The now famous 1989 NASA study has initially recommended at least one plant per 100 square feet of indoor space. Dr. Wolverton, the scientist behind the NASA study, has since recommended at least two “good sized plants” per 100 square feet in a home. Although, more is better if possible, according to his recommendation.
However, we should take this recommendation with a grain of salt. The answer is not only complicated, but also requires more information.
Do I Need 10 Air-Purifying Plants For a 500 Square-Foot Room?
If you translate this recommendation literally, it looks like your house needs to become an indoor botanical garden to truly reap the benefit of improved indoor air quality (IAQ).
As a matter of fact, there’s an app that can help you do that. It’s called Plant Life Balance. An Australian company has read through all the confusing science papers and developed an app (link to Apple version; link to Android version). With all the spotted, incomplete and at times confusing information that has resulted after the 1989 NASA study, an app like this seemed like a welcome resource (and a sigh of relief).
The app could become a little bit annoying for those of us in the United States since it uses meters instead of feet.
Here’s what happened when I tested it out.
First, the app is very simple and visually appealing. It takes only a few taps to get to the results. However, it took quite a few tries to get a perfect score for air quality. The app seems to also incorporates some of the recent research papers that tested productivity and mood of office workers when plants were introduced into the space. It’s certainly a nice bonus.
The image below represents what I encountered when using the app. Before I share the image, notice that I selected a large room of 8 by 8 meters, or 64 square meters. This translates to a space of about 26.2 by 26.2 feet, or 686 square feet. That’s about the total open concept space of my living room, dining room, foyer and eat-in kitchen.
For comparison, Dr. Wolverton would probably recommend about 13 to 14 total plants for this space. Here’s what the app told me I needed:
In order to get the “Perfect” score, the space needed 25 small plants, 20 medium plants, and 6 large plants. That’s a total of 51 plants. And even at the lower tiered score, the app was still requiring over 40 individual plants.
So, why 51 plants? Why not 13 or 14?
The creators of the app don’t mention much on their website about the 101 studies that were supposedly behind the math in the app, but it appears that all the information has been combined by reviewing the NASA study and studies done by RMIT University and University of Melbourne. These studies were were then later reviewed and discussed by an expert panel to reach consensus.
One of the scientists whose research has influenced the app, Dr. Dominique Hes at the University of Melbourne, actually recommends 1 medium-sized plant per about 24 square feet. This would certainly get closer to explaining why my 686 square-feet space requires so many plants.
The early NASA studies and those that soon followed were conducted in isolated, controlled chambers where a predetermined amount of toxins was injected once to measure how the plant would react. Of course, the real world doesn’t work that way. The typical HVAC system cycles the air in a home at least twice per hour, not to mention other factors that constantly change the quality of the air in a dynamic, living home.
It’s logical to assume that in a real living environment, more air-purifying plants are needed to achieve the same results that they would otherwise achieve in an isolated experimental setting. Dr. Hes also takes a practical building science approach to her research, which is definitely the direction that is much needed when it comes to trying to determine just how much air-purifying plants really help in real life.
Before this blog posts starts sounding like I’m trying to pitch you the app, I will mention one more thing about it before I move on. Once you determine what kind of plants you might want to add to your home, you can go back to the app to visualize it in your space and put together the plants that will work best for you.
What If I Don’t Want to Add 50 Plants to Clean the Air in My Room?
It’s not easy to maintain this many plants in a home. Every plant has its own requirements, not to mention the increased potential for mold and humidity. While I can’t tell you exactly how many plants you need to add, there are some ideas that may help you cut down on the number of plants you need in your home.
Use Activated Charcoal
The way you plant your air-purifying plant may even perhaps be more important than the plant itself. The 1989 NASA study used an activated carbon filter with the soil. This system allowed one plant to remove the same amount of VOCs that it would take fifteen plants to remove if it were not for the activated carbon, or activated charcoal.
