How many house plants do you need to clean the air in a small flat?


AS YOU may know from my bio, I cohabit my small flat in London with more than 500 plants. I am therefore fascinated by the promise of a plethora of health benefits from gardening in the great indoors. With the current flowering of interest in the hobby, the internet is awash with handy advice for the “10 best air-purifying plants for the home” and species marketed as “Air so pure”.

Being a stats geek, I wanted to calculate exactly how much the concentration of plants in my apartment could clean the air. This turned out to be a rather deep rabbit hole…

The seminal work on this subject came from NASA in 1989, after it investigated using plants as filters in space stations. In the study, researchers placed individual plants in tiny chambers filled with air contaminated with volatile organic compounds (VOCs). This group of chemicals includes known carcinogens like the benzene in cigarette smoke and the formaldehyde in paints, and it has been consistently linked to poor health outcomes.

NASA found that there was a significant improvement in the air quality in the chambers over a 24-hour period. Many headlines about how houseplants clean the air, including claims of a 90 per cent reduction in indoor air pollutants, have come from citing this three-decade-old paper.

While the study was well designed, there are a few caveats. Firstly, the “90 per cent” claim isn’t for all plants, or even all pollutants, but a single stat for the ability of ivy to remove benzene from the air. Results for other plants and contaminants aren’t as impressive, coming in at less than 10 per cent in some cases. So the way this study has been reported – including by me, I regret to say – exaggerates the actual ability of plants to clean the air.

“I like a challenge, but increasing my plant collection tenfold would mean entirely dispensing with furniture”

Secondly, this experiment only looked at three VOCs. There are hundreds of other VOCs, as well as other ways to measure pollution, such as carbon dioxide and dust levels. So using this study as a definitive measure of overall “purity” in the indoor atmosphere is very tricky indeed.

However, perhaps the most important proviso is that the experiment was done in tiny, sealed chambers. So while it did a great job of demonstrating that plants can filter air, it didn’t provide hard evidence that they can filter enough of it in a room setting to keep up with the level at which these pollutants are generated.

Fortunately, since NASA’s study, dozens of others have investigated these issues. A review of these studies found that you would need between 10 and 1000 plants per square metre of a building’s floor space to remove VOCs at the same rate as happens when air from indoors moves outside anyway.

So in my 50-square-metre flat, I would need around 5000 plants for them to be as good as simply opening a window. I like a challenge, but increasing my existing collection tenfold would mean entirely dispensing with furniture. But what about other measures of air quality? In lockdown, I have been stuck indoors, breathing out about 30 grams of CO₂ per hour. Research published by the University of Birmingham, UK, and the Royal Horticultural Society found that you would need 15 ivy or peace lily plants to reduce just 10 per cent of one person’s contribution.

While fitting 150 such plants in my flat is theoretically possible, given some sort of living wall set-up, it should be pointed out that these two species were the best-performing plants in the study. With bromeliads, for instance, you would need more than 1000 specimens to do the same job.

I should have twigged this, really, as even mature tropical trees are estimated to absorb only 22 kilograms of carbon dioxide per year. I would therefore need more than 11 rainforest giants in my flat just to tackle the pollutants I breathe out. That’s a lot of plants, even for me., 7 July 2020
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