A historical overview of the ozone exposure problem

Ozone can be found in essentially all locations in the troposphere. Too muc h exposure of vegetation and humans to this potent oxidizing gas can prove toxic. Reports of human toxicity to ozone first appeared in the 1800's from accidental occupational exposures when ozone was first discovered. Ozone w as recognized as damaging field vegetation with a report of altered leaf mo rphology in grapes in the 1950s. Ozone is the major oxidant component in ph otochemical smog, and is produced by reactions of volatile organic compound s and oxides of nitrogen with sunlight present. Soon after the inception of the U.S. Environmental Protection Agency (US EPA), the Agency set a genera l "oxidants" standard (which included ozone) in, 1971. A primary standard w as created to protect human health and a secondary standard to protect agai nst agricultural losses, ecological damage, and other losses. Ozone concent rations have decreased steadily over the last two decades in some areas of the U.S., but have increased in other areas. Several aspects of ozone expos ure need further characterization, including better determination of rural concentrations and the relationship of outdoor to indoor concentrations. Oz one is one of the six criteria air pollutants requiring a formal reexaminat ion of the new findings of effects on health and vegetation on a periodic b asis, a process that leads to the publication of an US EPA criteria documen t. As a result of further study concerning ozone effects, significant chang es were made to pollution standards in 1979 and 1997. This toxicant has rem ained a major air pollutant of concern in the U.S. despite regulation and i ntense study over several decades.