Opposition is mounting to the U.S. Environmental Protection Agency’s proposed more stringent limits for ground-level ozone, or smog.

EPA says the current National Ambient Air Quality Standard for ozone of 75 parts per billion over an eight-hour period does not adequately protect public health and proposed to tighten the limit to a range of 65 ppb to 70 ppb in November 2014. EPA also seeks public comments on a 60-ppb standard or leaving the benchmark at 75.

The proposal’s critics, including construction groups, contend that a tougher smog standard could jeopardize funding for road and highway projects. “Regulatory uncertainty hits [contractors] the hardest, because as state and local governments don’t know ... if they will be in attainment or not in attainment, projects get delayed,” says Nick Goldstein, American Road and Transportation Builders Association vice president of regulatory affairs and assistant general counsel.

Moreover, many counties have not yet been able to comply fully with the current standard, set in 2008. At present, 227 counties are designated as non-attainment for the 75-ppb standard, according to EPA. If the standard were changed to 70 ppb, 358 counties would be in non-attainment. At 70 ppb, 558 wouldn’t meet the test.

The American Association of State Highway and Transportation Officials will support whatever ozone number science shows would protect public health, but there are some challenges associated with a more stringent standard, says Shannon Eggleston, AASHTO program director for environment. In areas deemed to be not in attainment, local transportation officials are required to show that planned construction projects will stay within a certain emissions limit and to do so again when the projects are under construction. As a result, Eggleston says, “Project delivery can be impacted,” and costs can go up.

A Senate hearing on June 3 underscored the divide between Democrats and Republicans. Environment and Public Works Committee Chairman James Inhofe (R-Okla.) said, “A new standard at this time is not only irresponsible, but also impractical and economically destructive.” The panel’s top Democrat, Barbara Boxer (Calif.), praised the testimony of witness Dr. Gregory Diette, who represented the American Thoracic Society. He said recent science shows ozone exposure is deadly, especially to children, the elderly and adults with asthma.

Kanathur Srikanth, director of transportation planning, National Capital Region, Transportation Planning Board, said a new standard would pose “fresh challenges” and that EPA will need to help counties comply.

Several GOP lawmakers have introduced a handful of bills that among other things, would require EPA to focus on helping counties that are not in attainment with the current NAAQS first, before tightening the standard. But those bills have yet to be marked up, and could face an uphill battle in the Senate, which includes strong environmental and public health advocates like Boxer and Sheldon Whitehouse (D-R.I.).

ARTBA’s Goldstein says that instead of piecemeal bills, lawmakers should take another look at the Clean Air Act. “It’s not working in terms of standard-setting,” he says. “It’s a heavy lift legislatively, but that’s what legislators should be doing.”

EPA plans to finalize the NAAQs by Oct. 1.