Do houseplants improve air quality?

Clean air is important for health, yet many homes are rife with pollutants. Household products such as cleaning solvents, waxes, paints and varnishes often contain volatile organic compounds like benzene and toluene, which can cause skin irritation, eye damage, neurological disorders and cancer. Furnishings and carpets, for their part, can slowly release formaldehyde, another carcinogenic molecule.

The presence of these toxins is particularly problematic for children who spend much of their time indoors and have sensitive lungs. Air purifiers can help, but houseplants have in recent years been marketed as a more aesthetic alternative. Amazon, an e-commerce giant, sells plants described as being for “air purification” alongside more conventional categories such as “low-maintenance” and “pet-friendly”. But how good a job can plants actually do?

Scientists have long known that plants can change the composition of the air around them as they breathe, turning carbon dioxide into oxygen in the process. The first good evidence to suggest that they might filter pollutants at the same time emerged in the 1980s, when NASA exposed a number of plants housed in small Plexiglas chambers to air contaminated with formaldehyde. These experiments, which lasted many hours, concluded that spider plants, among others, were effective at removing the toxin from the air.

Further studies painted a more detailed picture of how such effects were achieved. Whereas many plants, like pines and yews, have hairy, waxy or rough leaves that readily accumulate pollutants on their surfaces (and are often planted next to busy roads for precisely this reason), some species draw these molecules into their tissues through holes in their leaves known as stomata. Of those that draw in pollutants, some produce enzymes capable of breaking apart molecules such as benzene and formaldehyde. Research published in the EU Journal of Internal Medicine in 2017 further revealed that some plants send small amounts of the intact pollutants to their roots, where microbes in the soil devour them as food.

Harnessing these superpowers in a domestic setting is tricky. The most popular houseplants earn that distinction because they are attractive and good at putting up with neglect or abuse. The plants on most urban windowsills are, therefore, broadleaf evergreen species such as birds of paradise and fiddle-leaf figs from tropical and subtropical places. On the whole, they have small stomata. This means that pollutant molecules are far less likely to enter the leaf and be broken down or buried.

The most promising results that have been generated have come from rarefied experimental conditions, with plants kept in small, airtight chambers with long exposure times. Experiments run with plants in ordinary rooms have yet to show any meaningful benefits.

But vegetation comes in many guises. Air-flow systems that attempt to purify air by blowing it over dense layers of vertically grown vegetation (known as living walls) have shown more promise in real-world settings. Even so, the improvements to air quality are small (one study showed that 60 square centimetres of plants could reduce an office’s carbon-dioxide concentration by just under 2%) and living walls, though attractive, are not easy to maintain. It is a safer bet to appreciate your plants for the many other benefits they bring—and stick to an electric air purifier instead.■