Glossary of Terms

Carbon Dioxide (CO2)

Carbon dioxide (CO2) is the most important greenhouse gas (GHG), referring to substances that trap heat in the earth's atmosphere and cause global warming. The largest portion of harmful GHG emissions is the CO2 released from burning fossils fuels: oil, coal, and natural gas.

Hydrocarbons (HC)

Hydrocarbons are a broad class of chemicals containing carbon and hydrogen. Those hydrocarbons that cause various forms of air pollution are also known as volatile organic compounds since they are forms of HC that are either gases or readily evaporate into the air. Many forms of HC are directly hazardous, contributing to what are collectively called "air toxics." these compounds can be directly irritating to the lungs and other tissues and they can also cause cancer, contribute to birth defects, and cause other illnesses. During daylight hours, and particularly during hot summer weather, HC reacts with NO X to form ozone smog (see box below). Controlling ozone is one of the major environmental challenges in the United States. Although progress has been made over the past several decades, many cities and regions still have smog alerts when ozone levels get too high.

Nitrogen oxides (NOX)

NOX refers mainly to two chemicals, nitrogen oxide (NO) and nitrogen dioxide (NO2),that are formed when nitrogen gas, which comprises 78 percent of air, reacts with oxygen during the high temperatures that occur during fuel combustion. NOX is truly a noxious pollutant in many ways. It is directly hazardous, an irritant to the lungs that can aggravate respiratory problems. It reacts with organic compounds in the air to cause ozone, which is the main reason for "smog alerts" that still happen too often in many cities and regions. NOX is a precursor of fine particles, which cause respiratory problems and lead to thousands of premature deaths each year. It is also a precursor of acid rain, which harms lakes, waterways, forests, and other ecosystems, as well as damaging buildings and crops. Airborne NOX also contributes to nitrification-essentially an over-fertilization-of wetlands and bays, leading to algae blooms and fish kills.


Ozone (O3) is a highly reactive form of oxygen that occurs naturally in various parts of the atmosphere but gets artificially produced in dangerously high concentrations due to emissions from cars, trucks, and other combustion sources.

Up in the stratosphere, ozone helps protect us from ultraviolet radiation. Loss of this protective ozone layer at high altitudes can lead to increased skin cancer. Such concerns have led to restrictions on ozone-depleting chemicals such as those once found in some spray cans and others that have been phased out of use in refrigerators and air conditioners (including automotive air conditioners).

Particulate matter (PM)

Fine airborne particles are an established cause of lung problems, from shortness of breath to worsening of respiratory and cardiovascular disease, damage to lung tissues, and cancer. Certain people are particularly vulnerable to breathing air polluted by fine particles, among them asthmatics, individuals with the flu and with chronic heart or lung diseases, as well as children and the elderly. Pm also soils and damages buildings and materials. It forms haze that obscures visibility in many regions. Soot and smoke coming from exhaust pipes are obvious sources of pm, but among the most deadly forms of airborne particulate matter are the invisible fine particles that lodge deeply in the lungs. Pm has been regulated for some time, but the regulations were based on counting all particles up to 10 microns in size (pm10). However, pm10 standards fail to adequately control the most dangerous, very fine particles. The U.S. EPA has recently started to regulate fine particles up to 2.5 microns in size (pm2.5), which better focuses on the most damaging category.

Sulfur dioxide (SO2)

Gasoline and diesel fuels also contain varying amounts of sulfur, which burns in the engine to produce sulfur dioxide (SO2). This gaseous chemical is another source of secondary particulate formation, and is itself a lung irritant as well as a cause of acid rain. So2 also interferes with the operation of catalytic converters. Some of the cleaner, reformulated versions of gasoline have very low sulfur levels. Most gasoline sold nationwide still has too much sulfur, but levels are being reduced under recently established EPA regulations.