WASHINGTON — The Obama administration on Thursday unveiled a major new regulation on smog-causing emissions that spew from smokestacks and tailpipes, significantly tightening the current Bush-era standards but falling short of more stringent regulations that public health advocates and environmentalists had urged.

The Environmental Protection Agency set the new national standard for ozone, a smog-causing gas that often forms on hot, sunny days when chemical emissions from power plants, factories and vehicles mix in the air, at 70 parts per billion, tightening the standard of 75 parts per billion set in 2008. Smog has been linked to asthma, heart and lung disease, and premature death.

The smog rule is the latest in a string of major new Clean Air Act pollution regulations that have become a hallmark of the Obama administration. Republicans and the coal industry have attacked the rules as job-killing regulatory overreach. In August, the E.P.A. proposed climate change regulations aimed at greenhouse gas pollution, which could shutter hundreds of coal-fired power plants. But with the new ozone rule, the Obama administration appears to have tempered its environmental ambitions and sought a politically pragmatic outcome that would sit better with business.

Even so, the E.P.A. estimated that the annual cost to the economy would be $1.4 billion, making it one of the most expensive regulations in history. But it said those costs would be vastly outweighed by annual economic benefits of $2.9 billion to $5.9 billion because of fewer premature deaths, missed days of school and work, asthma attacks and emergency room visits.