It is winter and time to close up our homes to keep the heat in and the cold winds out. It is also the perfect time to buy some more houseplants because, if you believe all of the headlines, they will purify the air in your home. This is especially important during winter months when we keep the doors and windows shut.
Do plants purify the air in our home?
Google showed almost 4 million hits when I asked this question. The good news is that the top of the list shows my previous blog post – and it answers the question correctly. Almost all of the other 4 million hits give the wrong answer.
In this blog I will review current knowledge and look at some new research. If you don’t want to read all of the details, just read the first two sections, which give a summary of known facts at this point in time.
Previous Discussions about Air Purifying Plants
The story of air purifying plants starts with a NASA study that showed plants can remove VOC pollutants from the air. It is quoted quite a bit, but interpreted incorrectly almost everywhere. I reviewed the study in detail in A Garden Myth is Born – Plants Don’t Purify Air
An overview of the science behind this topic can be found in Air Purifying Plants – Do They Work. This article also discusses an important study that looked at actual plants in an office setting, which showed no change in air purity after plants were added.
A popular TED video by Kamal Meattle made the rounds a few years ago. He reported on a study where plants were placed in a greenhouse on top of a building, purified the air in the building as it was circulated through the greenhouse. It turns out that when I contacted Mr. Meattle, he was unable to produce any research study. I guess you can say anything you want on a TED video. A complete review of this can be found here; Kamal Meattle – Plants and Air Purification.
New Research on Air Purifying Plants
There have been numerous other research studies done on plants in the lab and some of these have been posted in the comments section of the above posts. All of these have placed plants in a relatively small chamber, about the size of the plant, and monitored the change in a chemical that was injected once into the chamber.
Such studies all show that potted plants do remove chemicals from the air. Many people extrapolate this finding to the home and conclude that plants will also clean the air in our home. That extrapolation, seems obvious at first glance, but quickly falls apart when you have a closer look at the details.
Here are some of the problems when you compare small chambers to real homes.
Relative Amount of Air
All of the lab studies put a plant into a small chamber, about the size of the plant. This results in a relatively small amount of air per plant. In fact one study actually put several plants in a chamber at one time, because a single plant did not show any change in VOC level.
In the home we have a small number of plants in a relatively large space. In my previous review I concluded that to mimic the chamber in the home, “you need to have 50 small plants or 25 large plants for each 100 sq ft, and that is every room of the house., or 400 large plants in a 1,500 sq ft home.
Just because a plant reduces VOCs in a small controlled space does not mean it has any effect in the home.
Single Injection of Chemical
Every lab report I looked at injected the chamber with chemical and then monitored the levels over time. That is not what happens in the home. In the home all of our furnishing, including carpets, paint, cupboards, laminate flooring etc, are producing chemicals 24/7.
Think of it this way. Lets say one person leaves a room every minute. If you start with 100 people in the room, the room becomes empty in 100 minutes. But what if new people enter the room at a rate of 2 people per minute. Will the room ever become empty? No. And this second scenario is precisely the case in a home. VOCs are made faster than any plant can remove them.
Single injection experiments can’t be extrapolated to a home.
Plant Quality and Amount of Light
The chamber experiments use new plants that are in prime condition and are actively growing. Can you say the same for the plants in a home? Are they growing as vigorously as they did in their production greenhouse before you bought them? Not likely.
Lab experiments are also conducted under ideal light conditions. This is important because much of the plants ability to remove chemicals from the air depends on air moving into the leaf through the stomata, the natural openings in the leaf and these are more open in high light conditions. It also depends on a high level of biological activity inside the plant and that only happens in high light conditions.
The home on the other hand has virtually no light. We perceive our homes as having a lot of light, but consider this. Sunlight produces about 100,000 lux (amount of light) and a multi-bulb florescent fixture for growing plants is about 700 lux. The lighting in most homes is about 300 lux.
Plants that perform well in nature are receiving a much higher level of light than in the home. Plants in the chamber experiments get much more light than in our homes and so they remove chemicals much more efficiently.
Movement of Air
In a small chamber there is no air movement but the chemicals in air do diffuse from areas of high concentration to areas of low concentration. This is why a plant can remove molecules that are not immediately next to the plant. In a small chamber all molecules are relatively close to the plant.
What about a typical room? Plants are usually near the window because they need light. The pollutants are spread out everywhere in the room. Since much of the pollution is not near the plant, the plant becomes much less efficient at removing the pollutant. There is more air movement in a home, but nobody has measured how this affects the plants ability to remove chemicals.
In nature, there is continuous air movement, from both winds and thermal convection currents. Plants in nature are therefore much more efficient than in our homes.
