Houseplants and indoor air: what the evidence really says

You may have seen claims that a few houseplants will purify the air in your room. The real evidence is more modest, but plants can still make a space feel calmer and more pleasant, as long as you look after them and keep them safe for pets.

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Where the clean-air idea came from

• In 1989, NASA researchers looking for ways to clean air on space stations reported that plants could remove certain chemicals from the air of small sealed test chambers, Drexel University explains. That finding was later widely misread as meaning a few houseplants can clean the air in a home.
• Those experiments took place in small sealed chambers, which are very different from real homes and offices.
• In 2019, Drexel researchers Michael Waring and Bryan Cummings reviewed 12 studies covering 30 years and compared plants with normal ventilation.
• They calculated that it would take 100 to 1,000 plants per square metre of floor space to match the air cleaning of a building's ventilation system, or even a couple of open windows.
• Waring's conclusion: plants are great, but they do not clean indoor air quickly enough to have an effect.

What plants can do

• A study by RHS Science and the University of Reading asked 520 people to rate images of houseplants. Healthy, lush plants with dense green leaves were seen as most uplifting and beneficial.
• Participants rated unhealthy-looking plants as depressing, even worse than having no plants at all.
• That study measured perceptions, not actual air quality. One of the researchers noted that individual plants' physical impact on indoor air quality may be limited.
• So the honest summary is: plants can make a room feel nicer, which matters, but for cleaner air rely on ventilation and avoiding indoor pollution.

Keeping houseplants alive: light

• University of Missouri Extension says poor light is close behind poor watering as a frequent cause of houseplant failure.
• Most foliage plants do well in a bright spot with indirect sun; flowering plants usually need brighter light.
• Move plants gradually when changing their light, as sudden changes stress them.
• If a plant in a very sunny window develops pale or brown, crispy patches, try a slightly set-back spot or a sheer curtain.

Keeping houseplants alive: water and care

• University of Missouri Extension says houseplants are probably killed or injured more often by improper watering than by any other single factor.
• Learn to judge moisture by the colour and feel of the soil rather than watering on a fixed schedule.
• Soil that stays too wet becomes sticky and invites root rot; always use pots with drainage holes and empty water that collects in decorative outer pots.
• Do not repot a new plant straight away, and never feed a wilted plant: water it first and feed once it recovers (University of Missouri Extension).
• If indoor air is dry, University of Missouri Extension suggests standing pots on (not in) a tray of constantly moist gravel or sand.

Pets and toxic plants

• The ASPCA says eating any plant material may cause vomiting and stomach upset in dogs and cats.
• Some plants are far more dangerous. The ASPCA says lilies are extremely toxic to cats: any part of the plant, the pollen and even water from a vase of lilies can cause kidney failure.
• If you have pets, check every plant on the ASPCA's toxic and non-toxic plant lists before bringing it home.
• If a pet may have eaten a toxic plant, contact a vet immediately. The Pets and Animal Care module covers emergencies and spotting illness.