Stamford Plus magazine - Spring 2007
Hypoallergenic houseplants
By Sue Sweeney
Mar 5, 2007 - 8:52:53 AM

Allergies made you throw out all your houseplants and now your house looks like a NASA laboratory? See how you can have some green …without the sneeze.

It has, somehow, become established American medical doctrine that indoor plants aggravate asthma and certain allergies. To the contrary, institutions as respected as the Mayo Clinic and the Asthma and Allergy Foundation of America advise that most allergy and asthma sufferers can have a reasonable number of houseplants. Other research indicates that houseplants might improve air quality in an enclosed space. “The devil is in the details,” though, so it is good to know how to maximize indoor plants’ positive contributions while minimizing any unwanted side effects.

The problems that indoor plants could conceivably cause come from pollen in the flowers, dust on the leaves, and mold in the soil. Further, plants add humidity that can, in excess, encourage mold and dust mites. Lastly, sensitive individuals can get a rash (contact dermatitis) from direct contact with parts of some plants.

General precautions

For starters, many problems can be avoided by selecting only healthy plants that will thrive without fuss under average household conditions. If contact dermatitis is a concern, shop with a poison plant list in hand and consider doing a skin patch test with new plants.


Surprisingly, pollen is mostly a nonissue with houseplants. Medical experts say that it is wind-borne pollen that causes hay fever (an allergic reaction to pollen that inflames membranes in the eye or nose) and asthma (inflammation of the bronchial passages).

Wind-borne pollen is produced mainly by trees in the spring, grasses in the summer, and weeds like ragweed and mugwort in late summer and fall. Wind-borne pollen is light, powdery and produced in enormous quantities to ensure that some of it reaches the intended destination.

However, most plants, including most houseplants, are pollinated by insects, not wind. Insect-pollinated plants produce heavy, sticky pollen that adheres to insect legs but doesn’t readily become air-borne. Because insect pollination is more efficient, much less pollen is produced.

Thus, very few indoor plants, even in full flower, present a pollen allergy risk. Any conceivable risk (e.g., reaction to the flower’s odor or direct contact with the pollen) can be further reduced by keeping flowering plants out of the bedroom and wearing a mask when handling plants. Alternatively, the indoor plant collection can be limited to nonflowering plants.


According to the National Institutes of Health, dust contains allergy-causing molds, fibers and pet dander, as well as dust mites and their waste products; limiting dust exposure at home, particularly in the bedroom, helps dust-sensitive individuals cope with dust outdoors and at work.

Plants, being stationary, collect dust. Plants, however, can be kept relatively dust-free with a periodic sink or shower bath. Most plants, by the way, will appreciate the washing, which cleans the plant’s leaf pores and protects the plant against insect outbreak. Some plants, such as palms and Marantas (prayer plant family), regularly need water on the leaves for optimal color and health.

Avoid soaking the soil, and allow it to drain thoroughly so as not to encourage root rot. A few plants are at risk of crown rot if submerged, even briefly, in water, and thus should be avoided or kept dust free by another method. These include African violets, cyclamen, and moth orchids (Phalaenopsis). When dust is an issue, limiting the number and size of indoor plants helps reduce the cleaning burden. Placing houseplants where they facilitate the cleaning routine is also helpful. For example, hanging plants don’t get in the way of the vacuum, and plants by the kitchen sink are easy to wash. Hanging plants should be low enough to reach easily.


Molds are tiny fungi that reproduce via air-borne spores. Most molds break down dead plant matter (cellulose) for recycling into soil and, ultimately, nourishing new plants. Contact with molds, and the toxins and spores that molds produce, can cause allergies, asthma and other human diseases, some quite serious. There are many thousand of types of molds that health experts advise that it is generally not practical to try to sort the OK molds from the bad ones; the best approach is to treat all molds as suspect. According to the U.S. Environmental Protection Agency, even healthy individuals without known mold sensitivity should take mold exposure seriously.

Most molds prefer warm, damp, dark poorly ventilated conditions. Molds can’t be eliminated, but they can be controlled by eliminating their favored conditions. The most important condition to control appears to be moisture, but controlling food sources (e.g., dead plant material) and creating barriers to prevent mold spores from becoming airborne can also be important. Indoor plants are associated with several types of mold growth, but all can be controlled.

Mold grows on dead plant material, and a plant with an insect infestation can also be the source of allergens. Thus, dead leaves and flowers and ailing plants should be discarded promptly.

