The Environmental Protection Agency (EPA) in the US is tightening air quality standards in an effort to help improve public health.
The EPA's advisers recommended tighter cuts to help reduce smog
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.
However, scientists and health campaigners say the changes have not gone far enough.
Unlike stratospheric ozone, which forms a protective layer high above Earth's surface, ground-level ozone can harm people's lungs and aggravate conditions such as asthma, as well as increase susceptibility to respiratory infections.
Ground-level ozone is formed when nitrogen oxides mix with volatile organic compounds and are heated by sunlight. Man-made sources of these emissions include power plants, motor vehicle exhaust, industrial facilities, gasoline vapours and chemical solvents.
The new permitted ozone level has been reduced from 80 parts per billion to 75 parts per billion.
EPA Administrator Stephen Johnson said that by signing "the most stringent" ozone standard ever, the agency was meeting requirements of the Clean Air Act to periodically review limits.
The agency said it expected the new standards to be met as a result of programmes for reducing ozone-forming emissions from power plants in the east of the country and reducing similar emissions from diesel engines.
However, the EPA's own clean air scientific advisory committee had unanimously recommended setting a standard no higher than 70 parts per billion.
US-based campaigners Clean Air Watch say the reduction did not go far enough.
"Unfortunately, real science appears to have been tainted by political science," said Clean Air Watch president Frank O'Donnell.
"The Bush Administration is compromising public health to save industry money."
Industry representatives, who had lobbied against the change, disputed the environmental need for the change and said there were concerns that the cost of reducing emissions could hurt the economy.
The American Chemistry Council (ACC) said in a statement that there was "no clear and substantial basis" for tightening the standards, which would impose significant burdens on states.
Engine emissions will be targeted to reduce ground-ozone levels
Dan Ridinger, a spokesman for the Edison Electric Institute which represents 70% of the US electric power sector, said the new regulations were pointless.
"It looks to us like the rationale behind tightening the standard significantly skews and misrepresents the scientific record of ozone's health effects," he told the BBC.
"Ultimately, EPA is promising health benefits that people may never receive, but they will definitely end up paying for those benefits regardless of whether or not they get them at the gas pump and through higher energy bills," he said.
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).
It said those benefits included preventing cases of bronchitis, aggravated asthma, hospital and emergency room visits, non-fatal heart attacks and premature death.