer smog. Fog, black smoke and sulphur dioxide were trapped beneath a slow moving cold air mass for over a week. During this short period of time, over 4,000 people perished. Doctors attributed the deaths to heart and respiratory problems from over-exposure to pollutants. The pollutants were all from coal burning within the residential area. Since then, coal fires have been replaced by central heating and industry has moved away from urban areas. Air quality in London has never been worse than it was that fateful week.
What causes haze (visibility reduction)?
Haze is caused when sunlight encounters tiny pollution particles in the air. Some light is absorbed by particles while other light is scattered away before it reaches an observer. On days when there are more pollutants in that air, light is absorbed and scattered more. When this happens, visibility is reduced as well as the clarity and color of what we see. Some types of particles such as sulfates scatter more light, particularly during humid conditions. Haze is caused from a variety of natural sources such as wind-blown dust or soot from wild fires. Man-made sources include cars, electric utility plants, and factories.
Where can I go to learn more about pollution?
The Weather Underground has compiled a list of websites where you can go to learn more about air pollution. If you know of any that you think we should add, let us know!

Back to the Air Pollution page