Do houseplants really clean the air in a room?
― Matching that effect would take over 100 pots in a small room
Experiments really have found that plants can absorb air pollutants. But that was inside a small box with no air flowing in or out. In an ordinary room, the air exchanged through windows and gaps outweighs anything a plant does by a wide margin.
You've just moved into a new flat, and it still smells of new furniture. At the shop, you spot a houseplant with a tag reading "air-purifying plant."
You buy one and set it by the window, and the air somehow feels fresher. You vaguely recall hearing that NASA proved this in a study.
So how much air does that one pot actually clean in an hour? Look into it, and the answer turns out to be far smaller than you'd think.
There are only two reasons "cleaner air" doesn't work in a real room
The original experiment took place inside a container too small for a person to enter. In a small, closed space, pollutants steadily decrease even at a slow rate.
In an ordinary room, outside air constantly flows in through vents and gaps. That volume is more than 100 times what a single pot can purify.
It's not a lie that plants remove pollutants. The mistake was applying a result from a sealed box directly to the scale of a whole room. Let's look at this step by step.
The original story was about an experiment for a space station
In 1989, NASA researchers were looking for ways to keep the air clean aboard space stations, where opening a window to ventilate simply isn't possible.
So they placed houseplants inside a sealed chamber and injected it with organic gases released from furniture and paint. Over several hours, the gas concentration dropped. The result spread widely, giving rise to the claim that "plants clean the air."
Look at the left side of Figure 1. In this experiment, the air inside the container never exchanges with the outside. Pollutants have nowhere to go but into the plant and the soil, so they decrease, even if slowly. Later research came to suggest that microbes living in the soil around the roots do more of the gas breakdown than the leaves do.
Room air is fully replaced roughly every two hours
Homes in Japan today are required to have ventilation systems that exchange at least half the room's air every hour — a rule set in 2003 to prevent health problems caused by gases released from building materials.
For a 10-square-metre room, that works out to about 12 cubic metres of fresh air entering every hour. Meanwhile, a 2019 study pooled and recalculated earlier plant experiments. Working backwards from those results, a single pot cleans at most around 0.1 cubic metres of air per hour.
To match ventilation using plants alone, divide 12 by 0.1: you'd need about 120 pots. Look at Figure 2. Lining 120 pots across the floor of a 10-square-metre room leaves almost no room to walk. Studies estimate a need of anywhere from 10 to 1,000 pots per square metre.
At night, plants stop photosynthesizing and only respire. That does use a little oxygen, but the amount is thought to be far smaller than what a sleeping person in the same room breathes. For the same reason a plant's air-cleaning power is small, its air-worsening power is small too.
Not at all. Plenty of studies find that seeing greenery has a calming effect. Transpiration from leaves can also raise humidity slightly in a dry room. It's just that expecting a plant to substitute for an air purifier doesn't add up numerically.
Summary
It's true that houseplants remove air pollutants. But that effect is only visible in a small sealed space with no air exchange. In an ordinary room, the air replaced through ventilation is more than 100 times what a single pot provides. If you want cleaner air in a room, opening a window and cutting pollution sources at the source is the shortcut.
Plants do clean the air.
But the window does 100 times the work.
For how plants release water from their leaves to cool their surroundings, see "Why are forests cooler than cities?", and for how leaves manage taking in and letting out water, see "Why don't cacti die without watering?".
- Measure your room's floor area and ceiling height.
- Multiply the two to get the room's volume, and use half of that as a rough guide for "air exchanged per hour."
- Divide that number by 0.1 to get the number of pots needed to match ventilation. Compare it with how many pots you actually have.
Hold a single tissue near a vent or exhaust fan to see visually which way the air is flowing.
Want to go deeper? ― terms, formulas, and where this fits in the curriculumLabels show whether each part is middle-school-level or university-level material
- MSCovered in middle-school science
- HSCovered in high-school chemistry/maths
- HS+Advanced high-school content, or textbook sidebar material
- UniUniversity-level content (architectural/environmental engineering) not taught in high school
- ResearchNot yet settled even at university level — an active research question
MSTerms: this phenomenon has a name
- Volatile organic compounds (VOCs): organic substances released as gas at ordinary temperatures from furniture, paint, adhesives, and the like. Formaldehyde and toluene are examples.
- Air change rate: how many times the room's volume worth of air is replaced per hour. A rate of 0.5 means half the room's air is exchanged in an hour.
- Clean air delivery rate (CADR): a measure of air-cleaning ability, expressed as how many cubic metres of fully purified air can be produced per hour. This is also the concept used on air-purifier spec sheets.
MSHSChecking with a formula: how many pots does it take to match ventilation?
