The Obama administration on Thursday will unveil 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 has 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 current standard of 75 parts per billion set in 2008, according to people familiar with the plan but not authorized to speak on the record. Smog has been linked to asthma, heart and lung disease, and premature death.

The agency’s scientific panel had recommended a new standard of 60 to 70 parts per billion, and last year, the administration released a draft proposal which would have lowered the standard to a range of 65 to 70 parts per billion. Administration officials had sought public comment on a 60-parts-per-billion plan, keeping open the possibility that the final rule could be even stricter.

Since then, the nation’s business and manufacturing lobbies, including the Chamber of Commerce, National Association of Manufacturers, Business Roundtable and American Petroleum Institute have waged an all-fronts campaign to persuade the Obama administration to make the new standard as weak as possible. The groups were joined by dozens of mayors and local lawmakers, including many Democrats, who, in letters to the E.P.A. and the White House, said that a strict new ozone rule could lead to the closing of factories and power plants across the country.