In many homes, the room with the worst air quality is the basement. Not only is the air more contaminated than anywhere else in the home, but basement air rises into the rest of the house, spreading moisture along with mold, mildew, combustion gases, volatile organic compounds and other contaminants that can damage health.

While the number of single-family homes with full or partial basements is declining nationally, in colder regions most new homes still have basements. In New England, for example, 87% of new single-family homes built in 2013 had full or partial basements.

For homeowners with basements, here are four air-quality dangers lurking down below, and what to do about each:

Danger #1 — Moisture, mold, and mildew

Moisture in the basement leads to mildew and mold, and the best way to prevent their growth is through moisture control. Unfortunately, a damp basement is common. The sources of moisture include infiltration of rainwater or groundwater through the concrete foundation, interior sources such as bathrooms and unvented clothes dryers, and humid air from outside that condenses on the cooler basement surfaces.

How to prevent moisture, mold and mildew

• If moisture is leaking in to the basement, re-grading of earth around the exterior of the house may be necessary.
• Check to make sure all downspouts and gutters are discharging water at least four feet from the exterior wall.
• If a basement bathroom is unvented, consider adding a vent and fan that exhausts to the outside.
• A dehumidifier will help reduce moisture in the basement year round.

Danger #2 — Radon

Radon is a naturally forming, colorless, odorless and tasteless gas that breaks down into radioactive elements that attach to airborne particles. Radon enters a home through cracks in the floor or walls of the basement or foundation. As many as 21,000 Americans die every year from lung cancer related to radon exposure, according to the U.S. Environmental Protection Agency.

What to do about radon in the basement

• Every home should be tested for radon and, if necessary, a qualified radon mitigation contractor should be hired to assess radon levels and perform any necessary remediation work.
• Consider using a high-performance air purifier such as the IQAir HealthPro® Plus in the basement. Research by the American Association of Radon Scientists and Technologists showed that air filtration could reduce radon significantly.
• Air filtration should not be considered a substitute for radon remediation.

Danger #3 — Carbon Monoxide

Carbon monoxide (CO) is a highly toxic gas. It can be deadly at very low concentrations (300 parts per million). Carbon monoxide is a byproduct of combustion, so basement furnaces, water heaters and other combustion appliances can potentially be sources of carbon monoxide. Often, carbon monoxide is generated when an appliance is “backdrafting” as a result of a range hood or other high-power fan exhausting indoor air from the house upstairs.

What to do about carbon monoxide in the basement

• Prevention is the best strategy to prevent carbon monoxide from building up in your basement.
• Have appliances such as furnaces and gas water heaters checked regularly by a trained professional.
• Whenever possible, vent appliances to the outside of the home.

Danger #4 — Insulation

Whether a basement is finished or unfinished, insulation is necessary because as much as 25% of a home’s total heat loss can occur in the basement. But insulation can also present a threat to air quality and health, especially if it contains asbestos.

What to do about insulation in the basement

• In older homes, be aware that insulation may contain asbestos.
• If you suspect you have asbestos in the basement and it’s in good condition, the best thing to do is leave it alone.
• If repairs are needed, they should be performed only by a professional trained in methods for safely handling asbestos.
• Another potential problem with insulation is air leakage, also known as infiltration, which means outside air is leaking into the home. Air barriers such as drywall must be in direct contact with insulation or leakage will occur.
• Inspect and ensure that insulation is in direct contact with surfaces such as drywall or moisture can build up and heat is lost.
• Visit http://www.epa.gov/indoorairplus/technical/moisture for more information on construction specifications for managing moisture.

Whether it’s finished or unfinished, a basement is an important part of a home, affecting air quality both below ground level and in the rooms above. By recognizing the potential air quality dangers lurking in your basement, you can take action to keep the indoor air throughout your home clean and healthy.