When perusing houseplants for sale at a nursery or garden center, you see a few labels promoting their ability to improve air quality. Get a few long-lasting plants for their home, they suggest, and you'll be breathing easier.

This seems like a simple enough promise. Plants will take in carbon dioxide and expel oxygen, so it seems natural that they'll be able to freshen the air in your rooms. But there have also been suggestions that plants act as air purifiers, absorbing harmful airborne toxins.

The idea that houseplants can work as air filters stems from a 1989 NASA study, which sought to determine whether plants could help clean the air inside space stations. Researchers looked at 29 different plants and found that they were all capable of removing trace amounts of volatile organic compounds such as ammonia, benzene, formaldehyde, trichloroethylene, and xylene.

Ever since that study, the suggestion has persisted that houseplants are an easy way to improve indoor air quality. Danielle Blundell, writing for This Old House, says the chemicals in the report are commonly found in homes. For example, ammonia is found in household cleaners while formaldehyde is used in hardwood floor resins and some pressed wood products. The University of Minnesota Extension says the ability of plants to absorb certain pollutants is especially useful in energy-efficient homes, where volatile organic compounds are more likely to be trapped inside.

Though indoor plants can be a beneficial addition to the home, it is also easy to overstate their effect on air quality. In some circumstances, the plants may even promote allergens in the home.

The key reason that the NASA study does not translate well to earthbound homes is that planetary conditions are significantly different from those on a sealed space station. The American Lung Association says the study was based on a small sample size, and that the benefits from houseplants will vary based on factors such as sunlight and temperature. Steve Asbell, writing for the real estate site Zillow, notes how the NASA study recommends activated carbon filters as an "integral part" of any effort to use houseplants to improve air quality, since the best results occurred when fans pulled air through a charcoal filter in the soil.

The University of Minnesota Extension suggests that 15 to 18 "good-sized" houseplants can help improve air quality in an 1,800-square-foot home. Blundell suggests having at least two plants for every 100 square feet.

However, a 1992 analysis by the Environmental Protection Agency found that an excessive number of houseplants would be needed to have the same benefits shown in the NASA study. This review determined that a "typical house"—about 1,900 to 2,000 square feet in that year—would need 680 houseplants to achieve the same pollutant removal rate.

Plants can even have a detrimental effect on a home's air quality if you aren't careful. The American Lung Association says some types will emit volatile organic compounds of their own. A 2009 study by the University of Georgia Department of Horticulture used several of the same plants in the NASA study, as well as charcoal filters. The researchers found that some pollutants produced by the plants were a result of pesticides applied to them, while others were put off by microorganisms in the soil.

Soil conditions can affect air quality in other ways as well. The EPA says overwatering can cause the growth of mold and microorganisms, which may negatively impact homeowners with allergies.

Even if houseplants aren't a miracle method of improving indoor air quality, they can still bring beauty to your home. You can also take steps to avoid inadvertently fouling your air through indoor gardening. Asbell recommends removing dead or diseased leaves, avoiding pesticide whenever possible, watering only when the top half-inch of soil is dry, dusting off leaves, and placing a saucer under each plant.

You can also look at other methods of improving air quality beyond houseplants. The EPA says these include both natural ventilation, such as opening doors and windows, and mechanical ventilation such as HVAC systems and kitchen or bathroom fans. Efficient air cleaners with a high air circulation rate can also be effective.

The American Lung Association recommends that homeowners avoid smoking indoors and keep humidity levels under 50 percent. You should also test your home for radon and take steps to remediate this gas if its levels are too high.