House plants and indoor air quality

Let me begin with an admission.  I like indoor plants.  Throughout my career I have usually had one large plant in my work office.  Plants are nice to look at, and visitors often comment on the “nice plant.”  But while I like indoor plants, and suspect that they have some benefit in terms of psychological comfort to many, they do not appreciably improve indoor air quality any more than an old pair of socks that I would hang on a wall.  The sad thing about this never-ending saga is that those who are fooled into thinking that plants are good indoor air cleaners often avoid approaches and technologies that actually do effectively remove pollutants from indoor air.

In 2019, I gave a public seminar focused on myths related to indoor air quality.  I spoke about a lot of topics, from ion generators to essential oils. The audience was engaged throughout the seminar, asking many good questions.  But engagement quickly turned to enragement when I told them that plants do not clean indoor air. This issue is a visceral one for many on the side of purported VOC-munching house plants.  I have to believe that this response stems from years of repeated claims about the virtues of house plants as “air purifiers”, never with good evidence that can be translated to actual indoor environments.  There are some who grab on to something they saw on social media and communicate it widely without actually understanding the science.  And unfortunately, some researchers appear to seek a phyto-holy grail.  Many are responsible for bad research or flawed interpretation of results.  In all cases, none of these researchers are building scientists who understand spatial or temporal scale-up to actual building environments.

I recently did a Science Citation Index on peer-reviewed journal papers related to indoor plants for removal of VOCs or formaldehyde, with a lot of variations to try to capture as many papers as possible over the past 15 years.  I found 236 related papers.  Numerous papers have been published on pollutant removal by various types of plants.  Pollutants are dominated by volatile organic compounds (VOCs) such as toluene and benzene.  Formaldehyde, a ubiquitous indoor pollutant, has also been studied by many researchers.  So as not to be too critical, I will not mention specific authors here, nor will I mention journal editors who agreed to publish work of little relevance to indoor air quality.  Rather, I will generalize common problems with a large majority of papers.  Those with access to Science Citation index or other bibliographic databases can explore on their own.

For starters, let’s assume an 800 ft2 (74 m2) apartment with a ceiling height of 2.8 m (total volume = 208 m3) with a reasonable air exchange rate of 0.5/hr.  This translates to a volumetric flow rate of air into the apartment of 104 m3/hr (assuming similar indoor and outdoor temperatures and pressures, this is also close to the volumetric flow rate out of the apartment).  To reduce the concentration of a specific VOC by 50% (a measurable reduction) would require an air cleaner with a clean air delivery rate (CADR) of 104 m3/hr.  There are standalone air cleaners on the market that employ activated carbon and that can achieve this CADR for many VOCs.

So how does this relate to plants?  Well, here is the root of the problem (pun intended).  Research that has been completed on plants as air cleaners almost always suffers from one to all of these problems:

  1. Experiments are done in relatively small chambers that lead to MUCH higher plant (and potting soil) surface to chamber volume ratios than had the plant been placed in a large room, nonetheless an 800 ft2 apartment or even larger home.
  2. Experiments are completed with very chamber low ventilation rates, and some in completely sealed chambers.  Zero ventilation may be relevant to the international space station, but not to environments where nearly all of humanity spends the majority of its time.  Remarkably, many papers do not even state the chamber air exchange rate (ventilation rate), likely not recognizing the importance of this factor for back-calculating parameters that are relevant to assessing utility of plants in actual buildings. 
  3. Decay of pollutants injected into chambers is measured over long time periods, sometimes several days (required to actually measure decay).  Air generally stays in actual indoor spaces for tens of minutes to a few hours (not days), depending on air exchange rates of those spaces.
  4. Pollutant levels are often highly exaggerated by 100 to more than 1,000 times (or more) of what is typical in buildings.  This can affect the bioactivity of microbes in potting soil.

Consider the following statement in a recent paper … “A sealed perspex chamber with lid and fan was designed to ensure minimum leakage, proper aeration and distribution of benzene inside the chamber. Five different ornamental indoor plants were placed inside the chamber sequentially and exposed to a concentration of 5 ppm benzene for 30 h each. The leakage of benzene was checked beforehand.”

This type of approach is typical and of little relevance, unless the data are used to back-calculate parameters that can be scaled-up to actual buildings.  In this case, while spectacular results are presented, the scale-up of data to actual buildings shows these plants to be wholly irrelevant unless your home, workplace, or child’s classroom is packed with a deep jungle of plants, is extremely poorly ventilated, and trays of gasoline with an open surface are  emitting benzene into the indoor space.  I want to underscore the fact that this is not untypical of research done to show how houseplants can improve indoor air quality.  The research methods might have low experimental error, but the approach and outcomes are simply irrelevant (underscored x 100) to the real world.

What’s concerning about most research on houseplants is that the work is published without analysis of data relevant to actual buildings or insufficient data for building scientists to extract relevant parameters like clean air delivery rate (CADR) for the plant.  Extraction of such parameters is relatively easy when appropriate data are collected and analyzed using a basic concept in environmental and chemical engineering known as a mass balance (follow the mass).  To be clear, there are a few papers (a very small minority) with sufficient data and methodological description to allow determination of CADR.  For those papers, the back-calculated clean air delivery rate (CADR) for different types of plants and VOCs tends to be extremely low, e.g., 0.01 to 0.1 m3/hr (0.5 m3/hr would be very generous given published data).  To put this in another way, that 800 ft2 (74 m2) apartment described above would need to have somewhere between 1,000 to 10,000 plants in it to remove 50% of VOCs (perhaps 200 plants under the most generous of assumptions).  And yes, a larger home would require many more plants than this!

