2. Nitrogen Dioxide

Nitrogen dioxide (NO2) and related nitrogen oxides (NOx) are produced when fuel is burned, especially in power plants and motor vehicles. NO2 also changes in the atmosphere to form acidic particles and liquid nitric acid.
Nitrogen dioxide seems to react with lung tissue. It can decrease the lungs' working ability, inflame the breathing passages and cause both coughing and chest pains.

3. Ozone

Ozone (O3) is the major harmful ingredient in smog. It is not emitted directly into the air but produced when gases or vapors made of hydrocarbons and nitrogen oxide compounds created by factories, gas stations, motor vehicles, paints and solvents mix with sunlight. Southern California, with its heavy traffic and long season of sun, is prime ozone breeding ground.

Ozone reacts with lung tissue. It can inflame and cause harmful changes in breathing passages, decrease the lungs' working ability and cause both coughing and chest pains. Children, the elderly and people with lung diseases are especially sensitive to smog. If you are planning to do outdoor activities, check your local smog/ozone levels to determine how safe it is to be outside.

This harmful ozone close to the ground should not be confused with ozone in the upper atmosphere (a.k.a. "the ozone layer") which protects us from ultraviolet radiation.

4. Sulfur Dioxide

Sulfur dioxide (SO2) is created when sulfur-containing fuel is burned, primarily in power plants and diesel engines. Like NO2, sulfur dioxide can also change in the atmosphere into acidic particles and sulfuric acid. Sulfur dioxide constricts air passages and also reacts with lung tissue. Even brief exposure to relatively low levels of sulfur dioxide can cause an asthma episode.

5. Particulate Matter

Particulates, also known as particulate matter (PM) includes microscopic liquid droplets and particles of soot, ashes, dirt, dust, acid aerosols, metals and pollens from vehicle emissions, factory pollution, mining and construction. Exposure to particulate pollution can cause wheezing and other symptoms in people with asthma or sensitive airways.

Particulate matter varies in size, from 10 microns (PM10) down to 2.5 microns (PM2.5), which is small enough (about 1/7th the thickness of a human hair) to be inhaled into the deepest parts of the lungs where they can cause the most damage. PM2.5, the very finest particles, are of greatest concern. In 1995, investigators found a 17% increase in mortality risk in areas with higher concentration of small particles. Only now is the Environmental Protection Agency (EPA) setting standards that regulate PM2.5.

6. Lead

Lead (Pb) has been known as a poisonous substance for many years. Thanks to past major reductions and now the elimination of lead in gasoline, there has been a significant decrease in public exposure to lead in outdoor air. Remaining air pollution sources include lead smelters, incineration of lead batteries and lead-contaminated waste oil. The most common sources of current lead exposure, however, are found around the house in old lead paint and soil.

Lead accumulates in the body, so repeated small doses can be harmful. Exposure to high levels of lead can damage the blood, brain, nerves, kidneys, reproductive organs and the immune system. Even lower level exposure can result in impaired mental functioning in children and raising blood pressure in middle-aged men.