Many biologists believe that regional air pollution is a serious anthropogenic (made by humans) threat to temperate forest ecosystems. The most dangerous impact on forests comes from ozone, heavy metals, and acid deposition. Ozone exposure reduces forest yields by stunting the growth of seedlings and increasing stresses on trees. Such damage can take years to become evident. Numerous studies suggest that both photosynthesis and growth decline significantly after one or two weeks of ozone at levels of fifty to seventy parts per billion (ppb), more than twice the normal background level of twenty to thirty ppb. During growing seasons average ozone levels are highest in the West (California, Nevada, Utah, and Arizona) and on the East Coast south of Pennsylvania.
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