In the Blue Ridge mountains, Jacquie and Eric Thurston fear a wood preservative used in their custom log cabin is filling the home with fumes linked to cancer, birth defects and blood disorders. They wonder if their dream house has become a health nightmare.

They built the cabin themselves, Eric cutting the poplar trees and treating them with a chemical called pentachlorophenol. Now, nearly a decade later and after learning last summer that they may have exposed themselves and their two daughters to a dangerous substance, the couple frets over the possible long-term health effects the preservative might hold.

The Thurstons are not alone. Across the United States -- from trendy town house to tenement row house, from Wall Street to K Street -- experts say there is a growing stack of evidence that shows Americans eat, sleep and work in structures that may be hazardous to their health.

In inner-city apartments, families whose only source of winter warmth is the kitchen's unvented gas range will fight the chill but choke their rooms with smog-like vapors resembling the air above a crowded Los Angeles freeway.

In high-rise office complexes, workers can grow groggy and suffer headaches from breathing a witches' brew of ozone and chemicals from copying machines, exhaust from underground parking and smoke from cigarettes.

And as cold weather forces people to seal doors and windows, scientists warn that the air at home may be anything but sweet.

"The greatest exposure to pollutants, for the bulk of the American population, is indeed indoors," says Harvey Sachs, a consultant to the National Indoor Environmental Institute. The nonprofit company, part of Emergency Care Research Institute, specializes in studying and treating the myriad causes of indoor pollution.

A study released recently by the Environmental Protection Agency pointed out that indoor concentrations of 11 toxic or cancer-causing chemicals were often 100 times greater than normally found outdoors. And the researchers found that between 60 and 98 percent of the people exposed to the compounds during the day -- at the dry cleaners or the gasoline station, for example -- later showed traces of the chemicals on their breath.

Because Americans typically spend between 80 and 90 percent of their lives in homes or offices, the extent of the hazard has become a national concern, in part because it adds, according to government estimates, between $15 billion and $100 billion yearly to the cost of U.S. health care.

The root of the problem is nontechnical: Americans started shutting off the flow of fresh air at the same time they began filling their buildings with synthetic compounds and materials that give off vapors of hundreds of chemicals.

The indoor pollution problems that blew in with the winter winds and spiraling heat costs of the 1970s -- when homeowners by the millions ran scurrying for storm windows, insulation and wood stoves -- is now a full-blown epidemic, public health officials warn.

"You can conserve too much energy, and you pay for it in the long run," James Repace, an EPA analyst, says of the current trend to turn buildings into Thermos bottles.

"The air will get so dirty there will be acute and chronic health effects. People will get sick."

Repace and Sachs agree that tighter buildings trap germs, smoke, dust and a host of chemical vapors from common products like cleaning fluids, aerosol sprays, formaldehyde insulation, mothballs and paints.

Other causes are more exotic -- and revolting. Scientists with the Consumer Product Safety Commission say the bodies of millions of cockroaches decaying in the walls of urban apartment buildings can form clouds of gas potent enough to trigger severe allergic reactions in some people.

Concern over indoor pollution, however, lay mostly in scientific circles until the early 1980s, when the controversy surrounding urea-formaldehyde foam insulation captured the public's attention. Now, with health-conscious consumers more aware of the hazards lurking indoors, Sachs and other environmental specialists find themselves in what's likely to be a growth industry for decades to come.

"Our job is to diagnose the cause and write a prescription to solve the problem," he says.

The list of possible indoor pollutants is almost endless: a recent federal study identified 150 chemicals that commonly contaminate homes -- 15 times the number typically found outdoors. While many cause only mild allergic reactions like sneezing or watery eyes, exposure to more toxic vapors can trigger irregular heart rhythms, dizziness and shortness of breath, among other things.

And simply by inhaling the vapors of formaldehyde, insecticides, benzene or asbestos dust even at levels of a few parts per million, a person may be exposing himself to known or suspected causes of cancer, as well as risking genetic damage or injury to the unborn.

Other compounds can damage the liver, lungs, kidneys or heart. And modern science knows virtually nothing about what happens when a person breathes a combination of these chemicals.

