Written by David Roberts, MPH, Co-founder, Mara Labs
Passive houseplants do not measurably clean room air. Research converting decades of chamber studies into real-room terms found you would need 10 to 1,000 plants per square meter to match ordinary ventilation. Normal plant care does not raise indoor mold.
Key Takeaways
- ✓A single plant filters far less than one window
- ✓Ventilation and filtration do what plants cannot
- ✓Potting soil holds fungi, room air stays unchanged
- ✓Standing water in saucers and cachepots is the risk
- ✓Plants measurably calm people, not room air
Do houseplants clean the air in a real room?
No. Not at any number a person would keep in a home.
In 2020, Bryan Cummings and Michael Waring reviewed 12 chamber studies and translated 196 experimental results into clean air delivery rate, the standard metric used to rate air cleaners. [1]
The median result was 0.023 cubic meters per hour per plant. Matching the air exchange a building already provides would take 10 to 1,000 plants per square meter of floor space. [1]
A cracked window outperforms a room full of pothos.
The belief traces to a 1989 report by B.C. Wolverton and colleagues, funded jointly by NASA and the Associated Landscape Contractors of America, the interior landscaping trade group. [2]
Plants sat in sealed chambers, often smaller than a cubic meter, with a single compound injected and its decay tracked across 24 hours. [2]
The measurements were accurate, but scaling those results to a 1,200 square foot house is not the same.
What removes pollutants from indoor air?
Three things, in a specific order. The EPA ranks source control first, ventilation with clean outdoor air second, and filtration as a supplement to both. [3][4]
Source control means removing whatever is emitting: the new pressed-wood shelf, the aerosol, the unvented gas range. Nothing else works as well, because nothing else stops the emission. [4]
Filtration handles what remains. HEPA media captures particles, activated carbon adsorbs gases, and most inexpensive tabletop units do neither well. [4]
| Approach | What it addresses | What it does not | Relative effect |
|---|---|---|---|
| Removing the source | The emission itself | Anything already off-gassed elsewhere | Largest |
| Ventilation | Gases and particles together | Works against poor outdoor air quality | Large |
| HEPA portable cleaner | Particles, smoke, spores, dander | Gases and odors | Moderate, room by room |
| Activated carbon | Gases and odors | Particles; loads up and needs replacing | Moderate, depends on mass |
| Potted plants | Trace amounts in sealed chambers | Room air at any realistic density | Negligible |
Do houseplants raise indoor mold levels?
When measured in occupied buildings, the results say no.
Torpy and colleagues sampled 55 single-occupant offices across two Sydney buildings in autumn and spring, assigning each office zero, one, two, or three plants. [5]
Plant presence made no significant difference to mould spore counts or species composition. Neither plant number nor species changed the result, and indoor spore loads ran below outdoor levels. [5]
This sounds counterintuitive, as potting soil is a dense fungal reservoir.
Haas and colleagues measured median counts of 9.5 x 104 to 5.5 x 105 colony-forming units per gram in commercial soils and potted-plant surfaces.
That is roughly ten times sub-surface levels, with Aspergillus and Penicillium well represented. [6]
Both are true. A reservoir inside a pot is not the same as spores in the air you breathe, and the bridge between the two is standing moisture.
What keeps potting soil from staying wet?
Drainage and watering interval prevent excess standing moisture.
The EPA's indoor air guidance names overwatering directly, noting that persistently damp soil encourages microbial growth that affects allergic people. [3]
The same guidance puts indoor relative humidity below 60 percent, ideally between 30 and 50 percent, as the threshold where mold growth becomes unlikely. [7]
A twenty-dollar hygrometer is the easiest way to assess moisture.
| Condition | Why it holds moisture | What changes it |
|---|---|---|
| Pot with no drainage hole | Water pools at the base with nowhere to leave | A drilled hole, or a nursery pot set inside |
| Decorative cachepot | Runoff collects unseen under the inner pot | Lifting the inner pot out after watering |
| Saucer left full | The soil wicks it back up for days | Emptying the saucer within the hour |
| Watering on a fixed schedule | Winter and summer drying rates differ sharply | A finger two inches into the soil instead |
| Peat-heavy mix in a low-light room | Dense media holds water the plant is not using | Bark or perlite blended through the mix |
| Pots grouped on an exterior wall | Cool surfaces raise local humidity and condensation | Spacing, and air movement across the group |
Is there a version of plant filtration that works?
