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The Garden Scoop

Houseplants and Indoor Air Quality

The last thing any homeowner wants this time of year is a cold draft from the outdoors.  We humans have become exceedingly good at sealing up all of our indoor spaces in the interest of trapping heat during the winter or cooled air during the summer.  These type of improvements in home construction, insulation, and the overall sealing of our “building envelop” have gone a long way in reducing our energy costs, and our carbon footprint. 

However, there is one often less thought of negative side effect to these great improvements in efficiency: our indoor air quality.  When we absolutely seal up our building envelop, it can trap harmful volatile compounds in the air that have fewer and fewer ways of getting out.  Among the harmful constituents of indoor air, there has been interesting research into the removal of volatile organic compounds (more commonly referred to as “VOC’s”) by ordinary houseplants. 

VOC’s are emitted as gases from certain solids and liquids, many of which are common place around homes. The list of VOC’s includes a variety of chemicals, such as formaldehyde and benzene, which are emitted from a wide variety of products, numbering in the thousands. Everything from common building materials, such as plywood and particle board, to home furnishings such as carpeting and wallpaper, to commonly abundant liquids like paints, cleaners, air fresheners and pesticides have been found to emit VOC’s.  Many of these chemicals have been directly linked to short and long-term adverse health effects. 

The trapping effect from our highly sealed built environments can result in a much higher concentration than older, draftier homes would accumulate. EPA studies have found that VOC’s are consistently higher indoors compared to outdoors, sometimes up to ten times higher.  On average, people spend about 90 percent of their time in indoors and sixty-five percent of that time is spent in our homes.  So, anything we can do to lower VOC levels at home can have a significant long-term effect on our exposure.

In the 1980’s, NASA conducted studies looking at the value of houseplants for indoor air pollution abatement with the interest of using these plants in space to remove air pollution from living quarters.  These studies included many common houseplants, such as spider plant (Chlorophytum comosum), peace lily (Spathiphyllum wallisii), dragon tree (Dracaena marginata) and Snake plant (Sansevieria trifasciata) to name a few. They looked specifically at the plants ability to remove benzene and formaldehyde from the air finding that all plants included in the study significantly reduced the air pollutants.

Gas exchange from foliage during normal plant processes such as photosynthesis and respiration was assumed to be the major mechanism in VOC absorption.  Interestingly, the NASA studies did one replication where all foliage was removed from the plant and still found a significant amount of VOC removal at play.  Subsequent research has further examined other modes of VOC uptake and clearly identified roots and potting material as a major point of uptake.  Specifically, microorganisms in potting mix played a major role in the VOC uptake.

Other later studies looked more specifically at how foliage uptakes VOCs.  Conclusions from this research indicated the leaf cuticle, the outer waxy coating of leaves, as another point of uptake.  Several studies found that pathway for uptake by plant foliage was dependent on the properties of the specific VOC in question. Certain VOCs were more readily assumed through the cuticle and others through the stomata, openings in the leaves for gas and water vapor exchange to facilitate plant processes, such as photosynthesis.

Some researchers were critical of NASA and other’s research on this topic using isolated gas chambers, citing the fact that a gas chamber is not representative of our homes.  Although we have gotten really good at sealing up buildings, we certainly aren’t as perfect as a NASA gas chamber.  To date, few studies have looked at specific conditions in homes, mostly due to the wide variability among home structures.

However, one study did attempt to quantify the number of spider plants needed to effectively purify an entire home, based on data collected in a sealed Plexiglas chamber.  These researchers concluded that approximately 70 spider plants in 8 inch pots were needed to continuously remove an average level of formaldehyde from a 1,800 sq ft. home.  I think this conclusion builds a strong case for expanding all of our houseplant collections this winter.