Houseplants have a reputation for cleaning indoor air, but how much pollution can they actually remove?

A review of 60 studies suggests the benefits are often much smaller than lab tests make them seem. In homes and offices, researchers also had a harder time showing how much the plants contributed.

Plants can help, the authors say, but the evidence cannot yet predict how much a particular setup will improve a particular room. Prashant Kumar, founding director of the Global Centre for Clean Air Research at the University of Surrey in England, led the team of 35 researchers from seven countries. They ran no new experiments and could not combine the earlier results into one overall figure because the studies differed too much.  “There is growing evidence that indoor greening can improve the spaces where we live and work, but much of the research has looked at individual benefits in isolation,” Kumar said.

People spend roughly 80% to 90% of their time indoors, the authors noted. Yet 60% of the studies in real buildings focused on offices, while only 10% looked at homes. Paint, furniture and cleaning products can release gases called volatile organic compounds, or VOCs, including toluene and formaldehyde. In short lab tests, plants often removed 60% to 80% or more of these two gases.

In real rooms, studies of potted plants reported drops of 5% to 30% in VOCs and tiny airborne particles. Results depended on plant numbers, room size and ventilation. One occupied office with plants growing in water instead of soil had 28.1% lower total VOC levels over 12 months than an identical room without plants.

A green wall at a primary school in Turin, Italy, lowered most pollutants tested. The improvements were smaller than expected, partly because the school had more natural ventilation during the COVID-19 pandemic.