Nanogirl Michelle Dickinson: Don't count on houseplants to clean the air

Research has found that houseplants do not absorb as many volatile organic compounds as once thought. Photo / 123RF

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Houseplants bring life and colour to our homes and offices, and for many years we have believed they have the added benefit of improving our surrounding air quality. Now researchers have looked deeper into that claim and found that houseplants aren't as efficient as we were led to believe and the best way to improve your air quality indoors is, in fact, to just open a window.

Using plants to improve indoor air quality is called biofiltration or phytoremediation. A common, scientifically-backed misconception passed on through magazine articles, gardening centres and websites is that, in addition to carbon dioxide, plants can absorb indoor air pollutants.

The claims mostly stem from a high-profile research project by Nasa carried out in the 1980s, which looked into ways to clean the air within space stations. They showed that common houseplants could soak up molecules known as volatile organic compounds or VOCs.

VOCs are organic chemicals widely used as ingredients in many household products. They are released from thousands of indoor items, including cleaning supplies, paints, air fresheners, printers, whiteboard pens, furniture and even dry-cleaned clothes. Studies have found that constant exposure to VOCs can lead to short and long-term negative health effects. As people in the developed world spend up to 90 per cent of their time indoors, and with indoor VOC levels on average two to five times higher indoors than outdoors, houseplants were pushed as a simple and decorative way to help protect our health.

Open a window to clean the air inside your home.

The challenge with many scientific studies is that headline-hitting summaries tend to be passed on, while the deeper - sometimes less exciting - details can be missed. A new paper published in the Journal of Exposure Science and Environmental Epidemiology this week focused on houseplant VOC studies carried out over the last three decades and found that the details were much more insightful.

In the Nasa study and others like it, a potted houseplant was placed in a small space - typically a sealed cubic-metre box - and a single volatile organic compound was injected into that box. The scientists then measured the reduction of the chemical in the box over several days. The boxes did show a reduction in the concentration of VOCs over time, which was thought to be due to the plant absorbing the compounds. This was heavily reported by the media.

The issue with the Nasa and other similar studies is that a sealed chamber in a laboratory environment has very few similarities to a home or office, which constantly produce VOCs rather than receiving a one-off dose. What was also not reported in the media was that the Nasa study was repeated with houseplants that had all of their leaves removed, and the amount of VOCs absorbed by leafless plants was similar, indicating that it might not be the plant removing the chemicals but microorganisms that live in the soil of the plant instead.

After reviewing many different studies, the new review calculated a "houseplant clean air delivery rate", and found that the rate at which plants dissipate volatile organic compounds was orders of magnitude slower than the standard rate of air exchange in a building. This means that the overall effect of potted plants on indoor air quality is insignificant compared with the amount of air movement a building naturally experiences.

The reviewers calculated that it would take up to 1000 small houseplants per square metre of floor space to clean the air at the same rate as an open window.

So the next time you buy a pot plant, admire it for its beauty and texture but don't spend more for its air cleaning qualities - just crack open a window for free instead.