Ozone is a toxic gas that is formed in the atmosphere when nitrogen oxides (from fuel-burning sources such as utilities and automobiles) and volatile organic compounds (from paints, inks, solvents and gasoline) react in the presence of intense sunlight.

Various factors affect the production of ozone. These include the quantity of reactive gases present, the volume of air available for dilution, air temperature and the amount of sunlight.

Because ozone is not directly emitted into the atmosphere - it is the byproduct of other pollutants mixing together under the right atmospheric conditions - it has been particularly difficult to control.

Effects of ozone include interference with normal lung functioning, aggravation of respiratory disease and eye irritation, vegetation damage and water pollution.

An area violates the federal standard if ozone concentrations exceed .12 ppm for one hour.

If an area exceeds that standard more than once a year - or an average of one day per year over three years - it has failed to meet the standard.

While progress has been made over the years, some areas - including parts of the Bay watershed - often exceed the ozone standard a dozen times or more a year. Last year, for example, Baltimore exceeded the ozone standard 14 times.

Recent research has suggested that the current ozone standard is not adequate to protect public health. The EPA is expected to announce a new standard later this year. The new standard is expected to be lower - in the range of .7 to .9 ppm.

If the EPA lowers the standard, it is expected to lengthen the amount of time it must be violated to constitute nonattainment. For example, an area may have to exceed the new level for 8 hours instead of one hour to exceed the standard.

Depending on what the standard is, huge areas may suddenly attain - or exceed - the ozone standard.