A 2005 study presented at the American College of Allergy, Asthma & Immunology found that a common houseplant removed 94% of airborne fecal particles and 78% of airborne mold from a sealed test chamber within 12 hours. The plant was English ivy (Hedera helix). The numbers traveled widely on social media in the years since, often with the framing that one plant in your bedroom can clean your air.
The study did happen. The numbers are real. But the gap between “sealed 6-cubic-foot chamber in a research setting” and “your actual bedroom” is bigger than the headline suggests. Therefore, this is the patient-useful version: what the research actually tested, what plants can credibly do for indoor air, and what to spend on instead if cleaner air is the goal.
What the headline gets right (and what it leaves out)
English ivy did remove 94% of airborne fecal-matter particles and 78% of airborne mold spores in the 2005 study. That part is accurate. The qualifier is methodology. Specifically, the test conditions matter more than the percentages.
That setup is important. A typical bedroom is roughly 1,000 cubic feet — about 167 times the volume of the test chamber. Furthermore, a real bedroom is not sealed; air exchanges through doors, vents, and gaps several times per hour. As a result, the plant-to-air ratio that produced 94% reduction in the chamber would require roughly 100 to 200 English ivy plants in a single bedroom to replicate.
Why one plant in your bedroom does very little
A 2019 systematic review in the Journal of Exposure Science & Environmental Epidemiology, led by researchers at Drexel University, looked at the entire body of houseplant air-purification literature. Specifically, the team analyzed 196 experimental results across 12 studies. The finding was unambiguous.
To equal the air-cleaning capacity of normal building ventilation, a typical room would need between 10 and 1,000 plants per square meter of floor space. In a 200-square-foot bedroom, that is roughly 100 plants on the low end and 9,000 on the high end. By comparison, a single English ivy plant in the corner is closer to a rounding error than a functional intervention.
This does not make houseplants useless. However, it means the mechanism that cleaned the chamber air in 2005 was the plant-to-air-volume ratio, not some unique property of English ivy. In a normal room, that ratio collapses.
What actually moves indoor air quality
If reducing airborne particles — mold spores, pet dander, cooking byproducts, dust mite fragments — is the real goal, three interventions outperform houseplants by orders of magnitude. None of them are exotic.
1. A HEPA air purifier sized to the room. A true-HEPA filter rated for your room’s square footage will pull 99.97% of particles 0.3 microns and larger out of circulating air. Brands like Coway, Levoit, and Blueair make credible units in the $100–$250 range that handle a 200-square-foot bedroom. Run continuously on the lowest setting; the cumulative effect is what matters.
2. Ventilation. Cracking a window for 5–10 minutes once or twice a day flushes accumulated indoor particulates faster than any plant can. Notably, this matters more in airtight modern homes than in older drafty ones. For homes near major roads, time the ventilation away from rush hour.
3. Source reduction. The cheapest air-quality win is removing the things that pollute the air in the first place. Specifically: fragrance candles, plug-in air fresheners, conventional cleaning sprays, and gas stoves all measurably degrade indoor air. (For the full list of cleaning sprays to swap, our breakdown of 7 toxic cleaning ingredients to avoid covers it.) Replacing those is often higher-yield than adding anything.
Where houseplants genuinely help (the realistic version)
Plants do not clean the air in any clinically meaningful way at typical home densities. However, they help in ways that are easier to overlook.
Humidity. Houseplants modestly raise indoor humidity through transpiration. In dry winter homes (especially with forced-air heating), this is a real benefit for sinus comfort, sleep quality, and skin. A grouping of 6–10 medium plants in a room produces a noticeable bump.
Psychological wellbeing. Multiple studies, including a 2015 trial published in Journal of Physiological Anthropology, show that interacting with indoor plants reduces measured stress markers compared with sitting in a plant-free room. The mechanism is psychological rather than air-chemistry-driven, but the effect is documented.
Modest VOC absorption at scale. Wolverton’s earlier 1989 NASA Clean Air Study did find that some plants absorb volatile organic compounds (formaldehyde, benzene, trichloroethylene) through leaf and root activity. The catch is the same as the ivy study — the effect requires significant plant density to register at the room level.
A visual prompt for healthier behaviors. Patients who keep plants tend to ventilate more often, clean more regularly, and notice indoor mold and dust earlier. The plant itself is not what improves the air; the attention it brings to the room is.
Six plants with credible supporting research
If you are adding plants for the modest documented benefits above (humidity, calm, low-level VOC reduction), these six have the most credible research behind them. Each is widely available and low-maintenance enough for most rooms.
| Plant | Best for | Light needs | Pet-safe? |
|---|---|---|---|
| English ivy | Airborne mold & particulates (per Spyers-Duran 2005 ACAAI) | Medium, indirect | No |
| Snake plant (Dracaena trifasciata) | Nighttime oxygen, low-light spaces | Very low to bright | No |
| Spider plant (Chlorophytum comosum) | Formaldehyde absorption (NASA 1989) | Indirect light | Yes |
| Peace lily (Spathiphyllum) | Benzene/trichloroethylene, humid rooms | Low to medium | No |
| Boston fern (Nephrolepis exaltata) | Humidity, formaldehyde | Bright indirect | Yes |
| Areca palm (Dypsis lutescens) | Humidity, larger room presence | Bright indirect | Yes |
If you want a real air-quality upgrade, do this
The honest playbook for cleaner indoor air, in priority order:
- One HEPA purifier per main living space. Bedroom first, then the room where you spend evenings. Match the unit’s rated coverage to your square footage. Run it continuously on low.
