It’s not just your heating bill you have to worry about in winter. Bunkering down at home during the chilly months causes us to breathe more polluted air than we would outdoors – despite what you may think.
“People want clean air – they’re thinking, well, I can go indoors. No! Indoor air is always worse than outdoors,” says Fraser Torpy, a bioremediation researcher with the University of Technology Sydney. “Every single time it’s ever been studied by anybody, indoor air quality is worse than outdoors.”
In winter, poor indoor air quality is exacerbated by the fact we keep our windows closed – meaning there’s no fresh breeze to dilute the nasty emissions from household appliances, cleaning products and indoor garages (the latter of which can release high concentrations of petrol vapour directly indoors).
So how can you remedy the problem without bringing in that frigid winter air?
The solution, Torpy says, may be houseplants. A recent study he co-authored found that a small green wall containing a mix of indoor plants was effective at removing toxic petrol fumes such as benzene – extracting 97% of the most toxic carcinogenic compounds from the surrounding air within eight hours.
“Of course, we’ve known for 20 years that [plants purify the air],” Torpy says. “We’ve got 60 papers on the topic! Our problem is … no one’s listening yet.”
But reaping the air-purifying rewards of houseplants involves more than just potting a fiddle-leaf fig and calling it a day. A review of 12 previous studies on the topic found you would need between 10 and 1,000 plants per square metre of floor space to remove air contaminants at the same rate as simply opening a window.
Torpy agrees that “you need quite a lot of plants” to effectively purify the air. Installing a green wall is the simplest solution, he says. They work because you can cram a lot of plants into them and because the bacteria in potting mix is a crucial part of the ecosystem that removes toxins from the air. “You need to get that potting mix exposed to the air,” he says – as it is in green walls.
“If we could have a four-square-metre green wall in every office, we’d be sweet. We’d be there,” Torpy says.
The results of the UTS study echoes findings from other researchers.
Dr Tijana Blanusa, principal horticultural scientist at the UK’s Royal Horticultural Society and a researcher at the University of Reading, says: “The answer to whether plants purify air is yes, but the impact is limited and very much influenced by the number of plants.”
She adds that the amount of light plants receive is also an important part of the story – because the more light a plant gets, the more active it is, and therefore the more toxins it can remove from the air.
As for the types of plants that work best, Torpy says quantity and quality are more important than species.
“Just put in plants that work in your conditions,” he says. “Some plants develop a bacterial community that’s better at removing vapours than others. But the difference between the best plant and the worst plant is not that much.
“And then if you want the best plant, you can buy two. Buy four. Buy six. Buy eight. More is better.”
Installing a green wall at home may sound intimidating, but Dominic Hooghuis from plant care brand The Plant Runner says it can be a DIY job.
Hooghuis has installed a number of green walls in office buildings and apartment units, but for those who consider themselves handy, he recommends a panel system called Arch Bar Trellis Mesh or a “plug and play” mounting pots brand from the US called Wally Grow.
“In a green wall, you could probably fit 50 plants in a two-metre-square space,” Hooghuis says. “[Homes typically have] so much wasted vertical space … so you can cram a lot into a wall that’s two metres tall.”
If you’re not in a position to go full green wall, there are other ways to squeeze a lot of plants into your home. Hooghuis recommends making use of vertical spaces such as shelves, adding a few tiered plant stands, or getting a terrarium to “fit multiple species into a small space”.
And while the type of plant you go for doesn’t matter that much for air-purifying purposes, it may affect your wallet.
To get in as many plants as possible on the cheap, Hooghuis recommends devil’s ivy, which is easy to propagate – meaning you can eventually make multiple plants out of your initial purchase. Just clip off an end, ensuring there are some leaf nodes on your clipping, and put it in some water. Monstera adansonii (AKA “the Swiss cheese plant”) and heartleaf philodendron are also great for propagating, he says.
Torpy says even if you can’t fit enough plants to suck all the nasties out of the air, there are still benefits to having a modest amount of houseplants.
“A small number of plants is better than having no plants – it’s significantly better than having no plants,” he says. “[But] the more the better. More and more and more.”
As Hooghuis sees it, the number of plants you can get in a home is limited only by your ability to tolerate clutter.
“I’ve seen people with hundreds of plants – next-level sort of stuff. There’s more plants than anything else.”