The wood-burning fireplace warming the den is cozy, but it can belch as much pollution as an old diesel bus. That vintage wood stove cranks out heat, but it can contribute to asthma attacks in the neighborhood, or even inside the house.

When fuel oil prices soar, as they have this year, more Mainers turn to wood heat. But wood smoke contains fine particles that can cause health problems. In some locations, especially river valleys, residential wood smoke is a major source of winter air pollution.

Now the U.S. Environmental Protection Agency is preparing regulations meant to tighten emissions limits for new residential wood heaters. For the first time, these regulations will cover pellet stoves and outdoor boilers — heat sources that have become popular in Maine.

Also likely to be included are new indoor furnaces and cookstoves.

The proposed rules don’t involve any existing heaters. And new fireplaces, both factory-built and masonry, will continue to be exempt, at the urging of industry.

But the pending changes are noteworthy, because they cover most of the wood-burning devices common in Maine and represent the first substantial changes since the EPA set performance standards to “certify” wood stoves, 23 years ago.

The draft regulations were scheduled to be released next month, but have been pushed back until January. The compliance date for wood stoves and outdoor boilers is likely to be June 2014. Other standards will kick in by 2017.

The agency has been circulating details, which are being reviewed by interest groups.

Cleaner-burning technology could help reduce thousands of cases of asthma, respiratory symptoms and lost work days by 2017, according to EPA estimates.

But the standards don’t get at the biggest sources of wood smoke pollution: thousands of older wood stoves and alternative heaters still in use. An old wood stove, for instance, releases four times as many particles as a modern device, and because it’s less efficient, it burns more wood.

“Most of the stoves used in Maine and elsewhere are still pre-1990,” said John Crouch, public affairs director for the Hearth, Patio and Barbecue Association. “The old ones just don’t wear out.”

One solution developed by the EPA and Crouch’s group is to redirect money from air pollution fines to provide vouchers for residents who forfeit their old burners for new ones.

A program last year in Keene, N.H., used $106,000 to fund 86 change-outs. Residential wood smoke contributes a quarter of the winter air pollution in the town, which is flanked by mountains.

Ed Miller, vice president of health promotion at the American Lung Association in Maine, currently is working with a similar program in western Massachusetts. It provides vouchers for up to $3,000 to buy new wood, pellet or gas stoves. Miller wants to launch a similar program in Maine, but isn’t aware of any funding now.

A survey done by the association during the last heating oil price spike, in 2008, found a quarter of Maine households using wood as supplemental heat. One third of the stoves were more than 20 years old.

“Wood smoke is, in many ways, like second-hand cigarette smoke,” Miller said.

Overall, the new EPA proposals seek to strengthen emission limits to reflect today’s best available technology.

That shouldn’t be hard for certified wood stoves. The standards would cut fine particle emissions from a maximum 7.5 grams per hour to 4.5 grams for stoves using non-catalytic technology. Catalytic stoves would go from 4.1 grams to 2.5 grams. The tougher standards have been in place in Washington state since 1995.

Pellet stoves would have to meet the new wood stove standards, and the EPA estimates that two-thirds already do. In addition, the EPA wants certification tests for pellets, to assure mills are making premium fuel with clean-burning qualities.

Factory-built fireplaces, which make up 90 percent of all units, also came under scrutiny. Washington has had an emissions limit since 1995. Some communities, such as Denver and parts of California, ban new open-burning fireplaces altogether.

The EPA has a voluntary limit of 5.1 grams per kilogram of wood burned. Some manufacturers meet that standard now, but the EPA concluded that extending it to all prefab units isn’t cost effective.

Crouch’s group argued against a mandatory limit, saying most factory-built units aren’t used for serious heating.

“It’s more of a Sun Belt product,” Crouch said.

A major target of the new standards is outdoor boilers, technically known as hydronic heaters. Neighbors frequently complain about the smoke they generate. The heaters are subject to bans and regulations in some communities and emission limits in some states, including Maine.

Some manufacturers have voluntarily begun making heaters that meet the EPA’s proposed standards, which limit particulate emissions, carbon monoxide and visible smoke.

These changes are pending as the hearth industry recovers from a drop in sales during the recession, despite federal tax credits meant to create incentives. The trade group’s latest survey counted nearly 1 million units shipped nationally last year. Seven out of 10 units were gas. Cordwood heaters made up 25 percent of the total; pellets were 5 percent.

Roughly one-third of the wood- and pellet-stove owners use their units as the major heat source, lighting a fire an average of 120 times a year. Fireplace owners reported burning 52 times a year, the survey found.

The survey also determined that more than half of all U.S. homes have a fireplace or freestanding stove, and that fireplaces rank second among the features buyers want in new homes.

Beyond dirty wood burners, the EPA identified another obstacle to cleaner air.

Manufacturers can develop the best technology, but it only achieves the design potential when operated properly, with a hot fire fueled by dry, seasoned wood.

In a discussion of its draft options, the EPA notes that a typical prefab fireplace emits 12 grams of particulates per kilogram of wood. That level can be cut to 1 gram, the EPA says, if owners would close the glass fireplace doors.

Staff Writer Tux Turkel can be contacted at 791-6462 or at:

tturkel@pressherald.com