Plants don't improve indoor air quality but cut outdoor pollution

New Delhi (PTI): As large parts of north India grapple with dangerous levels of smog, two new global studies seek to settle the debate on effective 'green' antidotes -- one concluding that potted plants don't affect indoor air quality and the other affirming that trees near factories and other pollution sources reduce outdoor air pollution.

A study by Ohio State University, published in the journal Environmental Science & Technology on Wednesday, found that adding plants and trees to landscapes near factories and other pollution sources could reduce air pollution by an average of 27 percent. Another study by researchers at Drexel University in the US said claims about the ability of plants to improve the air quality are vastly overstated.

"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," Michael Waring, an associate professor in Drexel's College of Engineering, said in a statement. A closer look at decades of research suggesting that potted plants can improve the air in homes and offices reveals that natural ventilation far outpaces plants when it comes to cleaning the air, the researchers explained. According to the World Health Organization (WHO), India is home to six of the top 10 polluted cities in the world with air quality index (AQI) in the 'severe' or 'severe plus' categories several times in a year in the National Capital Region (NCR).

Waring and one of his doctoral students, Bryan Cummings, reviewed a dozen studies, spanning 30 years of research, to draw their conclusions and published their findings on Wednesday in the Journal of Exposure Science and Environmental Epidemiology. The key finding is that the natural or ventilation air exchange rates in indoor environments, like homes and offices, dilutes concentrations of volatile organic compounds -- the air pollution that plants are allegedly cleaning -- much faster than plants can extract them from the air. The study by researchers from The Ohio State University, however, shows plants may be cheaper options than technology for cleaning the air near a number of industrial sites, roadways, power plants, commercial boilers and oil and gas drilling sites.

The researchers collected public data on air pollution and vegetation on a county-by-county basis across the lower 48 US states. They then calculated what adding additional trees and plants might cost. In 75 per cent of the counties analysed, it was cheaper to use plants to mitigate air pollution than it was to add technological interventions -- things like smokestack scrubbers -- to the sources of pollution, the team found.

"The fact is that traditionally, especially as engineers, we don't think about nature; we just focus on putting technology into everything," said Bhavik Bakshi, lead author of the study and professor at The Ohio State University. "And so, one key finding is that we need to start looking at nature and learning from it and respecting it.

There are win-win opportunities if we do -- opportunities that are potentially cheaper and better environmentally," Bakshi said in a statement. Researchers at Drexel University noted that the high-profile experiment that seemed to create the "myth of houseplants as air purifiers" happened in 1989 when NASA, in search of ways to clean the air on space stations, declared that plants could be used to remove cancer-causing chemicals from the air.

However, the problem with this experiment, and others like it, is that they were conducted in a sealed chamber in a lab, a contained environment that has little in common with a house or office, the Drexel team said. They said the data from these studies was not interpreted further to reflect what the findings would be if the plant were in a real indoor environment with natural or ventilation air exchange. "Typical for these studies, 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 compounds (VOCs) was injected, and its decay was tracked over the course of many hours or days," the researcher wrote.

The review by Waring and Cummings takes the data from volumes of potted plant research one step farther, by using it to calculate a measure called the "clean air delivery rate" or "CADR". Many of these 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 according to Waring and Cummings's calculations, it would take between 10 and 1,000 plants 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. On the other hand, calculations by the Ohio team included the capacity of current vegetation -- including trees, grasslands and shrublands -- to mitigate air pollution. They also considered the effect that restorative planting --bringing the vegetation cover of a given county to its county-average levels -- might have on air pollution levels.

They estimated the impact of plants on the most common air pollutants -- sulphur dioxide, particulate matter that contributes to smog, and nitrogen dioxide. Their research did not calculate the direct effects plants might have on ozone pollution, because, Bakshi said, the data on ozone emissions is lacking. The analysis also didn't consider whether certain species of trees or plants would better "scrub" pollution from the air, though Bakshi said it is likely that the species of plant would make a difference in air quality.