WAUSAU, Wis. – An explosion in the number of outdoor wood-fired boilers to heat homes is creating a new air quality problem: Their smoke is choking neighbors.

Over the past three years, the Wisconsin Department of Health has received nearly 200 complaints related to thick smoke from the stoves that look like tiny sheds or old-fashioned outhouses with a short smokestack.

Some local governments are passing laws regulating the boilers, essentially banning them. Two years ago, Wisconsin’s top environmental official urged the U.S. Environmental Protection Agency to take action. Several states in the Midwest and Northeast have looked at the issue, too, after they started turning up in more urban areas.

“It was bad, just choking smoke,” said Karl Wojtalewicz, describing the problem that his business, The Wellness Spa near Whiting, experienced. “Somebody called the cops a couple of times because you couldn’t see the highway.”

He’s baffled as to why there isn’t more regulation of the boilers, which some describe as little more than a steel box with water circulated around it and a smoke stack.

Six states – Wisconsin, Michigan, New York, Pennsylvania, Ohio and Minnesota – have at least 10,000 wood boilers, said Paul Miller, deputy director of Northeast States for Coordinated Air Use Management, which represents air pollution control agencies in eight states, including New York.

The group estimates that 500,000 outdoor wood boilers – emitting nearly 900,000 tons of fine particulate matter – could be installed nationwide by 2010, more than double the number now in use.

About 27,000 wood boilers have been installed throughout Wisconsin in the past decade because of skyrocketing fuel prices, experts say. Some burn barely 50 percent efficient and some were installed too close to neighbors, critics say.

“They tend to smoke a lot,” said Neil Baudhin, an air quality supervisor for the state Department of Natural Resource. “If you can smell it, your nose is detecting some kind of chemical.”

Some people have complained that illegal materials, including trash and tires, get burned, Baudhin said.

Jerry and Jean Blenker installed a wood boiler at their rural Athens home about a decade ago, saving “thousands and thousands of dollars” in heating costs. The family cuts its own wood.

A neighbor who lives about a half-mile away complained, causing state regulators and police to check whether something other than wood was being burned, Jerry Blenker said.

“It is just smoke and I am allowed to burn firewood,” he said. “I am real happy with it.”

In an hour, an outdoor wood boiler can emit as many tiny particles of pollution as 1,800 natural gas furnaces, Miller said.

Courtney Welch, a policy analyst for the National Conference of State Legislatures, said three states – Connecticut, Maine and Montana – have passed laws regulating wood boilers, including setting emission standards. New Hampshire, Vermont and Rhode Island have pending legislation.

The American Lung Association of Wisconsin is exploring whether new laws are needed in Wisconsin, said Dona Wininsky, a spokeswoman.

“We have got calls from asthmatics saying they have smoke pouring into their homes 24-7. The problem is they are just so unregulated and they are appearing in a lot of these newer subdivisions where people are closer together,” she said.

Two years ago, then-DNR Secretary Scott Hassett wrote the EPA urging a national strategy. But no mandatory rules are under consideration with the agency.

Leslie Wheeler, a spokeswoman for the Hearth, Patio and Barbecue Association, a Virginia-based group that represents some manufacturers, said complaints about smoke are a black eye for the industry, which has worked with the EPA on voluntary standards so the boilers, which typically cost $6,000, burn cleaner.

“If they want to stay in business, they better try and meet these standards,” she said. “Eighteen months ago, there wasn’t a furnace that met the voluntary standard. Now, there are at least two that do.”

But the voluntary standard is too weak, according to Northeast States for Coordinated Air Use Management. The group recommends a two-phase standard that initially would lead to 25 percent fewer emissions than the EPA’s voluntary standard allows and eventually 50 percent fewer, deputy director Miller said.

“These things smoke because they are incredibly inefficient,” he said.

Central Boiler of Greenbush, Minn., a leading manufacturer of outdoor wood boilers, promotes them as environmentally friendly because they burn wood, a renewable resource, Vice President Rodney Tollefson said. A homeowner can recover the investment within five years, he said.

The company advises buyers in densely populated areas to make sure the height of the smokestack exceeds the roof lines of homes, he said. “I have never seen a situation where they can’t have a chimney high enough to resolve the situation.”

But governments are acting as more of the wood boilers appear on the landscape.

About a year ago, Lake Mills, a city of about 5,300 people in southern Wisconsin, passed an ordinance that essentially banned them by regulating the space needed, city manager Steve Wilke said.

“If every house had one, you would have so much smoke you couldn’t breathe,” he said.

Robert Lins and his wife, Meredith, of rural Cambridge, near Madison, spent one winter putting up with smoke from a wood boiler, which was installed about 200 feet from their home.

“I had pretty chronic bronchitis. I had headaches,” the 64-year-old retired doctor said. “A couple of nights we had to leave and go sleep at my father’s.”

The neighbor eventually sold the home, removed the boiler and moved to the country, Lins said.

The family told the Linses they heated their home for $14 a month.

“That was pretty impressive. But they are just not very healthy,” Lins said. “They don’t belong in any place where a person doesn’t have enough acreage to take care of the smoke.”