The peace lily is the plant most often named in “air-purifying” lists, and it did take part in the 1989 NASA study. The numbers are real, but they come from a sealed box, not a room. Here is exactly what the report measured for Spathiphyllum, what it means in a home, and how to keep the plant healthy.

For the full list of plants tested and why chamber results do not translate to homes, see what the NASA clean-air study tested. This page stays with the peace lily.

Which peace lily NASA tested

The report, by B. C. Wolverton, Anne Johnson and Keith Bounds, lists the plant as Spathiphyllum ‘Mauna Loa’. Plants came from local nurseries and stayed in their original pots and potting soil. Between tests they were kept in a greenhouse and fed with fertilizer.

Each test sealed one plant in a Plexiglas chamber for 24 hours under constant grow lights. One chemical was injected and the air was sampled at the start, at 6 hours and at 24 hours.

The peace lily results

The report gives two kinds of results. Year one used high concentrations of about 15–20 ppm. The authors noted these were “far above the levels commonly found in indoor atmospheres.” Year two used under 1 ppm and more sensitive gas chromatography.

Test (report table)ConcentrationPeace lily result over 24 hours
Trichloroethylene (Table 1)High, about 15–20 ppm27,064 µg removed; leaf area 7,960 cm²
Benzene (Table 2)High, about 15–20 ppm41,392 µg removed; leaf area 7,960 cm²
Formaldehyde (Table 3)High16,167 µg removed; leaf area 8,509 cm²
Benzene (Table 5)Low: 0.166 ppm at start0.034 ppm at end (79.5% removed)
Trichloroethylene (Table 6)Low: 0.126 ppm at start0.097 ppm at end (23.0% removed)

The controls matter as much as the plant rows. In the same low-concentration benzene test, a pot of potting soil with no plant removed 20.1%, and the empty leak-test chamber lost 5.3%. In the TCE test the soil control removed 9.2%.

What the report says does the work

The authors were surprised that low-light plants removed so much, given their low photosynthetic rates. They then tested pots with and without foliage. Their conclusion was that “the plant root-soil zone appears to be the most effective area” for removing these chemicals.

They identified common soil bacteria in the peace lily’s potting soil: Aureobacterium, Bacillus, Curtobacterium, Micrococcus, Pseudomonas and Streptomyces. They described these as microbes known to break down toxic chemicals when activated by root growth. The report’s summary recommended fan-driven activated carbon filters as “an integral part” of any plan to use plants against indoor pollution.

What the report did not test

Several claims attached to the peace lily do not come from this report:

  • Carbon monoxide. Not tested. The chemicals were benzene, TCE and formaldehyde.
  • A plants-per-room number. The report gives none.
  • Complete removal in a few hours. Samples were taken at 0, 6 and 24 hours, and no peace lily result reached zero.
  • Particles, dust or allergens. Not measured.

What this means in your home

A home is not a sealed chamber. Outdoor air keeps entering and furniture, cooking and cleaning keep releasing chemicals. Cummings and Waring (2020) converted 196 chamber results from 12 studies into clean air delivery rates. They found a typical single plant removed so little that 10 to 1,000 plants per square metre of floor would be needed to match normal air exchange.

The US EPA states there is “currently no evidence” that a reasonable number of houseplants remove significant quantities of pollutants in homes and offices. It recommends source control first, then ventilation, then air cleaners for particles.

So keep a peace lily because you like it. If you notice an odour or fumes, find and remove the source and open a window.

Growing a healthy peace lily

The EPA’s one plant-specific warning applies directly: overly damp soil may encourage microorganisms that affect allergic people. A well-grown peace lily is also a better-looking one.

  • Light. NC State lists peace lilies for partial to deep shade and says to keep them out of direct sun. Iowa State says they grow in low light but flower best in indirect sunlight. The RHS adds that in deep shade they “will rarely flower.”
  • Temperature. NC State gives 20–29 °C (68–85 °F) by day, with a night drop of no more than about 6 °C (10 °F). Growth slows at 4–16 °C (40–60 °F), and long spells below 4 °C (40 °F) damage leaves and roots. The RHS gives 12–24 °C (55–75 °F) as fine.
  • Water. NC State says to keep the soil moist but not soggy and let it dry between waterings. The RHS says water once the top few centimetres are dry, and use rainwater or filtered water because fluoride in tap water can damage the foliage.
  • Feeding. NC State recommends quarter-strength fertilizer and flushing the soil between feeds, because salt build-up can burn leaf tips.
  • Brown tips. NC State lists too much direct sun, underwatering and low humidity as causes.

Our peace lily care guide covers repotting and division. If yours has stopped flowering, see why your peace lily won’t bloom.

Pets and peace lilies

The ASPCA lists peace lily as toxic to dogs and cats. The toxic principle is insoluble calcium oxalates. Listed signs are oral irritation, intense burning of the mouth, tongue and lips, excessive drooling, vomiting and difficulty swallowing.

The FDA explains that peace lily is not a true lily. It does not cause the kidney failure that Lilium and daylilies cause in cats. Its crystals irritate the mouth and throat, and the signs usually go away on their own. If a pet chews one, call your veterinarian or the ASPCA Animal Poison Control Center on (888) 426-4435. Our toxic plants guide covers true lilies.

References