One of the more frequent queries we receive from our customers is “What is the best air purifying plant?" While studies have certainly shown some plants are better than others, the answer is not a simple one, and articles have been published recently contending that indoor plants don’t purify the air in any meaningful way at all. The answer, simply put, is complicated.

Spoiler alert.

The amount of oxygen released by plants is proportional to the amount and spectrum of light they receive. If you have a plant in a dark corner, it produces very little oxygen as compared to a plant in a full sun location (or under a grow light). Most of the studies we have read that conclude that plants have no measurable air quality are putting the plants from their study under conditions that mimic home lighting, using lighting sources that are antiquated in today’s environment such as CFL or fluorescent bulbs. Many of these scientific articles don’t even source their lights in their methodology. This includes all of the studies cited in the Atlantic article, A Popular Benefit of Houseplants is a Myth and the National Geographic article “Which houseplants should you buy to purity the air? None of them.” 

 

A Poorly Studied Field

Our response to plants purifying the air indoors is that there's just not enough consistent research out there, and the biggest problem is the variable of light. As you move a plant from low light intensity to higher light intensity, the rate of photosynthesis increases because there is more light available to drive the reactions of photosynthesis. So with the amount and quality/spectrum of light as a key part of the photosynthesis equation, replicating any research study in an office or home environment can be complicated unless you're using grow lights and have access to a spectrometer. In a nutshell, you need A LOT of plants in BRIGHT LIGHT to produce a measurable change in oxygen levels and VOC reduction. An indoor jungle under bright light conditions will most certainly have a measurable impact, while a few small plants on a bookshelf in indirect light will be much harder to quantify.

Plant lighting is its own large field of study but a good place to start is learning about µmol, or the numerical measurement of light photons. µmol represent a numerical value of the total number of light photons hitting the place of measurement. A good analogy to µmol reading is: if you wanted to know how much rain fell in a day, would you put a bucket outside for 24 hours and measure a full sample, or just put a bucket out for one minute and then try to judge how much rain fell over a 24 hr period based on that one minute sample? The raindrops per minute would be of little value, just like a one time light meter reading or looking out your window at noon is of little value to gauging your plant's access to light. You have to do the calculations to figure out the volume and quality of light over time as that is how you could determine how much photosynthesis and growth is happening for different plant varieties.

 

The Value of Plants

Because of the lack of good research and the variable light has on photosynthesis, at Greenery Unlimited we don't focus on "air purification" as the primary benefit of integrating plant life into the built environment. There was much lobbying done to include indoors plants as part of LEED certification, but the inability to quantify exactly how much air purification was happening when light levels are so variable depending on the location of the plant ultimately led to plants exclusions for the certification. That said, WELL certification and LBC certification have biophilia elements, with a focus on live plant material designed into the space as having emotional  and cognitive benefits. Part of the reason our sister company Greenery NYC is always harping on getting grow lights into our designs is that we know plants perform better in spaces with adequate lighting to support photosynthesis.

Instead of "air purification," we find it more useful to focus on the emotional/creative/productivity benefits that plants have on our daily lives. One of the most cited recent studies is the Human Spaces Report. The sample was 7,600 office workers in 16 countries, and among the conclusions drawn were the increase of feelings of well-being, creativity and productivity when natural elements are incorporated into office design. We never cite that NASA air purification study -- it was bad science that obscured the relationship of lighting to the study findings.

Some fun factoids from more recent studies that we think are worth reading:

• The 2015 Human Spaces report, found that nearly two-thirds (58%) of workers have no live plants in their workspaces. Those whose environments incorporated natural elements reported a 15% higher well being score and a 6% higher productivity score than employees whose offices didn’t include such elements. The report also found that employees whose offices included natural elements scored 15% higher for creativity than those whose offices didn’t include such elements.

• Employees’ productivity jumps 15% when previously ‘lean’ work environments are filled with just a handful of houseplants, according to 2014 research by the University of Exeter. 

• A 2010 study by the New University of Technology, Sydney, found significant reductions in stress among workers when plants were introduced to their workspace. Results included a 37% fall in reported tension and anxiety; a 58% drop in depression or dejection; a 44% decrease in anger and hostility; and a 38% reduction in fatigue.

• By absorbing sounds (rather than insulating against noise pollution), plants help to reduce the distracting effects of background noise. Positioning larger plants in multiple locations in the edges and corners of a room has a positive benefit, according to a 1995 paper by researchers at London South Bank University.

• Attention restoration theory suggests that looking at nature – and even just images of nature – can shift the brain into a different processing mode, making employees feel more relaxed and better able to concentrate.

• And even though we said we don't focus on air quality because of the proportional relationship to light, there is a good 2012 study from University of Technology, Sydney (UTS) that shows with appropriate light, plants do a great job of decreasing Co2, increasing oxygen and decreasing VOCs.

• Another study that just came out that is worth a read is from Reading University and the Royal Horticultural Society that focused more on the relative humidity increase that comes from plants, and how that increase in humidity makes healthier spaces by reducing the potential transmission of airborne germs and increasing general respiratory and skin comfort.

So what are the big take-aways here? A plant's access to light is the biggest determining factor in its ability to capture CO2 and VOCs and produce O2. Because most people put houseplants in lower-light spaces, those plants don't produce as much O2 as they could if they were in a brighter spot. If one doesn't have good access to natural sunlight, there are some amazing LEDs on the market now that are consumer friendly -- as simple as screwing in a light bulb!