A single hour of exposure to ozone at this level is sufficient to cause negative respiratory effects. During heavy exercise these people are at particular risk for short-term health effects. Many more individuals may be repeatedly exposed to lower ozone levels which may cause long-term adverse effects. About 64 million people live in areas with ozone levels greater than the maximum health standard. Ground-level ozone interferes with the production and storage of starches within plants, reducing their growth rates.
Ozone reduces the ability of trees and plants to fight disease.
The extent of forest damage is currently being studied. Ozone has been shown to damage various tree seedlings. The Clean Air Act of 1990 requires EPA, states, and cities to initiate a series of programs that will further reduce emissions of VOCs and NOX from cars, fuels, industrial and chemical facilities, power plants, and consumer and commercial products among other sources. Cleaner cars and fuels, new kinds of gasoline nozzles, enhanced vehicle inspection, and other programs will be phased in over the next decade. Ozone is naturally produced and destroyed at a constant rate in the stratosphere.