AG 2007 10 15 - Laserfiche WebLink

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"Several additional factors should be considered when selecting an appropriate level for a secondary standard. These include: 1) The fact that 03 effects are cumulative and have been shown to have carry over effects from one year to the next; 2) Some seedling tree species have sensitivities as great as annual crops and the importance of protecting against small percentages of biomass loss on an annual basis has been expressed by some within the scientific community; 3) Visible foliar injury impacts can occur within a growing season at very low levels of 03 exposure; and 4) The extent to which a secondary standard is precautionary in nature, given the possibility of 03 impacts acting in synergy with other natural and manmade stressors. Should a 3-year average of a 12-hr W 126 be selected, the level chosen should reflect the fact that annual impacts are still a concern for visible foliar injury, tree seedling biomass loss, and crop yield loss, so that a potential lower. level might be considered to reduce the potential of adverse impact from a single high 03 year that could still occur while attaining a 3-year average." Additional Comments on the Scientific Merit of a Secondary Standard for Ozone that is Distinct in Form from the Primary Standard Ozone is a general metabolic poison. In the light of EPA's consistently strong emphasis on effects of ozone and related photochemical oxidants on human health, how interesting it is to realize that the injurious effects of ozone and other photochemical oxidants were first discovered on vegetable crops'and forest trees and only later were discovered also to be injurious to human health and to be the principal cause of eye irritation in photochemical smog. Ozone and other oxidants cause stress in plants and thus predispose both individual plants and whole ecosystems to attack by natural enemies that include disease- and injury-inducing bacteria, fungi, nematodes, viruses, and insects. In some cases, exposure to high concentrations of ozone also decreases the resistance of plants to injury and damage by abiotic stress factors such as drought and frost: Different species and varieties of plants vary widely in susceptibility to ozone and other oxidants. Many species of crop plants, forest and shade trees, and some of the multiple-species of plants in natural ecosystems are more sensitive to injury and damage by ozone than most people. That is, many plants show visible symptoms of injury at concentrations of ozone that are considerably lower (40 to 60 ppb of ozone) than the 80 to 120 ppb of ozone that are generally recognized to cause ill-health in people. The injurious effects of ozone and other oxidants on plants and ecosystems are CUMMLJLATIVE in their effects rather than acute or chronic in their effects as is found for most health effects of ozone on people. n ° ~ ~-~. C-5