What Is Ozone?

Ozone is a form of molecular oxygen that consists of three oxygen atoms linked together. Ozone in the upper atmosphere (the "ozone layer") occurs naturally and protects life on earth by filtering out ultraviolet radiation from the sun. But ozone at ground level is a noxious pollutant. It is the major component of smog and presents this country's most intractable urban air quality problem.

Why Is Ozone a Public Health Problem?
Ozone is a severe irritant. It is responsible for the choking, coughing, and stinging eyes associated with smog. Ozone damages lung tissue, aggravates respiratory disease, and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone's harmful effects, as are adults with existing disease. But even otherwise healthy individuals may experience impaired health from breathing ozone-polluted air.
Elevated ozone levels also inhibit plant growth and can cause widespread damage to crops and forests.

Unhealthy ozone levels are a problem across the United States, with nearly 100 cities exceeding the U.S. Environmental Protection Agency (EPA) National Ambient Air Quality Standard. The standard is based on the highest ozone exposure sensitive persons can tolerate. Nine cities, home to 57 million people, are considered "severely" polluted, experiencing peak ozone levels that exceed the standard by 50% or more.

How Is Ozone Formed?
Ozone is not emitted directly but is formed in the atmosphere through a complex set of chemical reactions involving hydrocarbons, oxides of nitrogen, and sunlight. The rate at which the reactions proceed is related to both temperature and intensity of the sunlight. Because of this, problematic ozone levels occur most frequently on hot summer afternoons.
Hydrocarbons and nitrogen oxides come from a great variety of industrial and combustion processes. In typical urban areas, at least half of those pollutants come from cars, buses, trucks, and off-highway mobile sources such as construction vehicles and boats.

What's Been Done to Control Ozone Levels?
The Clean Air Act of 1970 gives primary responsibility to state and local governments for regulating pollution from power plants, factories, and other "stationary sources." EPA has primary responsibility for regulating "mobile sources," which include cars, trucks, buses, and aircraft.
The EPA vehicle emission control program has achieved considerable success in reducing both nitrogen oxide and hydrocarbon emissions. Cars coming off today's production lines typically emit 70% less nitrogen oxides and 80% to 90% less hydrocarbons over their lifetimes than their uncontrolled counterparts of the 1960s. The improvement came about in response to stringent regulations, which required auto manufacturers to develop systems capable of capturing excess gasoline vapors and cleansing tailpipe emissions.

Why Aren't Ozone Levels Dropping?
Ozone levels in many cities have come down with the introduction of lower volatility gasoline and as newer cars with improved emission control systems replaced older models. But although there has been significant progress since 1970 in reducing emissions per mile traveled, the number of cars on the road -- and the miles they travel almost doubled in the same time frame.
A second reason that ozone levels remain high is that emission control systems do not always perform as designed over the full useful life of the vehicle. Routine aging and deterioration, poor state of tune, and emission control tampering can all increase vehicle emissions. In fact, a major portion of ozone-forming hydrocarbons can be attributed to a relatively small number of "super-dirty" cars whose emission control systems are not working properly.

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