The Environmental Protection Agency has proposed a tougher standard for levels of ground-level ozone, or smog, contending that the change will prevent hundreds of premature deaths and tens of thousands of asthma attacks.

Environmental groups and their congressional allies praised the proposal, but leading Republican lawmakers strongly criticized the plan.

The proposal, which EPA Administration Gina McCarthy signed on Nov. 26, would trim the ozone limit to a range of 65 to 70 parts per billion (ppb), from the current level of 75 ppb. That standard was put in place in 2008.

McCarthy told reporters in a conference call that the Clean Air Act directs EPA to review the smog standard every five years to ensure the benchmark protects public health with an “adequate margin of safety.”

She said that ground-level ozone is “a dangerous pollutant.” She added, “It can have very serious consequences our families’ health.”

EPA estimates that a new standard of 65-70 ppb would avert 750 to 4,300 premature deaths and 320,000 to 960,000 asthma attacks in children.

Nick Goldstein, American Road & Transportation Builders Association vice president of regulatory affairs, said in an interview that his group is disappointed that EPA did not give serious consideration to keeping the current standard in place. He says that implementing a new limit is “like moving goal posts in the middle of the game.”

He adds, “We’re not in favor of tightening it any further because we think it does more harm than good at this point.”

One construction-related concern is that if a county fails to meet the ozone standard and thus falls into “non-attainment” status, the federal government could withhold some highway funding.

Goldstein says even the possibility of that action could put a highway project on hold. “It’s a sword-of-Damocles type of situation,” he says.

EPA projects that 68 U.S. counties that use smog-monitoring devices, excluding California, would exceed a 65-ppb level in 2025, down from 558 in 2011-2013.

EPA said that the proposal’s per-year public-health benefits would range from $6.4 billion to $13 billion in 2025 if the standard were set at 70 ppb, or $19 billion to $38 billion for a 65-ppb mark.

EPA said those benefits would far exceed the regulation’s annual compliance costs, which it pegged at $3.9 billion in 2025 for a 70-ppb standard and $15 billion for 65 ppb. 

The agency also said that states’ deadlines for meeting the newly proposed standard would range from 2020 to 2037, depending on how severe their ozone problems are.

In addition, EPA is proposing to toughen its “secondary” ozone standard to a range of 65-70 ppb. That secondary standard aims to protect trees, crops and other plants from smog impact.

The agency will accept public comments on the proposal for 90 days after it appears in the Federal Register. It said it would issue the final rule by Oct. 1, 2015.

Environmentalists praised EPA’s proposal. Allen Hernandez, a Sierra Club community organizer in California, said in a statement, “Progress towards safe, healthy air has not come fast enough, which is why today’s action by EPA is so critical.”

But senior congressional Republicans slammed the EPA plan. Sen. James Inhofe (R-Okla.) said that the proposed standard “will lower our nation’s economic competitiveness and stifle job creation for decades.”

Inhofe, who is expected to chair the Environment and Public Works Committee in the next Congress, said there would be “rigorous oversight” of the proposal next year.

EPA also is seeking comments on lowering the standard even further, to 60 ppb. McCarthy said that EPA science advisors had indicated the 60-ppb level “should be on the table.”