Washington ? The air in hundreds of U.S. counties is simply too dirty to breathe, the government said Wednesday, ordering a multibillion-dollar expansion of efforts to clean up smog in cities and towns nationwide.

The Environmental Protection Agency announced it was tightening the amount of ozone, commonly known as smog, that will be allowed in the air. But the lower standard still falls short of what most health experts say is needed to significantly reduce heart and asthma attacks from breathing smog-clogged air.

EPA Administrator Stephen Johnson called the new limits “the most stringent standards ever,” and he said they will require 345 counties – out of more than 700 that are monitored – to make air quality improvements because they now have dirtier air than is healthy to breathe.

All counties in Kansas meet the new standards. In Missouri, Clay County in the metropolitan Kansas City area will exceed the lower limit.

Johnson said that state and local officials have considerable time to meet the requirements – as much as 20 years for some that have the most serious pollution problems. EPA estimates that by 2020 the number of counties failing to meet the new health standard will drop to about 28.

About 85 counties still fall short of the old standard enacted a decade ago.

Some of those chronic polluters are far above the old limit – Los Angeles County and a large swatch of southern California, for example, and a long stretch from Washington up to New England on the East Coast. Some areas that would be newly included under the stricter standard include Indianapolis and Cleveland’s Cuyahoga County; Mobile, Ala., Jacksonville, Fla., El Paso, Texas, and Tulsa, Okla.

All of Florida and Oklahoma currently comply with the smog standard. Nine counties in each state are unable to meet the tougher requirement.

Johnson’s decision was met with sharp criticism from health experts, and some members of Congress accused the EPA chief of ignoring the science. The new standard goes counter to the recommendations of two of the agency’s scientific advisory panels – one on air quality and the other on protection of children.

The new EPA standard will lower the allowable concentration of ozone in the air to no more than 75 parts per billion, compared with the old standard of 80.

The science boards had told the agency that limits of 60 to 70 parts per billion are needed to protect the nation’s most vulnerable citizens, especially children, the elderly and people suffering from asthma and other respiratory illnesses.

“Today’s decision means millions of Americans will not get the protection that the law requires,” said Bernadette Toomey, president of the American Lung Association, which had strongly urged the EPA to follow the advice of the science boards.

Johnson said he took those recommendations into account, but disagreed with the scientists. “In the end it is a judgment. I followed my obligation. I followed the law. I adhered to the science,” Johnson said in a conference call with reporters.