A load of hot air?

Plants aren’t the cleaners NASA thought

Since NASA published a paper in 1989 claiming that houseplants can soak up pollutants, people have increasingly invested in greenery to clean their air.

But analysis suggests it could 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 NASA conducted its tests in sealed containers that do not mimic the conditions in most homes or offices.

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

NASA found that plants such as English ivy, bamboo, gerbera and peace lilies could remove benzene, formaldehyde and trichloroethylene; even today, garden centres recommend the plants for their air-cleaning properties.

However, an evaluation of dozens of studies by Drexel University in Philadelphia found that house plants have little effect at all.

In fact, natural ventilation is better at cleaning 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,” said Dr. Michael Waring, associate professor of environmental engineering in Drexel’s College of Engineering.

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

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

But the team’s calculations showed it would take three to 312 plants per square foot of floor space to compete with the air-cleaning capacity of a building’s air-handling system or a couple of open windows.

In contrast, NASA’S sealed experiment recommended one pot plant per 100 square feet (9.3 square metres). Waring added: “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.”

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