Indoor plants do not improve indoor air quality

By Dr. Liji Thomas, MDNov 10 2019

A new study published in the Journal of Exposure Science & Environmental Epidemiology in November says houseplants aren’t much good at removing pollutants from your home.  The researchers systematically reviewed a dozen other studies, looking at factors related to indoor air quality, and especially on the contribution of potted plants to the efficient removal of volatile organic compounds (VOCs).

Researcher Michael Waring says, “The best way to have a healthy home is to try to reduce indoor emissions, ventilate well (especially when doing high impact emissions like cooking), and using filtration for certain pollutants (e.g. particulate matter).”

Image Credit: Shannon West / Shutterstock

Air pollution

Air pollution is a hot topic in most urban or semi-urban areas the world over. According to Waring, indoor air pollutants can be classified into VOCs, from cleaners, consumer products, and building materials, which float in the air; semi-volatile organic compounds (SVOCs) from pesticides, flame retardants and plasticizers that stick to indoor surfaces for months or years; and particulate matter, solid or liquid, in the air.

These are important because of their links to asthma, allergies, and other breathing issues, besides cancer and heart disease. Pollution affects 4 out of 10 US residents.

Potted plants do take up carbon dioxide and release oxygen, as our biology textbooks taught us. In addition, a 1989 NASA study showed that houseplants do remove toxins. Many quasi-scientists have suggested, or even commanded, indoor plants as a tried and tested remedy for poor air quality inside the house. Every plant nursery website is sure to offer a long list of “plants which improve indoor air quality” or “purify indoor air”. However, their performance doesn’t justify these claims.

Experimental limitations in earlier studies

Why the misinformation, then? The scientists explain that most of these studies looked at VOC removal by potted plants in a controlled setting, like one injection of a VOC into a box with a potted plant, sealed away from the surrounding air while the rate of VOC decline is tracked over hours or days. While this helps to standardize procedures and compare between studies, it can be counterproductive when it comes to estimating how much air cleaning they produce. In real life, VOCs are constantly released and air volumes are much greater.

Earlier studies reported the rates at which the plants remove unwanted substances from the air, but the current study, in contrast, reports a clean air delivery rate – the amount of clean air expected from a given plant, and the number of plants that would be required to clean air within a given space.

The reality

On average, the researchers conclude, it might take at least 10 plants to completely detoxify air corresponding to the room area contributed by a single square meter of floor space. However, most larger buildings and offices already use air exchange systems, including natural ventilation, that bring in outdoor air and push out indoor air regularly. To match the performance of such systems, you would need between 100 and 1,000 plants per square meter – which would enable one air change per hour, like most of these systems do. On the other hand, increased ventilation can take energy to warm the air being introduced, and may bring in more outdoor pollutants like particulate matter.

Thus other plant-based studies are needed, for instance, to see if air can be purified through a “biowall” or “plant wall” consisting of plants growing in a porous medium through which air is gently drawn for purification. We also need to know how plants filter the air, and take up VOCs, and how this is useful for human life.

Plant cover fights industrial pollution

However, when it comes to industrial-scale pollution, another study from Ohio State University, published in the journal Environmental Science & Technology, says, “Despite the proliferation of control technologies, air pollution remains a major concern across the United States, suggesting the need for a paradigm shift in methods for mitigating emissions." In other words, smoke scrubbing isn’t doing anything to treat emissions from factories. So scientists are looking back to good old trees.

They looked at emissions and vegetative cover in each county in the US, and found that plants were eating up a good amount of these toxic chemicals.

Restorative planting – the new anti-pollutant strategy

The researchers also looked into how restorative planting would affect air pollution. Restorative planting refers to bringing back trees, grasslands and bushes to the average levels for that county where they have been lost. They identified sulfur dioxide, particulate matter, and nitrogen dioxide, as the most common pollutants that they needed to measure with respect to plant-related reductions. They found that restoration removed another 27% of these pollutants, by bringing down particles from the air and absorbing the gases. And in at least 75% of these counties, restoration of county-average tree canopy and undergrowth actually cost less than upgrading anti-emission technology in local industries. And it is applicable to a broad variety of settings – factories, roads, power plants, and oil fields.

This doesn’t mean that factories can go on polluting the environment. Industries should continue to invest in reducing the pollution produced by their processes. However, planting trees is a super-effective solution to a host of health issues.

Plants crucial components of a healthy environment

Says lead author Bhavik Bakshi, “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.” He adds, “One big reason why engineering has not done that [contributing positively to sustainable development] is because engineering has kept nature outside of its system boundary."

So plant more trees – and don’t throw out your indoor potted plants! Though they aren’t too efficient at cleaning the air indoors, they have valuable psychological benefits:  they brighten your mood, beautify your environment, distract you from gloomy thoughts and tempt you outdoors – and they do remove VOCs. However, more research is needed on what other benefits they actually offer in terms of the physical environment.