The US Environmental Protection Agency is tightening air quality standards in an effort to help improve public health.
It is lowering the amount of smog-forming ground-level ozone permitted in the atmosphere for the first time in more than 10 years. The EPA says the change could save 4,000 lives each year.
The EPA said the cost of implementing the standards, ranging from $7.6bn to $8.5bn (£3.7bn to £4.1bn), would be outweighed by health benefits, valued at up to $19bn (£9.3bn).
However, scientists and health campaigners say the changes have not gone far enough. “Unfortunately, real science appears to have been tainted by political science,” said Clean Air Watch president Frank O’Donnell.