Locked up at home nice and tight on a chilly January night. Take a deep breath.
But what exactly are you breathing?
Studies show indoor air can be more polluted than outdoor air, as much as five times more. And even a "leaky" house - one not so tightly sealed against escaping heat and incoming drafts - can have contaminants in the air.
Indoor air quality can be compromised by deteriorated insulation, molds growing in a wet basement, chimney backdrafts, improper ventilation, and air leaks in ductwork or other household systems. Add to that chemicals in household cleaners and sprays, fumes from new carpeting, stain-treated furniture, pests and pet dander, tobacco smoke and on and on.
Some airborne pollutants are more troublesome than others. Cleansers can irritate and poison. Molds, dust and mites can trigger allergies and asthmatic reactions. Odorless, colorless gases can kill.
However, experts say proper ventilation, air filtration and control or elimination of the source of potential problems work together to improve indoor air quality.
"We recommend that a home be maintained, dry, clean, ventilated, contaminant-free, safe and pest-free," said Rebecca Morley, executive director of the National Center for Healthy Housing in Columbia.
Homeowners seek cleaner air for any number of reasons - health, environmental and more.
Nobody is sure what's in their home's air - even in a well-maintained home and even if there's no musty, sour, dank or irritating odor.
Gases you don't smell - carbon monoxide and radon - can kill, said Morley. She advised having professionals test for carbon monoxide when servicing furnaces.
Radon, the nation's second-leading cause of lung cancer, is a naturally occurring but radioactive product of uranium breakdown, residing in rock and soil. Dissipating outdoors, it is generally at a harmless level. But if it migrates indoors, it can build up and expose occupants to dangerous concentrations.
Homes should be tested and, if needed, retrofitted to remove it, Morley said.
Many household furnishings, such as cabinets and synthetics, contain formaldehyde and other volatile organic compounds or VOCs, health hazards characterized by a "new" smell. Experts recommend leaving the product outdoors for a time and ventilating the indoor area well, or buying VOC-free products.
Common cleaners and similar products are irritants, some hazardous, said Dr. Hidayat Khan, a Columbia allergist.
"It is better to keep indoor use of chemicals to a minimum," he said, adding that frequent, good damp mopping and cleaning is effective in keeping ordinary dirt as well as potential allergens such as dust mites at bay.
Whatever products are used in a home, and whether or not anyone inside has breathing ailments or allergies, proper ventilation is crucial.
Good ventilation starts with an air exchange in which the indoor air is replaced with outdoor air about every three hours, preventing indoor pollutants from building up, experts say. Humidity should hover around 40 percent. A tropical rain forest atmosphere in the house encourages mold and other unsavory things, but desert-like air is unhealthy, too.
Running an exhaust fan that is vented to the outside for at least 10 minutes after a shower is a good way to lower humidity. The amount of moisture in a home can be raised by adding a whole-house humidifier. Another useful tool for homeowners is the furnace filter. Good air filtration goes hand in hand with good ventilation.
"There is no alternative to your filter system. You are not going to be pulling in the air you should be pulling in," said Tom Caviness, owner of Comfort Solutions in Baltimore. "If you don't have filters, you get dust, and dirt and skin cells and dander from your pets blowing around the house."
But filters are effective only when changed as needed, Caviness said. He advises customers to be prepared to change a filter every 30 days, more often if it's clogged by then, less often if it's still clean.
Air quality, aided by cleanliness in general, gets a real boost from frequent vacuuming. That's because carpets are a haven for just about everything.
Dust mites, microscopic critters that dine on skin cells humans shed, thrive in pillows, mattresses and, of course, carpeting and are the cause of many allergic and asthmatic reactions.
"Stuffed animals are dust mite condominiums," said Dr. Mary Beth Bollinger, associate professor of pediatrics at the University of Maryland School of Medicine and an allergy specialist.
She recommended battling dust mites by washing bedding in hot water weekly, dusting with a damp cloth, damp-mopping floors and reducing clutter. "And frequent vacuuming when the [allergic] person is not there. Vacuuming can stir things up," she said.
Sometimes people wanting energy savings end up with better air, too.
"A lot of people come in and say they want to improve their energy use. Improved air quality comes along for the ride," said Peter Van Buren, president of TerraLogos Green Home Services in Baltimore.
The company specializes in home energy audits, and Van Buren said as a practical matter, comfort, health, environment and utility savings are often a package.