Smog and haze hovers over Salt Lake City. (AP/Deseret News/Brian Nicholson)

The Obama administration tightened limits Thursday on a key air pollutant that causes urban smog, a move that officials say will alleviate suffering for millions of Americans who suffer from asthma and other respiratory illnesses.

The new restrictions announced by the Environmental Protection Agency will require scores of cities and counties to take steps to reduce levels of ground-level ozone, a byproduct of fossil-fuel burning responsible for hazy skies over many U.S. cities, especially in the summer. But the lowering of the ozone limit—from 75 to 70 parts per billion—angered environmentalists and public-health groups that had sought an even tougher standard.

Industry trade groups, who had warned of severe economic damage and job losses, criticized the rule but said it could have been worse.

EPA officials said the first change in the federal ozone limit in seven years was prompted in part by new scientific research linking exposure to development of asthma, particularly in young children. EPA Administrator Gina McCarthy said the stricter standard will improve the lives of tens of millions of Americans suffering from respiratory disorders.

“While the days are gone when cities like Los Angeles were so smoggy that people had trouble seeing across the street, science tells us that ozone is still making people sick and we still have work to do,” McCarthy said.

The issue of how to regulate smog, which can cause or aggravate such health problems as asthma and heart disease and is a suspect in premature death, has been a contentious one for decades. While the determination is supposed to be made solely on scientific concerns rather than economic ones, any lowered limit carries enormous economic consequences for states and cities across the nation. Communities that consistently fail to meet the standard could eventually face restrictions on certain kinds of industrial development.

The EPA’s decision drew criticism from all sides of the ozone debate, while some former regulators applauded the agency for seeking a balance.

“EPA has threaded the needle in strengthening the ozone standard,” said S. William Becker, executive director of the National Association of Clean Air Agencies, which represents state and local air regulators. “The agency has appropriately balanced the views of divergent stakeholders with the public’s right to breathe clean air.”

[READ: On smog, industry gets unexpected help from a black business group]

But environmentalists and groups such as the American Lung Association had pushed for a much tougher standard. In June 2014, EPA’s group of independent science advisers, the Clean Air Science Advisory Committee, had said the most vulnerable groups might still suffer adverse consequences under a limit of 70 ppb.

The committee “advises that, based on the scientific evidence, a level of 70 ppb provides little margin of safety for the protection of public health, particularly for sensitive subpopulations,” it wrote.

John Walke, clean air director for the Natural Resources Defense Council, faulted the administration’s decision Thursday.

“The president’s legacy is shaping up to be one of unprecedented leadership on combating climate change, but weakness on health standards for smog pollution,” Walke said.

American Lung Association president and CEO Harold P. Wimmer said in a statement that “while the updated standard is a step in the right direction,” setting it at 70 “simply does not reflect what the science shows is necessary to truly protect public health.”

EPA issued the rule in response to a 2013 lawsuit filed by Earthjustice on behalf of the ALA and other groups; on Thursday Earthjustice managing attorney David Baron said his firm would likely sue the agency over the revised standard.

McCarthy defended the EPA’s decision, said the agency followed its “obligation to  weigh the science, including the uncertainties” that remain. While some studies suggest that ozone than effect adults at levels as low as 60 ppb, “these studies do not show that these effects are harmful,” she said.

“Based on that uncertainty, I concluded that we should strive to reduce, but not necessarily eliminate, exposures to ozone concentrations as low as 60 ppb,” she said.

Earlier this month, according to several individuals, the White House Council on Environmental Quality discussed the idea of setting the standard at 68 ppb. But industry officials, who have campaigned to preserve the current standard of 75 ppb, said such a change would have a major impact on their operations.

Paul Noe, vice president for public policy at the American Forest and Paper Association, said his group determined that dropping the standard from 70 to 68 would increase the costs of compliance for pulp and paper firms 10-fold. It would increase the number of affected pulp and paper facilities from 41 to 114, according to the analysis, raising the total costs from $125 million to $1.1 billion.

The skyline of downtown Los Angeles seen through a thick layer of smog. (Reuters/Fred Prouser)

“We appreciate the fact we were given the opportunity to speak with [Office of Management and Budget], EPA and other White House offices about our concern about going further” below 70, he said.

But Noe said that since nitrogen oxide, the pollution most associated with smog, has been cut in half since 1990, and is set to be reduced by another third between now and 2025, it made no sense to reduce the existing standard given that many communities are still striving to meet it.

“We’re disappointed in the decision to reduce the standard before the current standard’s yet be implemented,” he said.

National Manufacturers Association CEO Jay Timmons said in a statement Thursday that while “the worst-case scenario had been avoided,” the EPA rule is still “overly burdensome, costly and misguided.”

“We know that this regulation could have been worse, but it still feels like a punch in the gut,” said Tom Riordan, CEO of Neenah Enterprises, Inc. and a NAM member. “Manufacturers are tough and resilient, but when Washington puts politics above job creation, we still pay a price.”

At an appearance last week before the Business Roundtable, the president emphasized that his administration was legally obligated to examine the federal smog standard on a regular basis. “The ozone rules date back to when I was I think still in law school, before I had any gray hair.”

“We are mindful that in some cases, because of the nature of where pollutants are generated, where they blow, that this can create a really complicated situation for certain local jurisdictions and local communities, and some states and counties end up being hit worse than others,” he said.  “And we’re trying to work with those states and those communities as best we can taking their concerns into account.”

While Obama has sided with environmentalists repeatedly throughout his tenure, the question of how to regulate ozone is one of the few areas in which the White House has often been at odds with its liberal base. In September 2011 the president pulled back an EPA proposal to tighten the standard, arguing it did not make sense to increase the regulatory burden on the private sector “particularly as our economy continues to recover.”

According to records on OMB’s Web site, Howard Shelanski, a top official of OMB who reviews proposed regulations, personally attended meetings last month on ozone that administration officials held with the National Association of ManufacturersAmerican Petroleum Institute and Marathon Oil Corp., but sent deputies to sessions with environmental and public health advocates such as the American Nurses Association, Earthjustice, the American Lung Association, Sierra Club and NRDC.

Also in Energy & Environment:

Scientists declare an ‘urgent’ mission — study West Antarctica, and fast

E.O. Wilson explains why parks and nature are really good for your brain

Everything you need to know about the surprisingly cold ‘blob’ in the North Atlantic ocean

For more, you can sign up for our weekly newsletter here, and follow us on Twitter here.