It's a classic winter challenge: We head indoors to stay warm and batten down the hatches to rein in wickedly high fuel costs. But by shielding ourselves from the brutal elements, we inhale air that is not only stale but downright dirty.

Ironically, the more energy-efficient we make our homes, the less they "breathe." Tight seals, excellent insulation and pollution from inside and out make our indoor air up to 100 times dirtier than the haze outdoors, according to the Indoor Air Quality Association.

Luckily, we can take simple, inexpensive steps to clear our indoor air:

Check the basement for moisture, pipes for cracks, and drains and gutters for clogs

Wherever you spot water accumulating, fix the problem. Leaked water generates mold. "Mold is an important source of allergens and, in rare instances, can cause pulmonary mycotoxicosis," a type of poisoning, says Homer Boushey, M.D., a pulmonologist and past president of the American Thoracic Society. A little bit of gunk on your bathroom grout is probably not going to hurt you, but big colonies need to be killed off. Once a mold infects the lungs, "it's very difficult to treat," warns Kent Pinkerton, Ph.D., a pathologist who specializes in environmental health.

Change or wash your furnace filter at least once a month

Don't skimp by buying cheap filters with wide-gauge screens. Instead, read the label and select a high-efficiency electrostatic filter such as the 3M Filtrete, $6 to $20, Lowe's, Home Depot and Target, or a High-Efficiency Particulate Air (HEPA) filter, which screens out at least 99.97 percent of very small particles. Test the effectiveness of your furnace filter by removing it and pouring sand on it. If the sand goes through, it's junk, says Glenn Fellman, executive director of the Indoor Air Quality Association.

Air out your house!

Try to open up the windows each day for 10 minutes to let lung-plugging particles escape, suggests Pinkerton. If your roast becomes toast, chase out smoke by opening two opposing windows. Put a fan (vented outdoors) in one of the windows. This creates a wind-tunnel effect that wicks out the bad air and sucks in the good, explains Paul McRandle, senior research editor for The Green Guide, a New York-based environmental consumer publication.

Clean up, already!

"One of the most important things you can do is just keep your house clean," says Susanna Von Essen, M.D., a pulmonologist and professor at the University of Nebraska Medical Center in Omaha. Get rid of all dust-collecting clutter, which is a magnet for dust mites and dirt.

And reassess wall-to-wall carpeting. "Carpets are a really good sink for particulates, especially cat dander and house dust mites. Every time you walk across them, you send these things up into the air," says Dr. Boushey. Life can be made simpler and healthier by letting floors go naked and damp-mopping at least once a week. Increase vacuuming frequency, too, until sneezing and wheezing abates.

Don't burn stuff indoors

Never, ever, allow anyone to smoke in your home. Also, ban incense and cut back on crackling fires. Fireplace fires don't save money, anyway. "It feels warm next to the fire, but lots of warm air from your house is being sucked right up the chimney," says McRandle.

Don't fall for questionable gadgets

Most mechanical air cleaners do a crummy job of removing dust, smoke and other pollutants, according to Consumer Reports. Their researchers found that the cleaners that worked best were big models designed to purge pollutants from the whole house. But those are costly and require professional installation and yearly maintenance. Steer completely clear of all "ionizing" models, which produce ozone, an infamous asthma aggravator and lung irritant. The theory behind air cleaners seems sound, "but they haven't been shown to make any difference for people with lung disease," says Dr. Boushey. "It's much better to cut down on the production and source of pollutants" than to try to clear the air.