US toughens stance on ozone level
It is lowering the amount of smog-forming ozone permitted in the atmosphere for the first time in more than 10 years.
Health and environmental campaigners wanted more stringent limits but industry has lobbied against them.
Industry leaders say complying with the new standards will prove expensive.
Ozone can harm people's lungs and exposure can aggravate conditions such as asthma and increase susceptibility to respiratory infections.
The new level is reduced from 80 parts per billion to 75 parts per billion.
However, the EPA's clean air scientific advisory committee had unanimously recommended setting a standard no higher than 70 parts per billion.
The committee had estimated that reducing smog to that level could prevent almost 4,000 premature deaths and about 7,000 hospital visits.