Documents: Special Interest: Horticultural Therapy:

Asthma and Houseplants
by Jerry Filipski
by Jerry Filipski


Gerald (Jerry) Filipski is the gardening columnist for the Edmonton Journal, a position he has enjoyed as a freelance writer for the past 12 years. Jerry also writes for Canadian Gardening, the new Alberta Gardener as well as for the lifestyle magazine of P&O ferries. Jerry also does numerous public speaking engagements including some major gardening conferences and workshops as well as question and answer sessions for Wal-Mart and Rona.

February 4, 2001

To lower stress, many physicians recommend a relaxing hobby like gardening. But cultivating plants indoors may also lower the risk of asthma, allergies and "sick building syndrome."
The Environmental Protection Agency cites indoor air pollution as one of the top five public health threats in America, and the main culprit in the 60 percent rise in asthma over the last decade. Now researchers are looking to plants -- common houseplants -- for a solution to polluted indoor air.
The energy crisis of the 1970s led many Americans to superinsulate their homes and offices against energy loss. Man-made materials like particle board, synthetic fibers and plastic -- which harmfully emit formaldehyde, trichloroethylene (TCE), benzene and other volatile organic compounds (VOCs) over years of exposure -- have also become ubiquitous indoors. Paints, varnishes, household cleaners, adhesives, carpeting and tobacco smoke are other common VOC-emitters. And while air filters are adequate at capturing particles like dust and dander, they do little to eliminate noxious VOCs.
Though debates have raged for over a decade as to just how effective houseplants are in improving indoor air, studies in Europe and the U.S. have reported that particular plant species can lower VOC levels.
In 1984, NASA senior research scientist Dr. Bill Wolverton tested houseplants for their ability to maintain clean air for future habitable lunar bases. Testing in sealed chambers, Wolverton found that philodendrons and golden pothos were excellent formaldehyde controllers; gerbera daisy and chrysanthemums were impressive benzene purgers; pot mums and peace lilies were highly rated for TCE removal. His initial findings suggested that one to three mature plants were enough to improve the air in a 100-cubic-foot area. Wolverton's research also concluded that it wasn't just plants doing the clean-up work, but the microbes that were specific to the plants' roots. Another 1989 NASA study concluded that tested houseplants removed up to 87 percent of toxic indoor air within 24 hours. A 1994 German study further reported that one spider plant could, in six hours, detoxify a 100-cubic-foot room laden with formaldehyde. Further tests showed English ivy benefited smoking areas, bamboo palm eliminated carpet odors, and the snake plant cleared household cleaner smells.
Wolverton's personal favorites are the lady palm, peace lily and "Janet Craig" dracaena -- "because they're easy to grow, resist insects and work wonders" for a variety of pollutants. His latest book, How to Grow Fresh Air: Fifty Houseplants that Purify Your Home or Office (Penguin Books, $15.95), additionally lists the areca and bamboo palms, English ivy and rubber plant as top pollutant controllers. Because indoor air varies, a good mixture works best, researchers advise.

The Skeptics

But other researchers are unconvinced. For years, the EPA's Office of Indoor Air Quality has remained skeptical, as has Skip Boat, writer for Indoor Air Quality Update. "In lab studies, you have a meticulously controlled environment. Under a tightly controlled situation, using plants may have some merit, but people bring in contaminants when they enter a room," Boat argues. Indeed, this was former EPA Indoor Air Division Director Robert Axelrad's primary complaint. He said that NASA's studies did little to simulate the air changes of a typical home or office. "Our calculations indicate that a much higher density of plants would be required (hundreds of plants in the typical house) to achieve these results," writes Axelrad.
And while the EPA says removing the sources of contamination and increasing ventilation are the best cures for polluted indoor air, a 1993 New England Journal of Medicine study found that increased ventilation did not solve the problem of sick building syndrome.
The Plants for Clean Air Council (PCAC) adds that low indoor humidity is another significant health problem. Dry, winterized air, the group says, irritates nasal membranes, triggers asthma and congestion, and makes people more susceptible to viruses and allergens. Wolverton advises keeping indoor humidity levels between 35 and 65 percent, which plants help maintain. "But humidity levels in excess of 70 percent can also result in indoor air quality problems," including mold spores and mildew growth, Wolverton says.
"Growing plants hydroponically (in water) overcomes some of these problems. Wolverton says that filters using hydroponic plants, fans and activated charcoal will be on the market within a year, and will improve air purification two hundred-fold. "Hydroponically-grown plants [don't produce] mold spores, and are easy to maintain," he adds. Wolverton further found that in home tests, rooms devoid of plants had airborne microbe levels 50 percent higher than plant-filled rooms.
Whether they clean the air or not--and there's plenty of evidence that they do--houseplants will continue to decorate the homes of people who appreciate this relatively painless way of bringing the outside inside.