We calculate how many pots it would take for the clean air a plant produces to equal the clean air brought in by ventilation.
| In symbols | N = n × V ÷ C |
| In words | Pots needed = air change rate × room volume ÷ one pot's clean air delivery rate |
| Where it comes from | This is a mass balance for pollutants entering and leaving the room. Since cleaning power is measured as "clean air produced per hour," we solve for the number of pots at which the plants' rate matches ventilation's rate. |
| Symbol | Meaning and unit |
|---|---|
| N | Pots needed (pots) |
| n | Air change rate (per hour) |
| V | Room volume (cubic metres) |
| C | One pot's clean air delivery rate (cubic metres/hour) |
| Floor area of a 10-sqm room | about 10 m² |
| Ceiling height | 2.4 m |
| Home air change rate (legal minimum) | 0.5 per hour |
| One pot's clean air delivery rate (generous figure worked back from studies) | about 0.1 m³/hour |
| Room volume | 10 × 2.4 = 24 m³ |
| Air exchanged by ventilation per hour | 24 × 0.5 = 12 m³ |
| Pots needed to match that | 12 ÷ 0.1 = 120 pots |
| Pots per square metre of floor | 120 ÷ 10 = 12 pots |
Twelve pots per square metre sits at the low end of the "10 to 1,000" range studies suggest. Even giving plants a generous benefit of the doubt, you'd have to pack the room with pots just to match ventilation.
HSHS+Why did it look so "effective" inside the sealed container?
HSInside a sealed container, the rate of pollutant removal is proportional to the current concentration. So the concentration halves, then halves again, at regular time intervals — the shape of an exponential function from maths class. The smaller the container, the shorter the halving time, even with the same plant.
HS+In a room, pollutants keep being released from furniture and the like. The concentration settles wherever the release rate balances the removal rate from ventilation and plants combined. That concentration is set by "release rate ÷ (ventilation rate + plant rate)," and ventilation makes up almost all of the denominator. Adding the plant's rate barely moves it.
UniThe "well-mixed model" that determines indoor concentration
In architectural environmental engineering, indoor concentration is handled using a well-mixed model, which assumes the room's air is uniformly mixed. Steady-state concentration and its change over time are derived from a mass balance of emission rate, ventilation rate, and removal rate. The effect of air purifiers or plants is evaluated as a clean air delivery rate, added in the same units as the ventilation rate. In Japan, a 2003 revision to the Building Standards Act required mechanical ventilation systems achieving an air change rate of at least 0.5 per hour in homes, as a measure against sick building syndrome.
📖 For the derivation of the formula and further reading: Ventilation (Japanese Wikipedia) / Sick building syndrome (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How much does forcing air through the soil with a fan help? Devices that force air through the soil of a pot or green wall using a fan are being studied. How much they help in real buildings is still being worked out.
- Which microbes are breaking down the gases? Microbes living in the soil around the roots are thought to play a major role, but their full range of species and function is not yet known.
- How large is the effect on mood and concentration? Many studies find that greenery improves mood, but results are not consistent on how large or long-lasting the effect is.
In other words, this article too reflects "the explanation as currently understood." Even the figures for plant performance vary with experimental conditions.
Where this fits in the curriculum (by level)
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science, "Plant structure and function" | Photosynthesis, respiration, and the night-time oxygen question |
| HS | Maths "exponential functions," Chemistry "gases" | How concentration halves repeatedly in a sealed container |
| HS+ | Advanced chemistry (mass balance) | How concentration settles where release rate balances removal rate |
| Uni | Architectural environmental engineering (indoor air quality) | Well-mixed model, clean air delivery rate |
| Research | Environmental microbiology, environmental psychology | Soil microbes, psychological effects of greenery |
| ― | Everyday relevance | Don't block vents, open windows, dealing with new-furniture odours |
- Cummings, B. E. & Waring, M. S. (2020) Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies. Journal of Exposure Science & Environmental Epidemiology, 30, 253–261.
- Wolverton, B. C., Johnson, A. & Bounds, K. (1989) Interior Landscape Plants for Indoor Air Pollution Abatement. NASA.
- Ministry of Land, Infrastructure, Transport and Tourism (国土交通省), "On sick-house measures under the Building Standards Act"
- Ministry of Health, Labour and Welfare (厚生労働省), "Indoor concentration guideline values for chemical substances in indoor air"
- Wikipedia, "Ventilation" (換気, Japanese)
※This article is a general-audience science explainer. The figures given are approximations meant to illustrate the underlying mechanism. If odours or health symptoms persist, ventilate thoroughly and consult a medical provider or your local health office.