MISSISSIPPI STATE -- When cold weather arrives, human nature is to close up the house and turn on the heater, but sometimes air quality loses in the battle to stay warm.

Dr. Frances Graham, housing specialist with the Mississippi State University Extension Service, said moisture, pets, smoking, molds and carbon monoxide affect indoor air quality.

"The key to healthy indoor air is proper cleaning of the home, adequate ventilation and maintenance of the heating system," Graham said. "Poor indoor air quality can lead to many respiratory problems, sickness and even death in extreme cases."

Carbon monoxide is an odorless gas that interferes with the blood's ability to deliver oxygen to the body. It is a major pollutant given off by wood and gas stoves, fireplaces and gas space heaters. At low concentrations, it can cause fatigue, headaches, dizziness, nausea and confusion. At higher concentrations, it can cause death.

"Indoor heating systems need not present a threat to health," Graham said. "The key is proper maintenance and use of the systems, along with adequate ventilation."

Inspect and repair air-handling systems, including chimneys and furnaces, and have a professional tune-up the central heating system annually. Allow adequate ventilation in rooms using gas space heaters by keeping the room's door open and a window slightly ajar.

Airtight houses with combustion heaters are especially at risk from carbon monoxide poisoning if they don't have an adequate source of outside air.

"Chimney backdrafting can occur as a house becomes depressurized from exhaust fans or competition for air among combustion appliances," Graham said. "For example, a kitchen exhaust fan can draw enough air out of the house to overpower the draft in a fireplace chimney, drawing poisonous gases back into the house."

Carbon monoxide detectors can provide some warning of dangerous gas levels, but Graham warned that people should not depend solely on the detector.

Other threats to indoor air quality are year-round. Mold and mildew often grows in bathrooms or dark, humid places. Other than being an unsightly nuisance, these molds can cause difficulty breathing or asthma attacks.

A few molds can be toxic. Infants and children are at the greatest risk as their respiratory systems are not completely developed. Eliminate molds and mildews by ensuring all areas of the house are clean and dry, and by cleaning infected areas with a bleach and water solution.

Dust mites pose another respiratory threat. To reduce exposure to these mites, wash bedding at least once a week, remove bulky curtains and extra rugs, keep only washable stuffed toys, and dust and vacuum regularly.

House pets can be a problem if owners are allergic to them. The most common pet allergen is hair, but pet dander can be a problem. Take steps to prevent pets from lowering indoor air quality. These include bathing the pet often and grooming it outside, cleaning the house regularly and not letting the pet sleep in the room with the allergic person.

Smoking also does major harm to indoor air quality. In addition to the cancer-causing nature of cigarette smoke, it can cause respiratory problems in those who breathe it. To avoid contamination of indoor air, do not smoke indoors or around those with respiratory problems.