FREEPORT — Fire chiefs and lawmakers are working to protect the system of volunteer firefighting that has served rural America for more than a century but is threatened by an ambiguity in President Obama’s health care law.

Small and rural fire departments from California to Maine, which has one of the country’s highest percentages of volunteer and on-call firefighters, rely on volunteers to avoid the cost of paying them to be on duty in between fighting fires.

The volunteers are considered employees for tax purposes, a classification that grew out of an ongoing effort to attract firefighters by offering them such incentives as stipends, retirement benefits and free gym memberships.

That leaves open the question of whether the volunteer firefighters fall under the health care law’s requirement that employers with 50 or more employees working at least 30 hours a week must provide health insurance for them. Fire departments say they can’t afford to pay for insurance.

“Most of these are operating on a shoestring budget — holding pancake dinners to raise money to put enough gas in the truck so they can respond to the next fire, the next medical call,” said Dave Finger, director of government relations for the National Volunteer Fire Council.

Faced with the cost of insurance, or being fined if they fail to provide it, departments would likely be forced to reduce the number of hours that firefighters can volunteer or to eliminate the benefit programs, officials said. That has both fire chiefs and lawmakers raising concerns.

Darrel Fournier, fire chief in Freeport, said his department is bracing for what could be significant costs under the health care law. He expects he’ll have to provide coverage for the five firefighters he employs part time. That would cost the town — and ultimately taxpayers — about $75,000, or a penalty of $150,000.

Additionally, in a busy winter with lots of fires, emergency calls and accidents, he said his roughly 50 volunteers could work more than 30 hours a week, meeting the threshold under the law that would require him to provide health insurance for them as well.

To avoid the penalty, Freeport could cut back on the number of hours that part-time and volunteer firefighters have to work. But that would mean finding more volunteers to make up the difference, something the department and others across the country already struggle to do, Fournier said. When he started in Freeport in 1972, there was a waiting list of 25 people. Now, after three months actively recruiting in the community, Fournier said he’s lucky that he’ll soon be interviewing nine potential volunteer firefighters.

“It’s pretty amazing how this law is touching different operations,” he said in an interview in Freeport’s brick firehouse, where yellow fire trucks and ambulances were lined up awaiting the next call. “I’m not sure everyone thought that through.”

The question is expected to be answered when the Internal Revenue Service releases final regulations this year before the provision takes effect in 2015. A Treasury spokeswoman said the department is taking the concerns into account as it works toward the final regulations, but wouldn’t comment on what they’re likely to include.

In the meantime, Maine’s U.S. senators are backing a recently introduced bill aimed at ensuring that volunteer firefighters and other emergency responders are exempt from the health care law requirement. Republicans point to the confusion as another example of the problems with the law, which has been plagued by a fumbled rollout and criticism over canceled health care plans.

“This is yet another adverse and unanticipated impact of Obamacare,” said Maine Republican Susan Collins.

City and town leaders are also discussing ways to keep their staff under 50 employees to dodge the costs, Maine Municipal Association officials said. School superintendents are making sure their substitute teachers aren’t working more than 30 hours a week to avoid having to cover them, Collins said.

Others point to factors that they say show the concern is overblown. Many small towns only have about 20 to 30 volunteer firefighters, likely making them too small to be affected by the law’s mandate. And though Fournier described a busy winter with a lot of work for firefighters, they often fall short of working enough hours to be affected by the insurance requirement. Most volunteers also receive insurance coverage through their primary work as teachers, business professionals or city employees.

It’s too early to ring the alarm, concluded Trish Riley, an adjunct professor of health policy at the University of Southern Maine’s Muskie School of Public Service.

“With any major piece of legislation like this, there are bound to be unintended consequences,” she said. “But you can’t leap to conclusions before the federal government has the opportunity to address it.”

Finger said he’s confident the issue will ultimately be addressed either through the regulations or legislation and is urging fire departments to avoid taking any immediate steps. But he realizes that the political tension over the health care law means that even a small change can turn into a big fight.

“The tough part is, anything having to do with this law is just so partisan that it’s difficult to take action on,” he said.