Here is how results in papers can be highly deceiving and what happens when one puts results in context.  Shown in the plots below are not data from an actual paper, but reflect approximate results from many studies.  Plot A shows the normalized concentration (pollutant concentration divided by its initial value after injection) plotted against time in hours.  In this case I have assumed a sealed chamber so that the entire decay can be attributed to pollutant removal by a plant in a 200 L chamber.  Approximately 95% of the pollutant has been removed over 72 hours.  That’s great, right?  Run out and grab a couple of potted plants for your home, office, or child’s classroom.  Not so fast.  The clean air delivery rate (CADR) back-calculated from this analysis is only 0.0125 m3/hr.  Compare this with, say, a CADR of 500 m3/hr (300 ft3/min) for portable HEPA filters that remove particles (4 to 5 orders of magnitude lower for the plant in this analysis).  Now, if one uses 0.0125 m3/hr in a mass balance to estimate the % reduction of a pollutant due to use of indoor plants in the apartment descried previously, we get plot B.  Seventeen plants buys you about a 0.25% reduction in pollutant concentration.  It takes over 80 plants in the apartment to get to a 1% reduction in pollutant concentrations (levels) and we do not even achieve 2.5% with nearly 200 plants.  Note that these concentration reductions are less than or approach the experimental error for specific VOC measurement and analysis, i.e., we would have no confidence that we could actually see a difference in pollutant levels, even with what would appear to be an apartment converted to a dense greenhouse.

If one wants to give plants the benefit of the doubt it is instructive to look at ozone, a pollutant that ought to be easily removed by chemical reaction when ozone molecules come in contact with plant surfaces or potting soil.  One research team (proudly from Portland State University) did very well-controlled experiments using five common indoor houseplants touted for improvement of indoor air quality.  They back-calculated appropriate parameters to allow a scale-up analysis for actual buildings.  Applying their results to a 1,800 ft2 home suggests over 90 houseplants are needed to remove somewhere between less than 1% to less than 10% of ozone.  Not to dwell too much on my old socks, but I could hang 90 old pairs around the house and see ozone removal benefits that might exceed those of the house plants that were tested based on reactions with squalene from my skin oil left on the socks.  In either case, there would be unwanted reaction products formed and released to indoor air. And that leads me to the topic of unintended consequences.

The jungle that we have created comes with some additional problems.  Plants emit sesquiterpenes and other easily oxidized terpenoids that can leave a long-lasting stench.  Those who have ever been inside a home that was once a marijuana grow house know what I am talking about.  It can be difficult to sell such homes.  And those terpenoids can be pretty reactive with a little ozone around, e.g., in the summer ozone season or if one also myths into using an ion generator as an air cleaning device.  Related reactions can lead to light and heavy carbonyls that can irritate the upper respiratory system or worse, and that can also linger on surfaces with a very slow release over weeks to months.  Furthermore, watering all of those plants will affect moisture balances in the indoor space, with possible mold problems as a result.

I have seen some propose the addition of granular activated carbon into the soil that supports the plant with forced air flow through the soil’s root zone.  This would allow VOCs to be adsorbed to the activated carbon and biodegraded by microorganisms in the soil.  This is really not as much of a houseplant as it is a more conventional packed-bed biofilter, which could work in theory.  However, even if a highly generous 100% of VOCs are captured in the soil bed, one would need to pass 100 m3/hr of air through the bed to get the same results as the commercial air cleaner described earlier.  That’s a lot of air flow. For an 8 inch diameter planter it equates to an air speed of almost 1 m/s through the bed.  Water would have to be replenished frequently due to evaporation, and things could get messy with moisture, soil particles, bacteria, and activated carbon released from the bed.  To avoid these complications a smaller volume could be used, e.g., 0.5 m3/hr, but that gets us back to a jungle of house plants. 

Any way you trim this (pun intended), using plants to reduce pollutant levels indoors is simply not practical.  Appreciate plants for their beauty, but don’t ask them to perform miracles.

Alas, it is time to go water my indoor plants.

One thought on “House plants and indoor air quality

  1. Mr Anonymous

    Unfortunately my PhD was on this exact topic (it was a studentship so already decided) – my viva in 2021. About a year in I realised the same thing, and wanted to write a paper just as prof waring has done.
    However my supervisor and funding body were a plant based company and thus negative work would not have been appreciated or suitable for publication. I have published three research papers, with the results exactly as I found I.e need hundreds of plants at a minimum – unfortunately as the research is rather flawed ( as you mentioned above) none are in any high/medium impact journals.
    I have learnt a lot about indoor air quality during my work, but feel my potential career in academia is now pretty much finished because of the topic of my PhD and the research I have undertaken and published.
    Just wondering if you have any helpful suggestions for me moving forward? Also have really enjoyed your recent presentations.

    Like

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