The U.S. Environmental Protection Agency reports that air in sealed-up houses, schools and offices can have concentrations of volatile organic chemicals (VOCs) up to 10 times higher than those outdoors. Poor indoor air can sicken occupants, with chemicals like formaldehyde and benzene causing fatigue, headaches and longer-term health hazards. The cumulative effects of indoor air pollution are not trivial, given that most Americans spend roughly 90 percent of their lives inside. Plants are just one line of defense against “sick building syndrome.” It’s best to start by controlling sources: avoid furnishings that off-gas chemicals; select low- or no-VOC paints and building supplies; and banish toxic cleaning products, scented laundry products and candles, clothes dry-cleaned with tetrachloroethene, and alleged “air fresheners.” Pollutants can remain in household air long after an aerosol can is sprayed or a dryer sheet used. They linger most in houses that are well-insulated and sealed, and lack venting systems to provide an adequate exchange with outside air. Efforts to control sources can improve indoor air, but they won’t entirely eliminate pollution. Even our own bodies generate what scientists none too delicately call bioeffluents. These aren’t the emissions that might follow a big chili dinner but are atmospheric pollutants like acetone naturally generated by metabolic activity. Whether chemical vapors come directly from us or from the stuff that fills our homes, having plants indoors can markedly reduce pollutants’ damaging impact. The plants deliver airborne toxins to soil microbes that break them down. Houseplants also mitigate dry winter air by serving as natural humidifiers. The drier the air, the more moisture they release; talk about accommodating houseguests! According to B.C. Wolverton’s book “How to Grow Fresh Air,” plants even “release phytochemicals that suppress mold spores and bacteria found in the ambient air.” (This beneficial effect can be undermined by operator error, though, as I can attest: Overwatering will turn houseplants into mold factories. Spare yourself this hard-earned lesson and water plants sparingly in winter months.) Many findings in Wolverton’s book trace back to NASA studies done in the early 1980s, when the agency began experimenting with how to maintain healthy air in sealed-chamber settings – an obvious need on space flights. They tested different varieties of plants to identify which were most effective for specific toxins. In terms of tackling pollutants, the plants are remarkably specialized. Palm plants, for example, are exceptionally good at reducing chemicals like xylene and ammonia. The peace lily does poorly with xylene, but is a standout at removing acetone. While the detailed breakout Wolverton provides is interesting, the bottom line appears to be that diverse ecosystems are best – indoors as well as out. Plan on an assortment of plants and keep good air circulation among them if grouped.= Wolverton suggests that people bring plants into the immediate spaces indoors they frequent most – what he terms the “personal breathing zone” of 6 or 8 cubic feet near one’s desk, bed or favorite chair. Concentrating houseplants in these areas can ensure that residents derive the greatest benefits from plants’ ongoing service purifying the air. Houseplants offer a salve for the eyes and lungs throughout the year. But in the depths of mud season, their effect is especially salubrious – for body and soul.