But in Texas and Southern California, already among the smoggiest areas in the country, the science is unclear, even conflicting. Smog there could get slightly better or become more severe, the analysis said.
Nonetheless, researchers said state officials should be factoring in the impact of global warming as they make plans to try to reduce smog, calling it a "climate penalty."
"These findings also indicate, that, where climate-change-induced increases in (smog) do occur, damaging effects on ecosystems, agriculture, and health will be especially pronounced, due to increases in the frequency of extreme pollution events," the analysis concluded. However, the prediction came with a caveat: the researchers did not take into account efforts to reduce smog that are already under way because of stricter environmental regulations.
EPA stressed that the document did not represent the agency's policies or position on global warming. On Friday, the agency is scheduled to officially respond to a Supreme Court ruling that said greenhouse gases could be regulated under the Clean Air Act if they pose a risk to human health or welfare.
The Bush administration has resisted linking global warming to public health problems.
The ill-effects of smog, however, have been long been recognized. Smog is formed when nitrogen oxides and volatile organic compounds released from cars, industry and plants mix in sunlight. It can irritate the respiratory system, reduce lung capacity and aggravate asthma. Global warming would make smog worse because it would cause plants to release more smog-forming organic compounds and spark more lightning storms, which create nitrogen oxides naturally.
Barry Wallerstein, executive director of the South Coast Air Quality Management District, which regulates air pollution in Orange County, Calif., and parts of Los Angeles, said the link between global warming and smog should compel EPA to control greenhouse gas emissions.