Soil naturally harbors molds, but remember that it is the contact with mold and spores that causes the problems. The Asthma and Allergy Foundation of America advises that soil fungi are not a problem unless the soil is disturbed. The Mayo Clinic advises spreading aquarium gravel over the soil to help contain mold spores. Others have said that fine sand works as well. One source says that the soil cover should be as much as 2” thick to be effective.

Sensitive individuals can use gloves and a mask to avoid contact with the soil fungi and any airborne spores when transplanting. Alternatively, many florist and plant nurseries will do the repotting for you. In any case, use only commercially sterilized potting mix.

Overwatering should be avoided. Not only is overwatering the number one killer of houseplants, it can lead to excessive mold growth on the plant, in the soil, in the plant’s drainage saucer and under the plant in the carpet or flooring if there’s a spill or leak. Mop up any spilled water, do not let plants stand in water, drain excess water out of saucers and regularly check for under-pot leakage.

Finally, plants are said to produce the least mold in a sunny, well-ventilated room. And, of course, the bedroom is never the first choice.


Excessive humidity fosters mold and dust mites. Overly low humidity is also undesirable, as it dries out the mucous membranes that protect us from respiratory disease. The National Institutes of Health advise that, for those with allergies or asthma (and everyone else), the ideal level of relative humidity is between 30% and 50%.

Don’t judge indoor humidity by whether moisture forms on windowpanes. Since humidity is relative, moisture on windowpanes indicates only the difference between indoor and outdoor temperatures and the efficiency of the window’s insulation. The best bet is a simple hygrometer, a handy, inexpensive device that measures humidity. Since humidity levels vary during the day, a hygrometer that stores minimum and maximum readings is useful. Hygrometers are available in hardware and “gadget” stores, as well as on the Internet.

Many allergy and asthma advisers believe that houseplants must be avoided because the water given to the plants raises indoor humidity. This advice appears to be incorrect, or at least overstated, and doesn’t take into account the need to maintain minimum humidity levels.

It is my observation that, while indoor plants are very useful to protect against humidity levels under 30%, it is very difficult for indoor plants to raise the humidity into the danger zone over 50%. For many years, I kept records of the humidity levels I was able to achieve when I was maintaining a veritable indoor jungle and trying my best to raise my winter humidity with trays of pebbles, etc. Despite my best efforts, I was seldom able to get the humidity over 50%, and it was still sometimes below 30%.

Out of an abundance of caution, though, it is good to avoid overwatering and standing water, if only to reduce mold. If the hygrometer says humidity levels need to come down, improving kitchen, bathroom, and cellar ventilation seems to have the biggest impact.

If you do want plants that will have minimum impact on humidity, go for the “dry lovers” (see the accompanying box). The dry-loving plants require less watering, and so, at least, are less work.

Air quality

In the 1980s, a study by National Aeronautics and Space Administration (NASA) and the Associated Landscape Contractors of America demonstrated that, in a sealed environment, certain houseplants could reduce airborne toxic chemicals, including formaldehyde, benzene and trichloroethylene, that can be emitted by paint, computers, carpeting, cleaning fluids, etc. Later studies, again in small, sealed environments, showed that certain plants measurably added air-purifying negative ions and reduced airborne dust and microbes. Some feng shui practitioners claim that certain houseplants absorb excess electromagnetic energy emitted by computers and other electronic equipment.

There is no question that poor outdoor-indoor air exchange can lead to unhealthy levels of toxic substances indoors. The EPA view is that opening the windows or otherwise ventilating with old-fashioned fresh air works best. Pollen and dust allergy sufferers, however, do tend to seal up the home; likewise, “climatecontrolled” office buildings are sealed tight.

While the ability of plants to control indoor air pollutants, other than carbon dioxide, is not established to many scientists’ satisfaction, in a sealed environment plants may be worth a try in combination with other measures. Proponents of houseplants as “nature’s clean-air machine” advise using at least one plant per 100 square feet. Some claim that tropical plants with large, broad leaves, like the peace lily (Spathiphyllum), are best at filtering out dust and microbes.

Keeping toxic substances out of the home helps, too. This includes removing street shoes (a source of outdoor pesticide and petroleum residues) at the door, selecting nontoxic household cleaners and avoiding houseplant-related chemicals. Repotting once every year or two negates the need for concentrated fertilizers except for flowering plants. Wash off dust; don’t use a leaf shiner. Control pests by careful plant selection and culture, frequent washing and discarding the plants that don’t readily thrive.■

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