In 1989, NASA conducted its famous Clean Air Study to see whether common houseplants might purify indoor air by removing toxins in addition to absorbing carbon dioxide and releasing oxygen. It worked, and while plants are still capable of absorbing harmful toxins in the air, new research suggests that potted plants’ ability to improve air quality in the home or office is largely overstated and buries a more effective solution to keeping the air clean.

Writing in the Journal of Exposure Science and Environmental Epidemiology, researchers found that natural ventilation of indoor environments dilutes concentrations of potentially harmful air pollutants much faster than a plant is capable of extracting them.

“The best way to have a healthy home is to try to reduce indoor emissions, ventilate well (especially when doing high impact emissions like cooking), and using filtration for certain pollutants (e.g. particulate matter),” study author Michael Waring of Drexel University told IFLScience.

Where NASA and similar studies went wrong is that they conducted their experiments in sealed chambers in laboratories, which do not accurately mimic the many factors that influence our indoor environments.

“This is the first systematic review of the literature that examines [the] removal of volatile organic compounds (VOCs) by potted plants using chamber studies,” said Waring.

To come to their conclusions, Waring and his team systematically reviewed a dozen studies by taking the available VOC removal data from different studies and converting the findings into a common metric they nicknamed the clean air delivery rate (CADR). CADR was used to parameterize air cleaning indoors. Using that metric, the impact of plants’ ability to remove VOCs was compared to the removal ability of other kinds of ventilation, both natural and mechanical.

“In a small office, you would have to have somewhere between 100 to 1,000 plants to have the same air cleaning impact of ventilation at 1 air change per hour, which is a typical value,” said Waring.

To understand the effects of indoor pollution, Waring says to think of it in three categories. The first is volatile organic compounds, which are gas-phase molecules that are emitted from many sources such as consumer products, cleaners, and building materials. These tend to reside in the air itself.

“The second is semi-volatile organic compounds (SVOCs), which are in things like pesticides, flame retardants, or plasticizers. These tend to be 'sticky' and stick to surfaces indoors and may persist for months or years,” he added.

Lastly, particulate matter, which can be solid or liquid, may be found floating in the air. These indoor pollutants have been linked to asthma, allergic reactions, irritation, and other respiratory ailments.

“The best way to have a healthy home is to try to reduce indoor emissions, ventilate well – especially when doing high-impact emissions like cooking, and using filtration for certain pollutants like particulate matter,” said Waring.

That doesn’t mean you should chuck out your plants just yet. Waring says that although houseplants do not clean the air under typical settings, houseplants have many benefits, most of which are psychological.