Ozone is a colorless gas with a pungent odor. Whether it is helpful or harmful to the planet depends on where it is. In the stratosphere, miles above the surface of the earth, ozone forms a protective layer that screens out harmful ultraviolet radiation. At ground level, however, ozone is a pollutant, the main ingredient of smog.

Ground-level ozone is not emitted directly. Instead, it forms when nitrogen oxide and hydrocarbons react in the presence of sunlight. These substances, known as ozone precursors, have varied sources, but combustion is by far the most important. In the Bay Area, tailpipes produce more than 50 percent of the nitrogen oxide and hydrocarbons in the air.

Ozone harms vegetation and reduces visibility. It also affects human health, by irritating the eyes, for instance, and damaging the respiratory system. Children, the elderly, and people with asthma are particularly vulnerable to ozone pollution.

Both the state of California and the federal government regulate ozone, but the state's standard is stricter. California's standard, along with its strict rules on auto emissions and other air-quality controls, has succeeded in making the air in the Bay Area cleaner, and healthier to breathe, than it was two decades ago. In 1995, the Bay Area became the largest metropolitan area in the nation to attain the federal ozone standard. As the result of high ozone levels in 1995, 1996, and 1998, however, the region lost that status.

More Vehicles, More Ozone
The Bay Area's population is projected to increase 14 percent by 2020, bringing more cars, more traffic, and more emissions to the region. New vehicles must meet the California Air Resources Board's stringent emission standards, which means they will produce about 95 percent fewer smog-forming emissions than vehicles produced in the 1970s.

Even this dramatic improvement won't be enough to lay the problem to rest, however. The benefits of today's cleaner vehicle technology may well be unable to keep pace with pollution resulting from the region's steady increase in vehicle use. To reduce ground-level ozone, Bay Area residents will not only have to drive cleaner vehicles, they will also have to drive less.

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