(When shopping for potting soil enhancements, you’ll likely come across horticultural charcoal. Remember, not all horticultural charcoal is activated charcoal. Activated charcoal is powerful because of its high surface area, which makes it very potent when it comes to trapping toxins and creating pockets of much-needed air.)
Get Some Airflow Through Your Plant
The point of activated carbon in Wolverton’s studies was that it allowed more airflow through the potted soil. Airflow is essential to a plant’s ability to remove toxins from the air. By using compacted, heavy potting soil, the air is blocked because it cannot reach the roots of the plant, where microorganisms break down the toxins.
Since the success of using activated carbon, Dr. Wolverton has partnered with a Japanese company to further develop and maximize the system used in the original studies. As of right now, the system is called the Plant Air Purifier. It makes the plant 60 times as effective at removing toxins as it would be sitting in just regular potted soil. It works because it pushes air into the roots of the plant by means of fans that are conveniently built into the container, hidden from view.
By improving the air flow in the soil, the rate of transpiration will speed up in the plant. That basically means that more toxins will be able to be eliminated at a faster rate. It’s also recommended to use hydroculture pebbles – these have been shown to improve toxin removal by 25% or more. Another side benefit of moving air through the soil more quickly is that stagnant water is eliminated, and by doing this, mold is less likely to grow.
Buy the Best Air-Purifying Plants
The number of plants needed will depend on the total leaf surface area. Plants must first release water vapor from their leaves in order to absorb the indoor air and push it to its roots so that the toxins can be removed. Thus, the rate of water loss has become synonymous with the rate at which a plant can clean toxins from the air.
Even though I love my small window sill aloe vera plant, it’s nowhere near as effective as purifying the air as a large bamboo palm is.
Besides the size of the leaves, some plants are just better at filtering out the air. That’s because these plants have more microorganisms in their roots. Unlike humans, microorganisms actually love harmful volatile organic compounds (VOCs) because they can break them down to use as nutrients.
You can always go back to the NASA study to find out the full list of air-purifying plants and to determine which ones are the most effective.
If you’re interested in a real-life example of how plants can improve indoor air quality, I encourage you to check out Kamal Meattle’s TED talk. Meattle is a researcher and CEO from India who used plants to create the healthiest building in New Delhi. That’s not self-proclaimed. The Indian government has proclaimed this after its tests.
This achievement may not mean much elsewhere, but Meattle’s achievement is pretty impressive once you realize that New Delhi is the #1 most polluted city in the world and has a packed population of 22 million people. The air pollution is so bad in New Delhi, it’s been equated to smoking 50 cigarettes every day.
Meattle introduced 1,200 total plants into the building of 50,000 square feet. If you want to replicate the same results in your home, Meattle recommends three particular plants. These plants were one of the best performing plants in the NASA study, too.
The three plants that Meattle used are:
- Areca palm – recommends 4 shoulder-height plants per person.
- Mother-in-law’s tongue – recommends 6 to 8 waist-height plants per person. This plant is also called the snake plant.
- Money plant -recommends 6 to 8 waist-height plants per person. This plant is also called the pothos, or devil’s ivy.
Lower the Room Temperature
Temperature affects the transpiration rate (or, how quickly toxins can be removed) of a plant. A room with higher temperature will slow down the rate at which a houseplant transpires.
This makes sense since the plants from the NASA study are mostly tropical plants that have adapted to dry conditions. When temperature is high, these plants have a tendency to conserve their water instead of exchanging air with the surrounding environment. In fact, just dropping the temperature in the room from 76 ºF to 73 ºF can almost double the rate of water loss in the Areca palm.
The bottom line is this – if you want to get the most out of your own houseplants, keep your home cool.
It’s clear that plants have the ability to purify air. However, there’s no clear consensus on how many plants are needed to purify the air in your home. It’s probably safe to say that a normal home probably needs more plants than what the NASA study has initially shown under isolated, experimental conditions.
Hopefully the resources I’ve offered in this post will help you find the right number of air-purifying plants your home needs.