Do plants really clean the air in your home?

The short answer: yes, they do filter the air, and it’s been measured. The catch is that in a real home the effect falls a long way short of what you get from opening the windows for ten minutes. Plants do their bit; it’s just a small bit.

It’s worth telling where the idea comes from, because it’s a lovely example of how a real study ends up saying something it never said.

Where the air-purifying plants idea comes from

In 1989 NASA published a report on houseplants and indoor air pollution, written by the engineer B. C. Wolverton. The agency wasn’t thinking about your living room: it was thinking about space stations — sealed spaces where the air is never refreshed and anything released into it just keeps going round.

And that’s exactly how the experiment was set up: small sealed chambers, each with one plant inside and a high concentration of a single compound. Under those conditions the plants — and above all the microorganisms in the soil — absorbed formaldehyde, benzene and trichloroethylene in measurable amounts. The finding is real and the study is serious.

What happened next is that the headline took on a life of its own. ‘NASA says these plants clean the air’ was repeated for thirty years without the important part: in a sealed chamber the size of a wardrobe.

What later research says

In 2019, two researchers at Drexel University went back over thirty years of studies on the subject and redid the maths using the measure that’s actually used to compare air purifiers: the clean air delivery rate. Their review, published in the Journal of Exposure Science & Environmental Epidemiology, reached an awkward and very specific conclusion.

For plants to match what a home’s normal ventilation already does, you’d need between 100 and 1,000 plants per square metre of floor. In a 20-square-metre living room, that’s somewhere between 2,000 and 20,000 pots.

It’s worth reading that carefully, because it doesn’t say what it seems to: it isn’t that plants don’t filter. They do. What it says is that a real room isn’t a sealed chamber. The air renews itself — through gaps, through doors, every time someone comes in or goes out — and that renewal dilutes pollutants far faster than any plant can absorb them. The plant’s contribution is still there; it just gets drowned out.

So what does clean the air in your home?

Three things, in order of how well they work — and none of them expensive:

Ventilate. Throw the windows wide open for ten minutes, ideally two on opposite sides so the air can flow straight through. Ten minutes with air flowing through refreshes more of it than two hours with one window open a crack.

Extract it where it’s made. The cooker hood when you cook, the extractor fan when you shower. Getting rid of stale air where it starts stops it spreading through the rest of the house.

Remove the source. The compounds from the NASA study come from specific places: new chipboard furniture, fresh paint and varnish, air fresheners, smoke. Airing the room more than usual for the first few weeks after you put up a new piece of furniture does more than any pot plant.

And still, have plants

The fact that they don’t change the air in your home doesn’t make them a con. It just means the reason everyone kept repeating wasn’t the right one — and that the right one was in plain sight all along.

A home with something green in it is a nicer place to be. Looking after something that responds — that puts out a new leaf, that droops when you forget it and perks up when you water it — is a small, pleasant habit, the kind that lasts. And in a city flat, having something alive in sight is no small thing.

That’s reason enough. There’s no need to invent another one.

The three from the study, and why they’re still a good choice

The species NASA tested are still, as luck would have it, among the easiest to keep alive. Not because of what they filter: because of how little they ask for.

Snake plant (also known as mother-in-law’s tongue). The hardest of the three to kill. It copes with low light, weeks without water and general neglect. It’s a good bedroom plant simply because it fits in any corner and needs next to no attention.

Pothos (devil’s ivy). Grows almost anywhere, even in a glass of water, and lets you know when it’s thirsty by drooping its leaves. Perfect on a high kitchen shelf, where it looks its best trailing down.

Peace lily. The one that copes best with humidity, which makes it the obvious choice for a bathroom with a window. It flowers, too, which the other two don’t.

Start today: open two opposite windows for ten minutes. And put the snake plant on your bedside table anyway — it does its bit, and it looks gorgeous.