Potted plants may spruce up your living room, but experts warn claims of their benefits have been greatly exaggerated.
The “myth” is thought to date back to 1989, when NASA declared house plants helped to remove cancer-causing chemicals from its air stations.
Green-fingered fans went on to claim filling your house with plants reduced the risk of everything from allergies to asthma by “cleansing the air”.
To learn more, scientists from Drexel University in Philadelphia looked at a dozen studies spanning 30 years on the subject.
They found that while plants may go someway to removing air pollution, you would need up to 1,000 per square metre to compete with a few open windows.
“This has been a common misconception for some time,” study author Dr Michael Waring said.
“Plants are great, but they don't actually clean indoor air quickly enough to have an effect on the air quality of your home or office environment.”
Plants were thought to clear away volatile organic compounds (VOCs).
VOCs are found in many household items, including paint, moth repellent, cleaning products and make-up.
The US Environmental Protection Agency links the chemicals to eye, nose and throat irritation, as well as headaches, nausea, and even damage to the kidneys, liver and central nervous system.
Some VOCs have been shown to cause cancer in animals, while others are “suspected or known to cause cancer in humans”.
The Drexel scientists found plants may clear VOCs but “natural or ventilation” systems do it “much faster”.
The problem with the NASA experiment, and many others, is it was carried out in a sealed environment in a laboratory.
This is very different to the standard house or office, which are littered with windows and open doors.
“Typical for these studies, a potted plant was placed in a sealed chamber (often with a volume of a cubic meter or smaller)”, the scientists wrote in the Journal of Exposure Science and Environmental Epidemiology.
“Into which a single VOC was injected and its decay was tracked over the course of many hours or days.”
The scientists created a measurement called the “clean air delivery rate” (CADR), which they applied to most of the 30 studies.
In every case, the rate at which plants removed VOCs in a sealed chamber was “orders of magnitude slower” than the standard rate of a building, they found.
Between 10 and 1,000 plants per square metre of floor space would be required to compete with the “air cleaning capacity of a building’s air handling system” or even just a few open windows.
Based on this, the team concluded “plants’ overall effect on indoor air quality is irrelevant”.
“The CADR is the standard metric used for scientific study of the impacts of air purifiers on indoor environments,” Dr Waring said.
“But many of the researchers conducting these studies were not looking at them from an environmental engineering perspective and did not understand how building air exchange rates interplay with the plants to affect indoor air quality.
“This is certainly an example of how scientific findings can be misleading or misinterpreted over time.
“But it's also a great example of how scientific research should continually re-examine and question findings to get closer to the ground truth of understanding what's actually happening around us.”
Despite the damning results, house plants are not thought to do any harm.
They have also been linked to a better mood and reduced stress.