Our homes are not air tight even with the doors and windows closed. Outside air is always seeping into the home and pushing inside air to the outside. This movement of air is called infiltration.
Lab chambers have no air infiltration and nobody has looked at the effect of infiltration on the plants ability to remove VOCs.
Infiltration also reduces chemicals in the home and is almost certain to have a greater effect than any plants we might use.
Logic vs Extrapolation
It seems logical that the results from chamber experiments can be extrapolated to homes, but as you can see there are many reasons why such an extrapolation is illogical. Chambers and homes are so different that you can’t use results in one to verify results in the other. The mathematical model discussed below attempts to quantify the differences but there are significant unknowns.
This means that in order to prove that plants clean the air in our homes we need to actually measure the effect in our homes. An important office study has been discussed previously in Air Purifying Plants – Do They Work. A new study doing just this is discussed below. There are a few other studies that have also looked into this and the conclusion they have reached is that any reasonable amount of plants in a home will not clean the air.
Do Plants Purify the Air in Your Home?
Given all of the research on this topic, we must conclude that:
A reasonable amount of plants in the home or office will NOT purify the air.
New Chamber Study
A study from 2009, called Screening Indoor Plants for Volatile Organic Pollutant Removal Efficiency looked at twenty-eight ornamental species commonly used for interior plantscapes. The plants were placed in air-tight glass jars, chemicals were injected, and samples were analyzed to measure how the concentration of chemical changed over time.
“Of the 28 species tested, Hemigraphis alternata (hemigraphis or red ivy) , Hedera helix (common ivy), Hoya carnosa (porcelainflower or wax plant), and Asparagus densiflorus (Sprenger’s asparagus fern) had the highest removal efficiencies for all pollutants” [note: common names not in original quote]. A complete list of plants and removal rates can be found in the reference.
Interestingly, none of these are the plants are normally recommended on social media sites for purifying home air.
The study does point out the fact that some plants are better at removing one chemical vs another, “The efficiency of VOC removal varies substantially among species”. It therefore makes sense that you select plants for the pollutant you want to remove. Problem is that homeowners don’t know which pollutant they have.
Mathematical Model for Air Purification by Plants
The published paper contains a lot of good insight into the problem but I will just pick out a few items I found interesting.
“The model shown here suggests that plants with moderate to high VOC removal rates should be effective at lowering VOC concentrations in buildings, even when some air exchange with the outside occurs.” The paper goes on to say “When plants were tested in actual office rooms with occupants, rooms with interior volumes of 275 and 350 m3, containing 16 to 19 occupants and 22 to 25 plants per office, no reduction was found in benzene, toluene, ethylene, or xylene concentrations.”
The paper goes on to discuss various possible explanations why the mathematical model prediction does not match the actual real world findings. In short there are still too many unknowns.
“When the phytoremediation rate was measured in sealed rooms (60 m3) and compared with that of small chambers (1 m3) under otherwise identical conditions, the VOC removal rate in the rooms was 1/20th of that of the small chambers (Kim et al., 2009). Based on this result, to obtain an equivalent reduction in VOC concentration (i.e., 67%) in a 300 m3 house, it would require an unrealistically large number of plants (i.e., 360 or 750 plants depending upon the species.”
There are two important points here. Actual rooms and small chambers do not behave the same. Second, a large number of plants would be needed for them to have an effect on the air quality in a home.
Plants Affecting Air in Office Buildings
In this study called Evaluation of Indoor Air Quality and Health Related Parameters in Office Buildings with or without Indoor Plants, VOCs were monitored in a variety of office buildings both before and after placement of plants. Measurements were taken in both winter (sealed building) and summer (ventilated building).
“When plants were tested in actual office rooms with occupants, rooms with interior volumes of 275 and 350 m3 , containing 16 to 19 occupants and 22 to 25 plants per office, no reduction was found in benzene, toluene, ethylene, or xylene concentrations.”
“the individual application of ventilation or indoor-plant placement achieved good efficiency against high concentrations but performed poorly with low concentrations.”
“the concentration of formaldehyde in indoor air was hardly decreased by the individual application of ventilation or indoor-plant placement.”
Some chemical VOCs were higher in new buildings compared to older buildings.
Plants have little or no effect on the quality of indoor air.
Importance of Plants Cleaning Our Air
Plants do remove chemicals from the air and in nature the effect is not only large, but very important for the planet. But this is due to several factors. There is a huge amount of leaf surface out there so that it can have an effect. Air movement in nature is significant. Light quality and plant quality are high.
None of these factors exist in the home. Plants just don’t work in the home.
However, there is evidence that “indoor-plant placement could be one of the most appropriate and environmentally-friendly methods to alleviate mental stress.” Grow them because you like them.