They look nice, but what are they achieving?

Pot plants don't improve indoor air quality, researchers say.

They’re green but they don’t clean, it seems.

Dorling Kindersley: Rob Streeter / Getty Images

By Nick Carne

The idea that pot plants are as environmentally valuable as they are aesthetically pleasing is vastly overstated, according to engineers from Drexel University in the US.

Michael Waring and Bryan Cummings reviewed dozens of studies undertaken over three decades and concluded that indoor plants don’t really improve the air in homes and offices.

The reason is that natural or ventilation air exchange rates dilute concentrations of the nasties – volatile organic compounds (VOCs) – much faster than plants can extract them from the air.

“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," says Waring.

Writing in the Journal of Exposure Science and Environmental Epidemiology, he and Cummings suggest the myth of plants as purifiers began in 1989 when NASA, looking for ways to clean the air on space stations, suggested that plants could be used to remove cancer-causing chemicals from the air.

The problem, they say, is that this experiment, and some others that followed, were conducted in a sealed chamber in a lab, and the data was not interpreted to reflect the real world.

Their review takes the data from volumes of pot plant research one step further by using it to calculate a measure called the "clean air delivery rate" or CADR.

They say they made this calculation for nearly all of the studies and found in every case that the rate at which plants dissipated VOCs in a chamber was orders of magnitude slower than the standard rate of air exchange in a building.

"The CADR is the standard metric used for scientific study of the impacts of air purifiers on indoor environments, 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," Waring says.

Many of these studies did show a reduction in the concentration of VOCs over time, but according to the pair’s calculations it would take between 10 and 1000 plants per square meter of floor space to compete with the air cleaning capacity of a building's air handling system – or even just a couple of open windows in a house.

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