Soil Mold and Allergies/Asthma

Can soil molds on indoor plants cause health problems and how large a problem is it? There are a variety of molds that can live in and on soils of indoor plants. These molds can be responsible for a number of health related problems ranging from mild to severe allergic reactions and can act as asthma triggers. The questions is how concerned should we be?

What are molds?

Mold, mildew, and fungi are all terms generally used to describe a group of diverse plants which appear as woolly or powdery growth. Molds are everywhere, indoors and out, in every type of climate, and in every social condition. They grow in places we would not expect. They are elusive and will be more prevalent at different times of the day or night depending on the type of mold. A single mold can germinate produce hundreds of thousands of airborne spores in 4-9 days.
Mold needs moisture, food, which can be any organic matter, and preferably warm temperatures. Relative humidity below 40% discourages mold growth. Molds are found in soil and in decaying leaves, straw, grains, and wood, for example.
Mowing the grass; raking leaves, working in the soil, or repotting an indoor plant can bring on symptoms associated with mold allergy.

Symptoms and sensitivity

As with most allergies sensitivity to the allergen is the key. This is very much an individual situation. While some people may be sensitive to one specific mold, others may be sensitive to all molds. Which category a person falls in can only be determined by allergy testing.
There are generic symptoms that should prompt a visit to your physician for examination and testing. The number of symptoms that occur and the severity depends on how much mold is present, how long you are exposed, and how sensitive you are to mold. Some may experience only a few symptoms, while others will have all or most of the symptoms.
Often, mold allergy sufferers feel like they have a mild cold and take a cold medicine [most cold remedies will help a mold allergy reaction]. Most colds and flu are of short duration and usually much more severe than mold allergy reactions.

Symptoms can be:

Ear, Nose, and Throat

nasal congestion or sudden unblocking of congestion, nasal itching, runny nose, sneezing, hacking cough, ringing in the ears, earache dizziness, recurrent ear infections [especially in children], fluid in middle ear, imbalance, dry tickling throat, clearing throat.


blurred vision, pain, watery or tearing, eyelid twitching, glare hurts, itching, droopy or swollen, redness.


shortness of breath, coughing, wheezing, mucus, tightness in chest, recurring bronchial infections.

What the experts say

Lilly Byrtus of the Allergy Asthma Information Association recommends, "not having more than 12 houseplants if you are concerned about mold allergies. Smooth-leaved varieties are better than the fuzzy-leaved ones because the fuzz can end up contributing to the problem when it breaks off or traps dust." Lilly stresses, " it is important to watch the soil. Any sign of mold growth remove the plant." Lilly also recommends plants that like low humidity. Higher humidity may be healthy for many plants but that would include molds and they thrive under higher humidity conditions.
Dr. Stuart Carr, an allergist, says "I have seen patients with indoor plant mold allergies but I don’t see it as a large problem. Houseplants have never been large contributors to the problem of indoor mold sensitivities." Dr. Carr goes on to say that ," the American Thoracic Society says that the data is insufficient to suggest that indoor house plants are the cause of much of indoor mold problems."
Dr. Carr says, "few patients will have major problems from indoor soil molds. Most reactions occur when the patient is repotting or digging in the soil. This stirs up the molds and their spores. Outdoor molds are a much larger problem than indoor molds."
Dr. Carina Majaesic, a pediatric respiratory medicine specialist, sees cases of allergy triggered asthma. She says, "indoor soil molds can contribute to these asthma cases. If the patient tests positive to sensitivity of indoor soil molds, I recommend removal of the plants. The reason for this is that it does not take much of the allergen in a repeated exposure to create a chronic asthma condition." Dr. Majaesic recommends seeing your physician and getting allergy tested if you are in doubt.

Prevention and reducing exposure

The key here is to be vigilant to the appearance of molds on your houseplant soils. The first sign of mold calls for drastic action. Use of a fungicide is an option as is disposal of the plant, in severe cases. Some plant experts recommend using a pea gravel mulch on the top of the soil as a deterrent to molds. Grapefruit seed extract has been used as an effective antifungal. If you have a sensitivity and are repotting use a filter mask to reduce exposure to spores.

Tips for houseplant lovers

Here are a few ideas that may help in avoiding the problem of soil molds from ever starting:

  • choose houseplants that are tolerant of low humidity conditions and require less frequent watering.

    Examples of such plants are :
        -Aloe variegata
        -Brassaia actinophylla (Schefflera)
        -Cactus (many species)
        -Crassula argentea (Jade Plant)
        -Dieffenbachia amonena (Dumb Cane)
        -Hoya carnosa (Wax Plant)
        -Philodendron oxycardium (Heartleaf Philodendron)

  • check water requirements of plants carefully. Many will tolerate much less frequent watering in winter.

  • mulch soil surface with pea gravel

Being informed is half the battle when it comes to the problem of indoor soil mold. If you have any questions or are concerned you may have a sensitivity to molds you should see your physician.

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