Science is swiftly turning upside down the common notion that a fire built with wood is kinder to humans’ well-being than gas and other modern fuels.

From California to Sweden and China, researchers are reporting that wood smoke contains large amounts of harmful pollutants, including some of the same toxic chemicals found in cigarette smoke.

Those reports seem counter-intuitive. After all, wood is a natural substance, a heat source since prehistoric times and a seemingly safe alternative to dirty fossil fuels.

But natural does not necessarily mean harmless, and a growing number of published studies are associating wood smoke with asthma, other lung problems and heart disease — some of the same illnesses associated with smoking and with heavy exposure to car and truck pollution.

“Is it as toxic as something coming out of the tailpipe? We’re not sure yet,” said Robert Devlin, senior scientist in the environmental public health division of the U.S. Environmental Protection Agency in Washington, D.C.

But the new consciousness of wood smoke’s dangers is spurring scientific inquiries in communities across the West where wood heat is popular: in California, Montana, Idaho, Seattle and British Columbia.

Wood consists largely of two relatively harmless ingredients, cellulose and a strengthening substance called lignin.

If the wood burned completely, it would turn into simple water and carbon dioxide. But instead it forms what scientists call “products of incomplete combustion” — thousands of chemicals, including certain toxic and carcinogenic substances.

Many of the same chemicals form during the burning of other organic matter — whether waste from orchards and rice fields or tobacco leaves wrapped up in cigarettes.

That is why some scientists compare wood smoke to second-hand smoke and cigarette smoke.

“It’s not the nicotine in cigarette smoke that kills you. It’s the other stuff,” said Kirk R. Smith, a professor of global environmental health at the UC Berkeley School of Public Health who has studied smoke’s health effects around the world.

“The worst thing that you can do with this stuff is stick it in your mouth,” Smith said. “The next worst thing is to have it in your house. The next worst thing is to have it in your neighbor’s house.”

Researchers can rattle off long lists of dangers in wood smoke.

They often focus on the tiny particles — a mere fraction of the width of a human hair — that can lodge in tissue and blood vessels and disrupt lung and heart functions. Some are so small that they can pass right through the walls of blood vessels. Wood smoke also contains well-known cancer-causing chemicals such as benzene and formaldehyde.

Scientists have published dozens of studies on the human health effects of wood smoke. In 2007, a 40-page review of those studies in the journal Inhalation Toxicology concluded, “It is now well established … that wood-burning stoves and fireplaces as well as wildland and agricultural fires emit significant quantities of known health-damaging pollutants, including several carcinogenic compounds.”

Today, most U.S. regulators focus largely on the fine particles in wood smoke to measure its potential for harm, rather focusing on its cancer-causing ingredients as they did with tobacco smoke a generation ago.

The same is true in California.

“The main issue is that it has particulate matter. When it comes to particles, we treat all particles the same. We feel that all particulate matter is bad for you,” said Linda Smith, head of the health impacts section at the state Air Resources Board.

International public health officials have gone further. In 2006, wood smoke was labeled “probably carcinogenic in humans” by the International Agency for Research on Cancer, part of the World Health Organization.

But the 2007 journal review concluded that it was too early to formally link wood smoke and cancer, and that more research is needed.

An American Cancer Society advisory group recommended several years ago that the society not take a position on the issue, deciding “that the evidence linking wood burning smoke to cancer was much weaker than that for heart and lung disease,” the society’s statistics director, Kenneth M. Portier, wrote in an e-mail note.

Some activists believe California should act more aggressively and treat wood smoke just as it does second-hand smoke or fumes from diesel-burning trucks. The state has classified both “toxic air contaminants.”

Wood smoke deserves the same label, said Jenny Bard, regional air quality director for the American Lung Association in California.

“We’re going after tobacco smoke in all sorts of ways,” Bard said. “We’ve banned it from workplaces and restaurants. And the exposures to wood stove pollution can be so much more concentrated in localized situations.”

This story is a result of a partnership between the Center for Health Reporting and the Chico Enterprise-Record.