WASHINGTON Big industries are lobbying intensely to block tougher limits on air pollution, which is cited as the cause for hundreds of heart attacks, deaths, and cases of asthma, bronchitis and other breathing problems.

The Environmental Protection Agency is to decide within weeks whether to reduce the allowable amount of ozone commonly called smog in the air. 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.