WASHINGTON — Big industries are waging an intense lobbying effort to block new, tougher limits on air pollution that is blamed for hundreds of heart attacks, deaths and cases of asthma, bronchitis, and other breathing problems. A tougher standard would require hundreds of counties across the country to find new ways to reduce smog-causing emissions of nitrogen oxides and chemical compounds from tailpipes and smokestacks.

Groups representing manufacturers, automakers, electric utilities, grocers, and cement makers met with White House officials recently in a last-ditch effort to keep the standard unchanged. They argued that tightening it would be costly and harm the economy in areas that will have to find additional air pollution controls. Oil and chemical companies also have pressed their case for leaving the current requirements alone in meetings on Capitol Hill and with the Bush administration. On the other side are health experts who conclude that tens of millions of people, particularly the elderly and small children, are still being harmed by poor air quality. EPA has estimated a reduction could result annually in 2,300 fewer nonfatal heart attacks; 48,000 fewer respiratory problems, acute bronchitis, and asthma attacks; 7,600 fewer respiratory related hospital visits, and 890,000 fewer days when people miss work or school. The EPA must decided by mid-March on what to do.