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 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. 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 a 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. 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. Some Democratic governors had also urged the administration to put forth a weaker standard. Virginia Gov. Terry McAuliffe told the White House that he would like to see the looser 70-parts-per-billion standard. Labor groups, such as the AFL-CIO, and the International Brotherhood of Electrical Workers, have asked the White House to keep the standard at the 75 parts-per-billion level. The new rule, set at the weakest standard in the range recommended by E.P.A.’s scientists, suggests that the industry groups were influential. The industry lobbies said they were disappointed that the standard had been tightened at all, but pleased that the new rule represents the least stringent of the options on the table. “Today, the Obama Administration finalized a rule that is overly burdensome, costly and misguided,” said Jay Timmons, president of the National Association of Manufacturers. “For months, the administration threatened to impose on manufacturers an even harsher rule, with even more devastating consequences. After an unprecedented level of outreach by manufacturers and other stakeholders, the worst-case scenario was avoided,” he said. The new rules will still require the owners of factories and power plants to install costly new “scrubber” technology on their smokestacks, designed to clean out the polluting chemicals. Each scrubber can cost tens of millions of dollars. But environmental advocates still saw the new rule as a blow. “Disappointing is too mild a term,” said Frank O’Donnell, president of the advocacy group Clean Air Watch. “The big polluters won this time, for the most part,” he said, adding, “this decision will tarnish the president’s environmental legacy. The national clean air standards are the heart of the Clean Air Act. This decision is heart failure.” Although environmentalists have praised Mr. Obama for many of his environmental policies, the efforts to control smog have long been a sore spot for them. The E.P.A. had planned to release the new ozone rule in August of 2011, and Lisa P. Jackson, the head of the agency at the time, had planned to issue a stringent rule of 65 parts per billion. Ms. Jackson, whose son suffers from asthma, saw the smog rule as a centerpiece of her own environmental legacy. But as Republicans and powerful industry groups prepared to go on attack against the plan, Mr. Obama decided to delay its release, fearing that opposition to the regulation would hurt his re-election chances in 2012. At the time, Mr. Obama said the regulation would impose too severe a burden on industry and local governments at a time of economic distress. Ms. Jackson nearly resigned over the issue.