Pot plants barely clean the air, so best to just open a window, say scientists

Since Nasa published a paper in 1989 claiming that houseplants can soak up pollution and toxic chemicals, businesses and homeowners have increasingly invested in greenery to help clean their air.

But a new analysis suggests it could actually take more than 300 plants per square foot (1,000 plants per square metre) to gain a benefit any greater than simply opening a couple of windows.

The problem lies in the fact that Nasa conducted their tests in a sealed containers that do not mimic the conditions in most people’s homes or offices.

The space agency was primarily concerned about keeping the air fresh for astronauts cut off in biospheres or space stations, and helping to combat ‘sick building syndrome’ which had become a problem due to the super insulated and energy efficient offices of the late 1970s.

By the early 1980s, workers regularly complained of itchy eyes, skin rashes, drowsiness, respiratory and sinus problems, headaches and allergies, as they inhaled toxic chemicals from paints, plastics and fire-retardants.

Nasa found that plants such as English ivy, bamboo, gerbera and peace lillies, could remove benzene, formaldehyde and trichloroethylene, and even today garden centres recommend the plants for air cleaning properties

However a new evaluation of dozens of studies spanning 30 years by Drexel University in Philadelphia, found that house plants in a normal environment have little impact at all. In fact, natural ventilation is far better at cleaning the air.

“This has been a common misconception for some time. 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,” said Dr Michael Waring, associate professor of architectural and environmental engineering in Drexel's College of Engineering.

The team also calculated the ‘clean air delivery rate’ for plants in the studies they analysed and found that the rate at which plants dissipated the compounds was well below the usual rate of air exchange in a normal building, caused by the movement of people coming and going, opening doors and windows.

Many of the studies did show a reduction in the concentration of volatile organic compounds over time, which is likely why people have seized on them to extol the air purifying virtues of plants.

But the team’s calculations showed, it would take between three and 312 plants per square foot (10 - 1,000 per square metre) of  floor space to compete with the air cleaning capacity of a building's air handling system or even just a couple open windows in a house.

In contrast, Nasa’s sealed experiment recommended one pot plant per 100 square feet (9.3 square metres.)

“This is certainly an example of how scientific findings can be misleading or misinterpreted over time,” added Dr Waring.

“But it's also a great example of how scientific research should continually reexamine and question findings to get closer to the ground truth of understanding what's actually happening.”

The research was published in the Journal of Exposure Science and Environmental Epidemiology.