Is it true that … houseplants freshen the air in our homes and offices?

‘Plants take in carbon dioxide and release oxygen when they photosynthesise,” says John Tregoning, a professor at Imperial College London and author of Live Forever? A Curious Scientist’s Guide to Wellness, Ageing and Death. “If you’ve got enough of them, they should produce more oxygen than there was initially.”

It’s tempting to think of plants as a kind of natural air filter, cleaning the air in stuffy, poorly ventilated spaces. But the reality is different.

The average plant produces just a few litres of oxygen a day. A person, by comparison, consumes about 500 litres. So, to produce enough oxygen for one person, you’d need hundreds of plants. “Quite a mad number of plants to have in a room,” says Tregoning. You would also need plenty of light for photosynthesis to take place.

Tregoning points out that we shouldn’t look to keep increasing the oxygen level indoors infinitely: “It’s quite poisonous at high levels.”

He says people are right to think about air quality in their homes and offices. “Higher levels of carbon dioxide are going to make you feel a bit groggy. If you’re working with lots of other people, poor ventilation is also going to lead to faster transmission of colds and illnesses.”

But plants aren’t a substitute for good ventilation. If you want cleaner, fresher air in an office or home, the most important thing is to make sure the air is regularly replaced by keeping windows open, for example.

Still, he says, trees and plants can have a beneficial effect on our health. He points to a study from the 1980s that looked at patients recovering from surgery. One hospital ward had windows overlooking trees, while another faced a brick wall. Patients in the tree-facing ward self-administered less pain medication and recovered faster than those facing the wall.