Yes, when a fan pushes air through the root zone. Active green wall biofilters draw room air through the growing substrate rather than waiting for it to drift past the leaves.
A 2019 study of four plant species in these systems measured single-pass benzene removal between 45.5 and 59.5 percent. [8]
The variable is airflow through the substrate and its microbial community, not the foliage. A shelf of plants in still air does not have airflow.
Wolverton reached the same conclusion in 1989. His report proposed a plant unit with activated carbon and forced airflow, because the passive version does not accomplish this. [2]
What are houseplants good for?
The documented effect is on the person, not the air.
In a 2015 crossover study, participants who transplanted an indoor plant showed reduced sympathetic nervous activity, and rated themselves more relaxed than after a computer task. [9]
The study was small: 24 young men, one session each.
It also points somewhere reasonable. Plants earn their place through what tending them does for the person doing the tending, and that case is beneficial regardless of filtration claims.
Frequently Asked Questions
How many houseplants would it take to clean the air in a room?
Between 10 and 1,000 plants per square meter of floor space, according to a 2020 analysis in the Journal of Exposure Science and Environmental Epidemiology. That is a density no home reaches. Ordinary building air exchange already does the work faster.
Is the NASA houseplant study wrong?
Its measurements were valid for the conditions tested: sealed chambers, often under a cubic meter, with a single compound injected. The error came later, when those results were applied to houses and offices where air is constantly exchanging.
Can potting soil make you sick?
Commercial potting soil carries high fungal counts, including Aspergillus and Penicillium species. Field sampling across 55 occupied offices found that plant presence did not raise airborne spore counts. People who are immunocompromised or highly allergic have reason to handle soil with more care.
Do snake plants or pothos filter air better than other plants?
Differences between species appear in chamber studies, but all of them fall orders of magnitude below the removal rate of normal ventilation. At the density found in a real home, species choice makes no measurable difference to air quality.
Does a humidifier or a group of plants raise mold risk more?
A running humidifier adds far more water to indoor air than a group of houseplants transpiring. Either way, the number that matters is relative humidity. Mold risk climbs above 60 percent, and the EPA's recommended range is 30 to 50 percent.
Sources
[1] Cummings, B.E., and Waring, M.S. "Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies." Journal of Exposure Science & Environmental Epidemiology, 2020.
https://doi.org/10.1038/s41370-019-0175-9
[2] Wolverton, B.C., Johnson, A., and Bounds, K. "Interior Landscape Plants for Indoor Air Pollution Abatement: Final Report." NASA / Associated Landscape Contractors of America, 1989.
https://ntrs.nasa.gov/citations/19930072988
[3] U.S. Environmental Protection Agency. "The Inside Story: A Guide to Indoor Air Quality."
https://www.epa.gov/indoor-air-quality-iaq/inside-story-guide-indoor-air-quality
[4] U.S. Environmental Protection Agency. "Guide to Air Cleaners in the Home."
https://www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home
[5] Torpy, F.R., Irga, P.J., Brennan, J., and Burchett, M.D. "Do indoor plants contribute to the aeromycota in city buildings?" Aerobiologia, 29(3), 2013.
https://doi.org/10.1007/s10453-012-9282-y
[6] Haas, D., Lesch, S., Buzina, W., Galler, H., Gutschi, A.M., Habib, J., Pfeifer, B., Luxner, J., and Reinthaler, F.F. "Culturable fungi in potting soils and compost." Medical Mycology, 54(8), 2016.
https://doi.org/10.1093/mmy/myw047
[7] U.S. Environmental Protection Agency. "Mold Course Chapter 2: Why and Where Mold Grows."
https://www.epa.gov/mold/mold-course-chapter-2
[8] Irga, P.J., Pettit, T., Irga, R.F., Paull, N.J., Douglas, A.N.J., and Torpy, F.R. "Does plant species selection in functional active green walls influence VOC phytoremediation efficiency?" Environmental Science and Pollution Research, 2019.
https://doi.org/10.1007/s11356-019-04719-9
[9] Lee, M.S., Lee, J., Park, B.J., and Miyazaki, Y. "Interaction with indoor plants may reduce psychological and physiological stress by suppressing autonomic nervous system activity in young adults: a randomized crossover study." Journal of Physiological Anthropology, 2015.
https://doi.org/10.1186/s40101-015-0060-8
0 Comments