- Open windows for 5–10 minutes, twice a day. Morning and evening are easiest to remember. Skip rush-hour windows if you live near a major road.
- Switch to nontoxic cleaning sprays. Most conventional household cleaners measurably degrade indoor air. The brands worth switching to all match conventional cleaning power.
- Replace fragrance candles and plug-ins. The fragrance industry’s “trade-secret” loophole hides hundreds of undisclosed chemicals per scent. Soy or beeswax candles with disclosed-component scents are the cleaner swap.
- If you cook on a gas stove, ventilate every cook. Range hoods vented outside (not recirculating) are the bigger upgrade. If yours recirculates, crack a window when cooking.
- Add plants for humidity, stress, and behavior cues — not for air filtration. Six to ten medium plants across a home is the right ballpark. More is fine; fewer is fine; the goal is not air chemistry.
The hidden indoor-air culprits worth scanning your house for
Before adding any plant or purifier, audit what is already polluting the air. In a typical home, the five biggest contributors are usually quiet ones that have been there long enough to feel invisible.
Gas stoves. Burning natural gas releases nitrogen dioxide, formaldehyde, and ultrafine particles directly into the kitchen. A 2022 Environmental Science & Technology paper from researchers at Stanford and PSE Healthy Energy found that a home with a gas stove and poor ventilation can have indoor nitrogen-dioxide levels above EPA outdoor air-quality standards. Range hoods that vent outdoors are the fix. Recirculating hoods do not solve it.
Carpets and upholstered furniture. Newer carpets off-gas formaldehyde and other VOCs for months after installation. Older carpets accumulate dust mites, pet dander, and fragrance residues. Hard-surface flooring with washable rugs is the lower-particle alternative.
Synthetic fragrances. Plug-in air fresheners, scented candles, fabric softeners, and dryer sheets release phthalates and undisclosed fragrance compounds for hours after use. Indeed, the EPA has found that indoor air typically contains 2–5 times more VOCs than outdoor air, and fragrance products are a major contributor. Soy or beeswax candles with disclosed scent components, plus fragrance-free laundry products, drop the load substantially.
Mold around bathrooms and basements. Visible mold is the obvious case. Hidden mold inside walls or under flooring is harder to catch. If anyone in the home has unexplained sinus symptoms, fatigue, or asthma flares that worsen at home and improve when traveling, a mold inspection is worth the $300–$500. The 3 AM wake-up patterns we see in patients sometimes trace back to bedroom mold, not stress.
Garage-adjacent rooms. Attached garages leak vehicle exhaust, lawn-equipment fumes, and stored-chemical vapors into living spaces, particularly through shared walls and floor-level vents. Storing gasoline, paint, and pesticides outside the garage (or in a sealed cabinet) is a real reduction.
Reader questions on plants, pets, and low-light rooms
Is English ivy actually safe to keep around pets? No. English ivy is toxic to cats and dogs — the leaves and berries can cause vomiting, diarrhea, and in larger amounts, neurological effects. If you have pets, choose spider plant, Boston fern, or areca palm from the table above. They are pet-safe and have credible air-quality research behind them.
I have low-light rooms. What plants can I still use? Snake plant and pothos both tolerate remarkably low light. Pothos was not in the table because it is toxic to pets, though it remains among the most forgiving houseplants on the market. Specifically, snake plant also releases oxygen at night rather than during the day, which makes it a credible bedroom choice on that single metric.
Does having plants reduce my mold risk if I already have a mold problem? No. If your home has a visible or suspected mold problem, plants will not help — they may even contribute to humidity that mold thrives in. Address the moisture source first. Subsequently, a plant or two is fine.
What about the “nighttime oxygen” claim for snake plants? True but small. Snake plants use CAM photosynthesis, which means they release oxygen at night when most plants do not. However, the amount is biologically trivial for adult humans — you exhale far more CO2 each night than a snake plant absorbs. Notably, the plant is still a perfectly good bedroom choice; just choose it for the look and the low-light tolerance, not the oxygen.
Do air-purifying plants help with seasonal allergies? Mostly no. Pollen from outside, dust mites, and pet dander are the usual allergy culprits, and houseplants do not meaningfully reduce any of them. A HEPA purifier helps. Vacuuming with a HEPA-filtered vacuum helps more. Plants do not.
The realistic claim
English ivy did filter 94% of airborne fecal matter and 78% of airborne mold from a small sealed chamber in 12 hours. That part of the headline is true. The part it leaves out is that the result depended entirely on the plant-to-air ratio in that chamber, which is roughly 100 to 200 times denser than what you would have in a real bedroom. For genuinely cleaner indoor air, the upgrade order is HEPA filtration, regular ventilation, and source reduction — in that order. Keep plants for humidity, stress relief, and the fact that they make you pay more attention to the room. That is a real benefit; it is just not the one the headline implied.
Kaelyn Johnson is a Registered Dietitian from Southern California who has worked as a clinical dietitian and is now a freelance writer. She has written for WebMD, produced an e-book, and written case studies for healthcare practitioners.
