Though we cannot always see them, the air around us is constantly filled with thousands of different chemical compounds. But don't worry. This is not the work of some sinister government agency or an arch-supervillian (like that dastard Polluter), it's actually quite natural. The air we breathe is primarily composed of 78% nitrogen, 20% oxygen, 1% argon, and 1% other. Some of these compounds, like oxygen, are necessary for life. Others are hazardous to our health. Complicated, huh? We call these hazardous little devils air pollutants or air toxins. But, just to make the whole issue even more confusing, there are other compounds that are good in some locations and quantities but bad in others. For example, plants "breathe" carbon dioxide (CO2). In fact, they need it to live. But too much CO2 is becoming a big problem for plants and humans alike, which is why we talk about CO2 as a form of pollution. Throughout this website, the following pollutants will reoccur. Each one has a distinct impact, and it is important to recognize the difference between them. Therefore when a pollutant is mentioned somewhere else in the site you will be able to click back to this page to refresh yourself on its properties. That way, everybody stays happy and well informed.

The following eight pollutants were targeted based on several factors. First, the Clean Air Act calls for the creation of National Ambient Air Quality Standards (NAAQS) for pollutants considered harmful to the environment and public health. The Environmental Protection Agency's (EPA) office of Air Quality Planning and Standards then established a set of six principal pollutants, or criteria pollutants, that most gravely affect health and well-being. These six pollutants are nitrogen dioxide, sulfur dioxide, carbon monoxide, particulate matter, lead, and ozone. To this list we have also added carbon dioxide, the primary indicator of greenhouse gas emissions, and volatile organic compounds, the precursor to ground level ozone pollution.

Carbon Dioxide (CO2):


CO2 is an odorless, colorless gas that occurs naturally in the atmosphere at about 0.036% or 360 ppm.11 Carbon Dioxide is about 1.5 times denser than air. CO2 is thought of as a greenhouse gas in the upper atmosphere because it traps incoming heat from the sun and outgoing heat from the earth. This greenhouse effect is what keeps our planet warm and comfortable enough for life, but human activity has increased the concentration of this gas in the atmosphere far above its natural level, and the result is global warming.


CO2 is created by both natural and man-made sources. Our bodies create CO2 continually. As we breathe in, oxygen travels through our bodies and, through chemical reactions in our cells, is exhaled in the form of CO2. Plants use CO2 during photosynthesis to create carbohydrates from carbon dioxide, water, and energy from sunlight. This process helps them grow. Yet, as with all components of our ecosystem, a delicate balance of CO2 is necessary for a healthy environment.

Man-made processes are increasing CO2 levels. When any carbon-based fuel (such as wood, coal, petroleum, or natural gas) is burned in the presence of oxygen, carbon dioxide will be emitted. CO2 levels are also increased when we deplete tree habitats for new developments.12

Ozone (O3):


Ozone (O3) is a gas that forms when three atoms of oxygen are combined. O3 is a highly unstable gas that may smell sweet. In the upper-atmosphere, ozone is found naturally and protects the earth from harmful ultraviolet radiation. But ground-level ozone is a key ingredient in forming urban smog and is considered one of the greenhouse gases.

In terms of health and air quality, we are concerned with both ground-level, or bad, ozone and upper-atmospheric, or good, ozone.


Upper-atmospheric ozone (good ozone)
Roughly 6-30 miles above the earth's surface lies the stratosphere where ozone occurs naturally. In this layer of our atmosphere, ozone is formed when ultraviolet radiation dissociates some O2 molecules into two free oxygen atoms. These free oxygen atoms then combine with other O2 molecules to create O3. Since O3 is in turn dissociated by UV rays, a net balance is kept with continual formation and destruction of the oxygen molecules.13 In the upper atmosphere this "good" ozone protects life on Earth by absorbing some of the sun's ultraviolet rays. The discovery of a hole in the ozone layer in the last 20 years has been a source of great environmental concern.

Ground-level ozone (bad ozone)

Ozone also occurs in the part of the atmosphere directly above the earth's surface, called the troposphere. Unlike the stratospheric ozone layer, this ground level ozone is harmful to human health. Ground-level ozone is caused by the release of volatile organic compounds (VOC) and nitrogen dioxides (NO2) into the air. When these chemicals are released into the air they react with sunlight and heat to create ozone. Consequently, as temperatures rise, ozone levels rise as well.

VOC + NO2 + heat + sunlight = ground level ozone (O3)

Whereas NO2 helps form ozone, nitrogen oxide (NO)--a pollutant primarily emitted from motor vehicles--contributes to the destruction of ozone (see discussion of nitrogen dioxide). Therefore, areas with high NO emissions, such as urban centers, have lower concentrations of ozone than rural areas.

Ozone is also considered a greenhouse gas. Ozone absorbs infrared radiation emitted by the Earth's surface and thereby traps heat and warms the troposphere.14

Nitrogen Dioxide (NO2):


NO2 is part of a family of highly reactive gases called nitrogen oxides (NOx), which also include nitric oxide (NO) and nitrous oxide (N2O). NO2 is a smog-forming chemical that can smell slightly sweet and is reddish brown in color. NOx can react with SO2 and other chemicals in the air to form acid rain.


NO2 forms when fuel is burned at high temperatures. For example, when a nitrogen (N2)-oxygen (O2) mixture is heated at temperatures over 1100°C, the nitrogen and oxygen will combine to form nitrogen oxide (NO). If the cooling process is slow, the gases will decompose back to their original state. If the cooling process is fast, such as in the internal combustion engines of motor vehicles, the nitrogen oxides will not decompose but instead stay in the NO state. These gases are released into the atmosphere and combine with ozone (O3) to produce NO2. Most NO2 in the atmosphere is formed in this way, although some is released directly from the source.

