Nitrogen Oxides During high-temperature combustion, such as in a power plant or an automobile engine, nitrogen reacting with oxygen forms gases collectively called nitrogen oxides. Elevated levels of nitrogen oxides can irritate the lungs, contributing to bronchitis or pneumonia. Hydrocarbons Hydrocarbons are a diverse group of chemicals formed from hydrogen and carbon. Some are produced naturally by plants and animals, but many more are created through human activity. They may be produced by combustion or emitted as vapors from fuels, paint, dry cleaners, oil and gas production and refining, or chemical plants.
Hydrocarbons can be toxic and play a primary role in the formation of ozone.
Ozone Once in the atmosphere, nitrogen oxides and hydrocarbons interact to form ozone-three oxygen atoms linked together instead of the usual two. Since sunlight provides the energy to produce ozone, the amount of ozone in the atmosphere is lower in the early morning and peaks in the late afternoon. In the upper atmosphere a natural ozone layer screens us from harmful ultra-violet radiation, while ozone near the surface of the earth can impair our health. Humans appear to be among the most sensitive animals to ozone. Exposure to elevated levels damages our lungs and makes breathing more difficult.