Here are some common culprits:

* Unvented gas stoves. Although gas burns with a relatively clean flame, it puts off enough carbon monoxide and nitrogen dioxide to quickly fill a kitchen. Carbon monoxide restricts the blood's ability to carry oxygen, and nitrogen dioxide can be toxic. Exposure to even relatively low concentrations can blur vision, impair coordination, cause angina pain or fill the lungs with fluid. Studies have linked an increased number of respiratory infections in children to homes with gas stoves.

* Wood-burning stoves and fireplaces. Besides producing carbon monoxide, nitric oxide and nitrogen dioxide, these popular heat sources spew out tiny particles of ash, some of which cause cancer. Don't burn treated wood -- its smoke is toxic. And kerosene heaters can emit sulfur dioxide, which can lead to asthma attacks even at very low concentrations.

* Aerosol sprays, including insecticides, cleaning and health care products. Labels help identify possible dangers, but these products literally fog a home's air with chemicals, some of them dangerous.

* Paints, varnishes and solvents. Bring in plenty of ventilation when using them indoors. And keep these products tightly sealed when stored, if possible in a ventilated garage or storage shed. The same applies for many craft materials.

* Formaldehyde. Prospective home buyers should ask about the type of material that insulates a residence -- although urea-formaldehyde foam, called UFFI, doesn't always contribute to higher indoor concentrations. And look carefully at mobile homes, some building materials (principally particle board), furniture, carpets and draperies.

* Asbestos. A mineral once widely used in floors, ceiling tiles and pipe insulation, asbestos can cause lung disease. It's especially common in older buildings, so be careful when renovating or remodeling -- these projects can disturb the fibers and release dangerous dust into the air.

* Radon. This radioactive gas is linked to lung cancer. It occurs naturally in the earth and can be found in stone, cement, concrete or bricks. Patch any cracks in basement walls or floors and be wary of areas in the home with poor ventilation, such as crawl spaces. EPA's Repace describes radon exposure as the most dangerous of all indoor pollution hazards.

* Microorganisms. These germs, fungi and molds can cause allergic reactions, respiratory infections and other illnesses. The higher humidity of tightly sealed homes only increases conditions on which these living dangers thrive. Consequences can be deadly -- this type of indoor pollution was responsible for the fatal outbreak of Legionnaires' disease in 1976.

* Tobacco smoke. The smoke of cigars, pipes and cigarettes sends literally thousands of chemicals spiraling into the air. Smoking in cars or other confined spaces is especially harmful. There are good reasons for nonsmoking sections, but they work best if vented separately.

So how can someone tell if the house or office is a threat to health? "Use your nose," Sachs says.

"Use some common sense," Repace advises.

Lingering cooking or other odors and a general sense of mustiness can be initial clues your building isn't getting enough fresh air. Chronic allergies or other unexplained illness can be another. If the home seems too humid, it could mean it needs better air exchange.

Look around. Have materials that could produce chemical vapors been storied in the house? Removing pollution's source is a sure remedy.

And before deciding to seek professional help, you might want to try Sachs' simple solution: "Open your windows." Checking Your House

"We all wish there were a little machine we could hook up to tell us if the air is good or bad," says indoor pollution expert Harvey Sachs. Unfortunately, identifying what ails our homes or offices isn't that easy. Sometimes it's expensive.

The current charge for bringing in a team of technicians from the National Indoor Environmental Institute is $400 per day per person. Plus expenses. And the fees don't include fixing the problem.

So what can a typical solution cost?

Here are some ballpark figures and other tip:

* A formaldehyde analysis costs about $225. An institute scientist will inspect for urea-formaldehyde foam insulation, measure air levels and issue a certificate if concentrations fall within safe limits.

* Costs are about the same for a radon investigation, gauging ventilation rates and pesticide analyses.

* Be prepared to pay at least $550 for a general "environmental investigation."

* Installing an electric stove is usually cheaper than venting a gas model.

* Most homes don't need high-tech air-to-air exchangers, machines that ventilate a building without losing heat but cost as much as $3,000. "They're rarely needed in conventional houses," he says. But if you build a new, super energy efficient home, install a system to bring in fresh air.

* It can cost as much as $15,000 to remove urea-formaldehyde foam insulation from a house. And publicity surrounding the possible health risks has made some UFFI-stuffed homes difficult to sell.