In the presence of sunlight, NO2 also reacts with other pollutants, such as VOCs, to help form ground-level ozone. Once this ozone is formed, it is again scavenged by NO to form NO2. In the absence of other competing reactions, a "photostationary state" is formed where concentrations of NO, NO2, and O3 are all interrelated.15 Ozone levels therefore tend to be higher in rural areas where automobile driving is less frequent and consequent NO levels are low.

NO2 is primarily the result of gases from motor vehicle exhaust and stationary fuel combustion sources like electric utilities and industrial boilers. It can also be produced from gas stoves and heaters. NO2 absorbs light and can lead to the brownish haze over metropolitan areas.

In addition, nitrogen oxides contribute to the formation of acid rain. NOx, when in the presence of water vapor in the atmosphere, partially converts into nitric acid, HNO3. Nitric acid and sulfuric acid combine to create acid rain.

Sulfur Dioxide (SO2):


SO2 belongs to a family of gases called sulfur oxides (SOx). SO2 is a colorless, non-flammable gas that is odorless at low concentrations, but at high concentrations can have a pungent and irritating smell. SO2 can react with NOx and other chemicals in the air to form acid rain.


Roughly one-third of atmospheric sulfur compounds come from human-made sources.16 Of this one-third, 70% of annual SO2 emissions come from electric power plants that burn coal.17 Burning fossil fuels such as oil, converting wood pulp to paper, and smelting copper, zinc, and lead are also contributors to SO2 formation. Natural sources of SO2 include volcanoes and hot springs.

Generally, the highest concentration of SO2 is found near large industrial sources and power plants. The United States is the largest emitter of SO2 in the world, emitting some 26 million tons a year.18 When SO2 enters the atmosphere it is oxidized into SO3 which then combines with water vapor in the atmosphere to form acid rain.19 Both wet and dry deposition have been sited as damaging to vegetation and the degradation of soils, building materials, and water bodies.

Particulate Matter (PM):


PM can be either solid particles or liquid droplets. PM is measured in micrometers, with matter less than 10 micrometers in diameter posing the greatest health risk. Particles less then 2.5 micrometers in diameter are described as being 'fine' particles. These particles are easily inhaled and can become lodged in the lungs and produce respiratory illness.


PM derives from both man-made and natural sources. Natural sources of PM include volcanic ash, salt from ocean spray, forest fires, and dust from fields. Statistics show that natural sources produce roughly 14 times as much PM as man-made sources. Volcanic eruptions in particular produce large quantities of PM.20

Still, man-made particles can contribute to human health risks. Unlike remote natural disasters, man-made processes tend to be concentrated in areas where many people live. The burning of wood, diesel, and other fuels all produce PM. Various processes that take place in industrial plants, such as incineration, are also large sources of PM distribution. PM greater then 2.5 micrometers in diameter is usually the result of smoke and dust from industry and agricultural production, while particles less then 2.5 generally come from combustion of fossil fuels.

Lead (Pb):


Lead is a heavy, soft metal with a metallic blue hue. Pb has no characteristic smell or taste. Minute lead particles can get into the air and travel long distances. When inhaled or ingested, lead particles are toxic. Lead is a cumulative poison to the central nervous system and is particularly damaging to the mental development of young children.


The most common uses of lead are in non-ferrous smelters, X-ray equipment, pipe and tank lining, lead-based paints, the manufacturing of batteries, and for shielding radioactive materials due to its high density and nuclear properties. Atmospheric concentrations of lead derived significantly from gasoline additives used prior to the 1970s, before "unleaded" gasoline began to be manufactured. While leaded gasoline is banned in North America, it is still common practice in some parts of the world.

Carbon Monoxide (CO):


CO is a colorless, odorless gas that is poisonous in high concentrations. Carbon monoxide is less dense than air.


CO is created when the carbon in fuel is not burned completely. Most fuels we use, such as natural gas, are made out of molecules containing carbon. When there is not enough oxygen present during combustion to form carbon dioxide (CO2), CO is the result.

CO can come from appliances fueled by natural gas, liquid petroleum, oil, and kerosene. Approximately 60% of nationwide CO emissions comes from automobiles. According to the EPA, motor vehicles generate over 90% of urban CO pollution. The remainder comes primarily from industrial processes and fuel combustion in sources such as boilers and incinerators, from electricity production, and from natural sources such as wild fires.

Volatile Organic Compounds (VOCs):


VOCs are organic chemicals that, when released into the atmosphere, participate in photochemical reactions. While VOCs are nearly invisible, they often have a noticeable odor and turn into an evaporated state easily.


While VOCs can be emitted from natural sources, many are man-made, created by industrial processes. VOCs include chemicals such as benzene, toluene, and methyl chloroform. Chlorofluororcarbons (CFCs), hydrofluorocarbons (HFCs), and Perchloroethylene (the principle dry cleaning solvent) are VOCs that are greenhouse gases. VOCs are released from burning fuel, solvents, paints, and glues. VOCs can combine with nitrogen oxides to help form ozone.

Carbon Dioxide Fact Book. W.E. Kunger Associates, July 2000 (Oct. 2000).
Ristinen, Robert A., and Jack Kraushaar. Energy and the Environment. New York: John Wiley & Sons, Inc. 1999. p. 327.
"Scientific Assessment of Ozone Depletion: 1998, Frequently asked Questions about Ozone", World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44, NOAA, NASA, WMO, European Commission, UN Environment Programme,
Air Pollution Chemistry. Encyclopedia of Atmospheric Environment. http:// (Oct. 2000).
Ristinen, p. 306.
"Environmental Effects of Acid Rain." United States Environmental Protection Agency. (Oct. 2000).
Ristinen p. 315.
Ibid. p 307
Ibid. p. 310.



Air Pollution - Some Facts