During the study, Drexel College of Engineering professor Michael Waring, PhD and his student, Bryan Cummings, analyzed over a dozen studies on plants’ air-purifying properties that spanned 30 years of research. They found that NASA’s 1989 clean air study claim that plants could remove air pollutants wasn't entirely true.
That's because the study (and others that followed) was conducted in a sealed chamber lab, rather than a home or office building, and the results weren’t analyzed further to determine whether plants would have the same effects in a realistic indoor environment. While NASA’s results might have been profound for cleaning the sterile air in space stations, it looks like the general public took this information and ran right to the plant store.
"Typical for these studies," the researchers say, "a potted plant was placed in a sealed chamber (often with a volume of a cubic meter or smaller), into which a single volatile organic compound (VOC) was injected, and its decay was tracked over the course of many hours or days."
To further examine their findings, the two researchers created a measure they named a "clean air delivery rate” (CADR). When they dove back into 30 years of plant studies, they used this CADR framework to calculate the rate at which plants would dilute VOC’s in the air. In nearly all of the studies, the plants’ CADR was actually way slower than traditional methods of air exchange in buildings.
But before you berate your houseplants for not doing their job, Waring and Cummings acknowledge that many of these controlled plant studies did show a reduction in VOC’s overtime—it was just a very, very long time. They concluded it would take between 10 and 1,000 plants per square meter of floor space to purify the air at the same pace as an air conditioning system.