How many plants does it take to clean the air?

It takes a lot of plants to clean the air. 

A NASA study in 1989 found some interesting results when exposing plants to the air pollution chemicals known as volatile organic compounds (VOCs). It seemed that the plants in combination with the soil and soil microbes could reduce the amount of VOCs in the air. The findings were exciting because plants are more attractive, sustainable, and self-replicating than any air cleaning technology we could imagine.

However, looking closer at this study from more than 30 years ago it appears that the authors of the NASA study, and a few researchers since then, may have overstated the impact indoor potted plants have on air quality. More recent research has been indicating that you would need an impractical number of houseplants to make any difference.

Apparently we need to be outnumbered by plants indoors just the same as we are outdoors. From a visual perspective, much more than 20% visible vegetation can be distracting or overwhelming, so using plants to clean the air might just not be efficient. But let’s take a look at the available research on how plants interact with different pollutants and how many plants are needed to purify air.

 

Do plants remove VOCs?

An infamous review article in Nature from 2020 titled “Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies” by Bryan Cummings and Michael Waring at Drexel University looked at 196 different VOC removal tests with plants across 12 different published papers. Cummings and Waring pointed out several problems with the methods of the papers they reviewed, including using sterile sealed chambers just large enough to fit the plant alone and injecting the VOCs once and allowing the plant to sit with them. In the real world, indoor plants are in large rooms with people, furniture, building materials, and continuous sources of VOCs.

The term VOC refers to a very wide variety of substances that tend to become gasses at room temperature. The pungent smells of paints, glues, lacquers, and similar products represent high concentrations of VOCs in the air, as do other strong smells like alcoholic beverages, essential oils, or new synthetic materials. Most homes have constant low levels of VOCs from new clothing, furniture, building materials, or cleaning products. In general VOCs do not have an immediate impact, but exposure over time is linked to asthma in children and formation of potentially more harmful particle pollution. Some VOCs, such as benzene in gasoline, are considered carcinogens, so there is a lot of interest in finding ways to reduce them in the home.

Cummings and Waring found a very wide range of VOC removal rates. The impact of different VOCs was inconsistent across studies, as was the performance of different plant species. But they said that in 196 experiments, plants were able to clean an average of 0.062 cubic meters of air per plant per hour, which is only about 13% of the half a cubic meter we breathe at the same time.

Person breathing versus plant removing VOCs

A plant would need to clean many times the amount of air a person breathes to effectively provide air that has less pollution. But plants do have some impact, depending on their size, leaf shape, and individual characteristics, so let’s look at the studies on a few specific species.

How much pollution do arrowhead plants remove?

In one study, enough benzene to raise the concentration to 250 times the safe limit was injected into a tiny chamber with an arrowhead plant. The chamber was then sealed for eight days. The arrowhead plant in addition to its soil was able to purify about 1% of a cubic foot or a little more than a third of a liter of air an hour. The rate dropped by half when the arrowhead plant was in just water, showing that much of the removal was due to the soil and microorganisms in the soil. 

Arrowhead plant indoors on a stool

By comparison in one test our Molekule Mini air purifier was able to clear toluene, a close relative of benzene, at a lower concentration but a larger amount in a much larger chamber more than 100,000 times faster.

Benzene cleaned by Molekule Mini and arrowhead

*Removed 99.7% toluene in a 30 cubic meter chamber in 8 hours from an initial concentration of 1938 ug/m3

How much pollution do ficus plants remove?

One researcher subjected ficus to formaldehyde about ten times the safe limit for five hours and a relatively safe amount of toluene and xylene for 24 hours. The plants were in a medium-sized (one square meter) chamber. Interestingly, the different ficus species did admirably at removing the large amount of formaldehyde, getting to the neighborhood of 10 cubic feet or 300 liters per hour. It is not clear if those results translate to the much lower levels typical of households or if it is sufficient to reduce the impact of formaldehyde.

Ficus benjamina indoors by a window

Ficus benjamina, the species used to remove formaldehyde

Mini was able to remove formaldehyde more than 25 times faster, but ficus plants just might be able to remove enough formaldehyde to matter. Plants can use airborne formaldehyde in their metabolisms, possibly because formaldehyde is extremely simple and small. So putting a few potted ficus or other plants around may lower the concentration of formaldehyde, but primarily during the day when they are photosynthesizing.

Formaldehyde cleaned by Molekule Mini and Ficus

*Removed 56.2% toluene in a 30 cubic meter chamber in 8 hours from an initial concentration of 1071 ?g/m3

Ficus benghalensis at a plant nursery

Ficus benghalensis, the species used to remove toluene and xylene

Toluene and xylene were less impressive, however, and Mini was about 600 times faster.

