The quality of the air we breathe indoors becomes even more important in the colder months, when we naturally spend more time inside. But the list of things that pollute that air is a long one, featuring such varied culprits as combustion sources (oil, gas, kerosene, coal, wood and tobacco), building materials (deteriorated insulation, damp carpet), household cleaners, even our furniture and carpets.

The fault line: Problems associated with indoor air quality have increased since the late 1970s and early 1980s, when builders started adding large quantities of insulation to new houses to reduce energy costs without balancing it with adequate ventilation to exchange polluted indoor air with fresh outdoor air. Excessive moisture and high temperatures created mold issues. Carbon monoxide from gas stoves, dryers and furnaces, if not properly vented outside, can cause illness and even death.

"In return for saving energy, many of us aren't feeling that well," says Tim Burke, strategic-marketing manager of the White-Rodgers division of Emerson Electric Co., which makes air cleaners and humidifiers. Studies have shown that indoor air is five times more polluted than outside air, he says, and that asthma has been on the rise for the last 15 to 20 years.

The foe: "The particles that you can see floating in the air in the sunlight are not 'bad dirt,' " says Burke. "It's the small stuff you can't see that is more of a concern from a health standpoint, and you need to remove as much of it as possible."

The objective: Improving indoor air means filtering it to remove those small particles, adjusting the temperature to create a comfortable environment, and getting humidity levels right so the air isn't too wet or too dry, Burke says.

The ideal humidity range in the northeastern United States is 40 percent and 60 percent, based on tests conducted in Pennsylvania about 20 years ago, he says. Below or above that range, even in the driest air, mold and microbes can thrive. Temperature, of course, is a matter of personal comfort; strive for consistency, rather than creating hot or cold spots throughout the house.

The obstacles: In winter, heat tends to dry out indoor air, and if moisture isn't introduced to compensate, the result can be dry skin and popping drywall nails and screws. "Although we test homes extensively before we recommend solutions, I can tell what's wrong simply by looking at the pops or cracks in the crown molding, as well as dirt and dust caked around the return registers," Burke says. "Dust and dirt migrates through the home through ducts quickly, and humidity levels are consistent throughout."

The remedy: Take a whole-house approach to indoor air quality, the same as you might for indoor comfort after an energy audit, Burke advises. (Remember, too, that everyone has a different reaction to indoor pollutants; the odor of glue used in boxes or carpeting might make one person sick and not affect anyone else.)

If a house is too tight, consider installing ventilation-recovery units to promote regular indoor-outdoor air exchange. Set dehumidifying and humidifying units to work in concert rather than against one another. Look for building products with low or no volatile organic compounds to release into the air. Install thermostats or try to zone your heating system to ensure that every part of the house is comfortable and energy efficient. And make sure gas-fired units are regularly serviced and cleaned to ensure proper combustion.