Minnesotans could face tighter emission limits on their vehicles or tougher air-permitting standards for businesses under new ozone rules proposed Wednesday by federal regulators, state officials said.

"For Minnesota it is critically important,'' said Frank Kohlasch, manager of the Air Assessment Section at the Minnesota Pollution Control Agency.

The U.S. Environmental Protection Agency proposed tougher limits on smog-producing ozone, standards that would be among the costliest ever issued by the agency but could reduce childhood asthma and other illnesses.

The EPA offered to tighten the levels to 65-70 parts per billion, from the current 75 parts per billion for any measured eight-hour period. That change could prevent almost 1 million asthma attacks and an equivalent number of missed school days, the agency said.

If the EPA chooses 70 parts per billion as the new standard, Minnesota would likely remain in good standing with the federal ozone standard, Kohlasch said. Currently, the state's highest ozone reading is 67 from a monitor located in the Twin Cities, he said.

But if the new limit is set at or near 65 parts per billion, the state could find itself in "nonattainment status'' and would have to propose a plan to move back into compliance, Kohlasch said.

In that case, the state's options would include tailpipe inspections on cars, trucks, buses and other motorized equipment. In addition, the state might consider tightening emission standards on companies seeking new pollution permits. Kohlasch emphasized that those decisions won't be made until after a new federal ozone standard is set late next year. "We continue to be at risk because our numbers are close to 65 [parts per billion],'' he said.

Kohlasch said any new restrictions would be applied geographically. For instance, if the Twin Cities falls into nonattainment status, communities in outstate Minnesota wouldn't necessarily face new restrictions, he said.

Kohlasch also said Minnesota's ozone monitoring results could change before the new standard is set, so it's too early to identify a make-or-break ozone number.

Compliance will come at a high cost for the nation's power plants, chemical manufacturers and refineries. The EPA estimates businesses would need to spend $4.7 billion to $16.6 billion in coming decades, depending on which standard is selected when the rule becomes final, according to an EPA analysis released alongside the proposal.

"Tightening these standards could be the most expensive regulation ever imposed on the American public, with potentially enormous costs to the economy, jobs, and consumers," Jack Gerard, the head of the American Petroleum Institute, said in a statement.

The rules give President Obama a second chance to issue a nationwide safety level, as he focuses on environmental protection and addressing climate change in his second term. Obama tossed out a previous EPA plan to cut ozone in 2011 as lobbyists from oil companies, manufacturers and chemical makers pressed the White House for a pre-election reprieve. Industry groups argue that the current standard is sufficient.

Gina McCarthy, the administrator of the EPA, also offered to consider lowering the standard even more, or leaving the current rule in place, in the proposal posted on the agency's website Wednesday.

McCarthy stopped short of adopting the strictest limits recommended by her own outside science advisory panel of 60 parts per billion. Lowering the level that far would increase the costs of compliance dramatically. A previous EPA analysis estimated that would cost $90 billion by 2020.

Cost estimates for the proposal assume compliance by 2025 for most areas though some, especially in California, would be given more time due to their high ozone levels.

"We are concerned that EPA did not include 60 ppb in the range, though it was the clear recommendation of independent scientists as well as health and medical societies," said Harold Wimmer of the American Lung Association.

The EPA's independent science advisers this year recommended the administration set the standard at 60 to 70 parts per billion, and urged the agency to consider the lower end of that range.

Ozone, a gas with three oxygen atoms, protects the earth from harmful ultraviolet in the upper atmosphere. At ground level, it's formed when nitrogen oxides and volatile organic compounds are released from things like auto tailpipes and coal-plant smokestacks and then are "cooked" by sun. The gas, which is most problematic during hot, dry summers, causes respiratory and cardiovascular ailments including asthma, chest tightness and low birth-weight babies.

Staff writer Tony Kennedy contributed to this report