Toluene and xylene cleaned by Molekule Mini and Ficus

*Removed 99.7% toluene in a 30 cubic meter chamber in 8 hours from an initial concentration of 1938 ug/m3

The plants’ metabolisms either did not have a use for the larger and more complex toluene, xylene, or benzene molecules, or could not remove them as quickly for some other reason.

Plants and particle pollution

VOCs are just one aspect of air pollution. Plants have also been tested on their ability to remove particles, like from smoke or exhaust. Particle pollution is different from VOC and other gaseous pollution because particles are typically larger than individual gas molecules and can carry more free radicals and other reactive substances that react with your body and cause damage.

A plant can’t absorb particles and use them like they can with formaldehyde. Instead, they remove particles by just holding them on the surface of their leaves. Most research into plants and particle removal are concerned with the total surface area of a plant’s leaves. Other factors are also important, for example leaves with more wax or that have rougher textures can hold more particles

A medium sized fern or spider plant with their many long leaves have a lot of surface areas for houseplants at around 1,200 square centimeters total. By comparison our lungs have an internal surface area of 50 to 75 square meters, or around 500 times as much, and our lungs are constantly pulling in air with the power of our diaphragm muscles, which plants do not have. Similarly, the dense forest of fibers in an air filter offers a lot of surface area to trap particles.

 

How much pollution do spider plants remove?

An experiment placing spider plants in different occupied rooms for 2 months found the leaves accumulated about 2-4 times the weight of particles by area than aluminum plates in the same rooms, which means they are better than nothing. We don’t know what the air quality was like in these rooms, but the maximum amount of particles each spider plant accumulated was around 20 micrograms per square centimeter of leaf. A spider plant has somewhere around 2,000 square centimeters of leaves, which makes for 50,000 micrograms per plant (an aspirin tablet weighs 300,000-500,000 micrograms, in comparison) in 2 months. 

Spider plant indoors on a table

When the AQI is moderate at 100, there are about 9,000 micrograms of inhalable particles (PM10) in a small 250 square foot room. If the source of pollution is outside, particle pollution will be replaced at most once an hour or every fifteen minutes if the window is open.

Total weight of particles Molekule Mini removes compared to spider plant

*Estimated assuming an 8.5×11 inch aluminum plate

**Removed 89% of an initial airborne dust (0.5 um to 10 um) concentration of approximately 0.2 micrograms per cubic meter was removed from a 30 cubic meter chamber in 20 minutes. 

Compared to the overall amount of particles in the air, a spider plant doesn’t make much of a meaningful difference. It’s just a little less than the fraction of smaller particles that are deposited in your lungs, and around 1% of what is coming into a home from outside. Molekule Air Mini, by comparison, was able to remove 89% of dust particles in a small chamber in about 20 minutes.

Spider plants do look nice and are very easy to propagate so there’s no reason not to spread them around your home, and as long as the soil isn’t moldy they do not make air quality worse. They also have been shown to remove formaldehyde in one study, though the authors have not made the full data public.

We can use the particle deposition data above to get an idea of how the presence of other plant species might remove particles.

How much pollution do aloe vera and other succulents remove?

The same study that tested spider plants and formaldehyde also mentioned that spider plants were better at removing formaldehyde than aloe vera but without seeing the data we cannot know to what extent. 

Aloe vera plant on a windowsill with books

We can estimate how many particles might fall on it, however. A mature 6-month old aloe vera plant in normal soil has about 12 long leaves with about 600 square centimeters per leaf. The leaves stick straight up so let’s assume all 7,200 square centimeters capture particles. Plants vary widely in the ability to capture particles, with the most effective being able to capture around 75 micrograms per cubic centimeter. Let’s give aloe vera the benefit of the doubt and assume it captures particles as well as the best.

Total weight of particles Molekule Mini removes compared to aloe vera

**Removed 89% of an initial airborne dust (0.5 um to 10 um) concentration of approximately 0.2 micrograms per cubic meter was removed from a 30 cubic meter chamber in 20 minutes.

Even if aloe vera was particularly skilled at capturing particles, it doesn’t quite remove enough pollution to have a significant impact. At best it would remove particles similarly to a person who is out of breath. 

 

Plants are great to have around and offer many benefits, including a tiny amount of air cleaning, but they don’t compare to an air purifier or proper ventilation. Keep an eye on this blog and our Facebook, Instagram, and Twitter accounts for more on the science of clean air.

Written by

Haldane King is a molecular biologist by education, a statistician by training, and a researcher by nature. He spent 15 years in the market research world helping to grow all types of companies from pharmaceuticals to software to insurance. Haldane has researched the world of air quality, air pollution, and air purifiers at Molekule and now proudly attends to the molekule.com/blog blog.