According to an often-quoted study conducted by NASA, house plants have been found to purify the air and remove “up to 87% of the amount of toxins in the air”. Recent studies, however, have cast doubt on these statements finding that indoor plants actually have no significant effect on improving indoor air quality (IAQ). What is the real scientific evidence behind these claims, and why do the findings of these studies differ so much? Read on more to find out.
The link between plants and indoor air quality
Why are we interested in the potential of plants to improve indoor air quality? Generally speaking, plants are known for their ability to turn carbon dioxide into oxygen in a process called photosynthesis, thereby creating the foundation of life on earth. Photosynthesis includes capturing carbon dioxide (CO2) from the air, alongside water (H2O) from the air and ground, and light energy from the sun to create glucose (sugars) and oxygen (O2). However less well-known is that plants also release CO2 back into the atmosphere through respiration. Cellular respiration is the process by which plants combine O2 with food molecules, keeping necessary substances and discarding CO2 and water as waste products. Photosynthesis happens in the presence of sunlight; in its absence plants use cellular respiration to survive.
Existing research has identified certain benefits of plants in interior spaces:
- Indoor air quality. Studies funded by NASA showed that plants reduced levels of some indoor pollutants and can raise relative humidity to healthier and more comfortable levels indoors.
- Well-being. Numerous studies showcase plants to reveal a number of positive feelings associated with plants. Studies have also shown that plants can also have productivity effects and reduce mental fatigue.
- Health Improvement. Plants can have a decorative value through which they can even influence pain tolerance. Rooms with plants were found to be significantly more interesting, comfortable, and ornate than rooms without them. Plants can thereby serve as a distraction to help keep one’s mind off of discomfort.
Can plants really improve air quality?
There are some mixed assumptions that prove for and against why plants really improved air quality. Critical reviews, studies, and research can be accounted for both sides:
Zoric and colleagues proposed in their research to promote the role of plants in human lives, as well as to develop a system of assessment and potential improvement of air quality. They conducted their studies by placing eight plants (called Sansevieria trifasciata ‘Laurentii’) into a bedroom environment. Zoric et al. argue that based on their findings, the potted plants did not have a significant, positive impact on CO2, methane, and volatile organic compound (VOCs) concentrations in the indoor air.
Cummings and Waring wanted to look further and analyze whether plants have an effect in reducing indoor VOC counts and improving air quality. They investigated this by standardizing 196 experiment results into a metric useful for measuring the identified indoor air cleaning capacity. Upon analysis, they discovered that many of the analyzed studies did not assess the same VOC combinations. Still, they identified some positive air quality impact(s) from plants: they observed lower levels of formaldehyde or benzene in the rooms’ air. Even though these positive effects were identified, the levels of formaldehyde and benzene did not occur simultaneously, meaning that plants improved one aspect at a time. Moreover, they were able to see that the measured clean air delivery rate (CADR) varied so much between individual studies that it might actually be more likely to be based on the different removal measurement methodologies used in those studies.
Gubb and colleagues investigated how potted plants might actually have an effect in removing nitrogen dioxide (NO2). They conducted an NO2 chamber experiment on three common houseplant in different conditions: exposing the plants to light vs. having no light present, potting the plants in wet vs. dry substrate (the substance needed for plants to grow roots and absorb nutrients).The researchers found the following:
- They demonstrated that the potted plants were able to remove measurable amounts of NO2 under the presence of light. These light conditions represent the common indoor condition, showing the replicability of an indoor environment.
- They were also able to show that different common houseplants removed NO2 at differing rates based on the amount of leaf surface area. This outcome suggests different inherent capacities for NO2 removal.
- Based on their estimates from the NO2 removal chamber experiment, five plants in a small office could remove approximately 3 ppb of NO2 within an hour (as a comparison: the EU threshold for outside nitrogen dioxide levels is currently set at 40ppm in the annual mean, i.e. 40,000 ppb). NO2 removal by plants was found to be consistent during day and night.
Han and Ruan also conducted a systematic review that comprehensively examined the effect of indoor plants on air quality in both English and Chinese. They were able to use 64 English and 31 Chinese articles resulting in the following commonalities:
- The primary effects of indoor plants are air purification and air quality improvement.
- Secondary effects of the potential of the indoor plants are increasing air humidity followed by reducing room temperature.
- Air quality and microclimates can be improved using various plants. Certain purifying and humidity-increasing plants may reduce the need for mechanical ventilation, thereby reducing energy consumption.
Gonzales-Martin and colleagues finally critically assessed the state-of-the-art of the indoor air pollution problem and possible prevention strategies, along with the recent advances in physical-chemical (i.e. artificial) and biological technologies for indoor pollutants removal. They concluded that future biology-based purification systems would be more promising than artificial solutions, especially the variability in air pollutant concentrations. These systems would need to assure humidity control of purified air as well as incorporate CO2 removal.
The aforementioned studies indicate how controversial the positive effects of plants on indoor air quality are in the first place, and that if they exist at all, the strength of the effect is so unclear and in some cases so small that you would have to disproportionately fill a room with plants to achieve a measurable reduction in certain pollutants. This is an obviously unrealistic and impractical scenario, and would create other problems such as increasing the humidity too much and introducing the danger of mold. Citing the study from Gubb et al. for instance, five plants would need nearly two years to fully remove 40 ppm of NO2 from a room – allthewhile new nitrogen dioxide might be entering through ventilation, thereby cancelling out the effect.
There is still research to be done to learn more about how plants can improve indoor air quality, especially with regard to individual plant species‘ capacities to clean air, and the environmental conditions that favor or curb this effect. Based on these findings, new systems could potentially be designed with significant air cleaning capacities.
What can we do to improve indoor air quality?
If plants are not the solution, then what is? There are several ways in which one can improve the air quality where they live:
- Source control: Eliminating the source of pollution or reducing their emissions is the most effective way to improve indoor air quality. Source control is also the most cost-efficient approach to protecting air quality inside buildings.
- Improved ventilation: Increasing the amount of (clean) outdoor air entering the indoor environment is another approach to lowering the concentrations of indoor air pollutants. Opening windows and doors, or running a window air conditioner increases the outdoor ventilation rate. It is important to check outside air pollution levels, though. To enable that, Breeze Technologies runs a global air quality citizen portal where you can check local air pollution levels for free.
- IAQ monitoring and management: Breeze Technologies’s high-end sensors can provide real-time monitoring of all common indoor air quality parameters at relatively low cost. Deploying an indoor air quality management solution has never been easier! Contact us to find out more!