It’s claimed that houseplants are good for improving indoor air quality, but the scientific proof is not matching up, writes Aznim Ruhana Md Yusup
DURING the recent spell of haze, I came across several articles recommending indoor plants. Apparently, plants can help improve air quality inside the house. Acting like a natural air purifier, they absorb harmful chemicals and make us healthier.
But terrarium maker Ronnie Khoo of Ohsum Mossum reckons it’s all hogwash.
“Plants may filter toxins from the air but I wouldn’t rely on them specifically to clean the air at home, as there are a lot of variables for the plants to be effective. I also notice that plant care experts and trained horticulturists never talk about this, which in itself says a lot,” adds Khoo.
The idea that plants can clean the air stems from a Nasa study published in 1989. The American space agency wanted to find out if certain plants can remove a type of air pollutant called volatile organic compounds (VOCs) from inside buildings.
Researchers bought popular indoor plants for the experiment such as pothos, snake plant, gerbera daisy and peace lily, and added activated carbon into the potting soil. Meanwhile, the VOCs they tested are benzene and formaldehyde, among others.
Sources of VOCs include petrol and tobacco smoke, as well as household products such as paint, adhesives and detergent. Most scents are considered VOCs and as most of us would know, the best way to remove bad smells is to open a window.
But this sort of natural ventilation is just not possible in a closed environment such as a space station, which is where Nasa comes in.
LOST IN RETELLING
It was found that the plants — together with soil and activated carbon — can indeed remove the tested VOCs. Not all of it but enough for the scientists to say that plants are “one of the most promising means” of alleviating indoor air pollution.
However, like many scientific research, a lot of the details get lost when it’s retold to the public.
The first detail was that the tests were conducted in laboratories using airtight Plexiglass chambers the size of boxes, not rooms. This is normal for scientific studies, but clearly unlike real world conditions since we don’t live or work in small boxes that are completely sealed.
Many buildings in Malaysia are ventilated naturally using windows or wall openings that allow for the continuous exchange of indoor and outdoor air. So the removal of VOCs in this situation can be attributed to ventilation rather than plants.
Meanwhile, buildings with a mechanical ventilation system will have its own rate of air being changed in and out of a room. In this case, replicating Nasa’s result would require placing one VOC-filtering plant in every 0.1 square metres, says engineering professor Michael Waring.
The Drexel University academic explains to The Atlantic: “You would have to put 1,000 plants in a 3.1 metre by 3.1m by 2.4m office to have the same air-cleaning capacity of just changing over the air once per hour, which is the typical air-exchange rate in an office ventilation system.”
Another issue was that too few VOCs were tested, when there are other VOCs that can potentially infiltrate the indoor environment. Compound concentrations can also change over time, depending on usage of VOC-emitting objects.
Given these arguments, it seems that the degree in which plants can draw out VOC pollutants is negligible. The Nasa study also did not test the plants against other sources of indoor air pollutants such as dust and biological contaminants.
OXYGEN OUT AND IN
But with haze, the problem lies with quality of outside air. People are asked to close their windows and doors to stop the spread of haze particles. But in buildings with only natural ventilation, the indoor environment will end up becoming quite stuffy.
So when the outdoor air quality is worse than indoors, can houseplants help the situation?
According to Kamal Meattle, the answer is yes. He is an environmental activist and CEO of Paharpur Business Centre in New Delhi, India — a city which the World Health Organisation has found to have the worst air quality among any major city.
Meattle’s building has some 1,200 plants across six floors, including 400 in the greenhouse on the roof. The plants used are mostly snake plants, areca palm and pothos. These dangle from ceilings, line up walls and decorate corridors.
In a 2014 article by National Geographic, he explains that the plants are part of a larger air treatment system for the building that includes air washers and carbon filters. When plants release oxygen from photosynthesis, it enriches the cleaned and filtered air. The inside air is as fresh as Switzerland, he says.
But Meattle’s system is a lot more complicated than simply having indoor plants. Sceptics say much of the work was actually done by the air cleaning and filtration system, not plants. Even the oxygen produced may be neglible, since plants also breathe and use oxygen.
That said, there are many houseplant owners who are happy to vouch for their air cleaning properties, including online entreprenuer Nadrah Radzi.
“I started collecting houseplants when we moved to an apartment with good natural light. I have two young children and one of them has autism so thinking about their health, I started bringing plants in to reduce indoor toxins and improve the overall air quality,” says Nadrah.
She has 73 plants in her 65sq m home, including a fiddle leaf fig tree and areca palm in her living room. Her favourites are the trailing pothos and Scindapsus plants above her desk and bed. She keeps several sansevieria or snake plant varieties and two types of Monstera plants.
She credits the plants for reducing symptoms of sinus and eczema in her family, while also helping her sleep better at night. The apartment is also less dusty and the air is fresher. So while the scientific community studies and debates the inner workings of indoor plants, for Nadrah the benefit is obvious.
“Good health depends on more than just diet and exercise,” she says. “Your environment affects your health too. Plants are the best because they quietly work their magic in the background and the right houseplant can make a big difference to your mood, stress level, sleep and even breathing.”