Can plants improve indoor air quality?
As far back into my childhood as I can remember, my parents always had a large assortment of indoor plants. When I recently asked my mother, “Why?” the answer was: “…because my parents, and grandparents had indoor plants, …because they are nice, …because they beautify my rooms, …because I like a little bit of nature inside.” I gave my mother another reason: “Mum, they are also healthy, because they clean the air we breathe.”
I base this statement on several studies I read while researching for this book. The most comprehensive is, without doubt, a study by NASA. What interest has NASA in the quality of indoor air? Astronauts, at times, spend months in a small space station, surrounded by synthetic materials that constantly emit chemicals into the cabin. The same goes for other occupations, of course, that spend lengthy periods in sealed environments, such as submariners. A team of scientists researched the types and amounts of chemicals in the air of such confined spaces and looked for measures to reduce the health risk to crew members.
Something we knew all the time.
Now let’s assume a crew of astronauts wants to establish a base on the Moon or Mars where they may live for a very long time on artificial life support systems. NASA scientists found a very simple and effective way of treating and recycling air and water: plants. This in itself is not an astonishing scientific breakthrough, because we all know that plants are the lungs of Mother Earth. Astonishing, however, was the discovery that plants can remove many of the more than 300 chemicals found in the air of a spacecraft.
Plants and chemical pollution
The NASA study has proven that plants can remove airborne chemicals to some extent. But where do these chemicals go? A German study investigated the question whether the plants will just store the chemicals on their leaves and in the soil and possibly re-release them back into the air.
For the study, scientists of the German National Research Centre for Environment and Health exposed popular indoor plants to formaldehyde, one of the most common indoor air pollutant. They reported that enzymes in the plant leaves break down the toxic chemical into non-toxic components that can be used by the plant. This process is similar to the way a human liver rids the body from toxins. Research also shows that chemicals are translocated into the root system and the adjoining soil, where soil micro-organisms can break down the substances even further.
Plants and dust removal
Airborne chemicals aren’t the only health hazards indoors. As we know, fine particles, or particulate matter as it is called, has also a significant impact on our health. The microscopic particles can penetrate deep into the lungs and can cause respiratory or cardiovascular disorders. Do the Earth’s lungs, the plants, experience something similar? I don’t mean to ask whether they will begin to cough and wheeze or get a heart attack, but do the fine particles enter the tiny pores of the leaves (stomata) through which a plant breathes?
Scientists of the Washington State University say ‘yes.’ Their experiments led them to conclude that foliage plants can reduce indoor dust levels by up to 20%. And you don’t have to plant a jungle either. The plants they added to an office room occupied only around 5% of the volume. But, be friendly to your plants and occasionally wipe off excess dust from the leaves.
Do plants promote biological pollution?
A concern could be the growth of fungi in the soil and on decaying plant matter. As long as the plant is healthy, however, this is not the case. The plant has its own defences against micro-organisms. It releases small amounts of its own disinfectant essential oils to control or destroy bacteria and fungi that invade the space between the leaves. We make use of this ourselves whenever we use disinfectants or room deodorizers that contain natural plant oils from, for example, citrus or pine trees. Experiments have shown that plants significantly reduce the number of microbes in indoor air. For example, when pots of citrus trees were added to a room, the air became almost sterile.
Do plants contribute to high humidity?
Another point of concern is the increase in humidity levels. Yes, plants transpire and increase the amount of water vapour in the air. The good news is that the increase depends very much on the humidity level that is in the room in the first place. On humid days, the rate of evaporation is very low and the humidity increase is only marginal. On dry days the plant ‘sweats’ significantly. This is to our benefit, because the indoor air is very often below the comfortable humidity level, as during the heating period. Cooling a room with an air conditioner also reduces the humidity significantly. Just have a look at the amount of water that drips out of the back of the condenser in the air conditioner unit. Dry air is unhealthy. Besides dehydration, dry air promotes cracked skin and lips, the drying of the mucus in airways and sinuses, and is a recognized trigger for asthma attacks. Allergens, bacteria and viruses can get easily past the dried-up defense mechanism. There is evidence that cases of the common cold are more frequent when the humidity is low.
The best indoor plants to improve air quality
All studies and experiments showed significant reductions of volatile chemicals in the indoor air when plants are present. The rate at which the plants metabolizes the substances depends on the growing conditions, such as the available light, the temperature, the humidity, and the nutrients that are available to the plant. Please be aware, however, that plants don’t remove the chemicals completely and different plants have different capabilities. A particular plant may be very good in removing formaldehyde, while another is better in destroying benzene. Then there is also the question of which plants to select for experiments. The NASA researchers used indoor plants common in the US, while the German scientists selected the plants as preferred by the citizens of Germany.
Of ten species tested, the German National Research Centre for Environment and Health identified the following plants as overall good performers, i.e. they are effective in removing all types of chemicals: weeping fig (Ficus benjamina), umbrella tree (Schefflera arboricola), oakleaf/grape ivy (Cissus rhombifolia), peace/madonna lily (Spathiphyllum sp.), and devils ivy (Epipremnum pinnatum/aureum).
In his book Eco-friendly house plants, retired NASA research scientist Dr. B. C. Wolverton grades 50 house plants according to their ability to remove chemicals from indoor air. I have listed 20 top performers for this category in the attached text box. The list is an overall rating of a plant’s effectiveness in removing chemical pollutants in general. Other plants may be excellent achievers in metabolizing a particular chemical, but didn’t make the list in overall performance.
Best plants that remove chemical vapors from indoor air.
Bamboo Palm (Chamaedorea seifrizii)
Boston Fern (Nephrolepis exaltata “Bostoniensis”)
Dwarf Date Palm (Phoenix roebelenii)
English Ivy (Hedera helix)
Florist’s Mum (Chrysanthemum morifolium)
Gerbera (Gerbera jamesonii)
Kimberley Queen (Nephrolepis obliterata)
Rubber Plant (Ficus robusta)
Areca Palm (Chrysalidocarpus lutescens)
Corn/Happy plant (Dracaena fragrans “Massangeana”)
Dracaena “Janet Craig” (Dracaena deremensis “Janet Craig”)
Schefflera/Umbrella Tree (Brassaia actinophylla)
Peace/Madonna Lily (Spathiphyllum sp.)
Weeping Fig (Ficus benjamina)
Dendrobium Orchid (Dendrobium sp.)
Dumb Cane (Dieffenbachia “Exotica compacta”)
Ficus Alii (Ficus macleilandii “Alii”)
King of hearts (Homalomena wallisii)
Lady palm (Rhapis excelsa)
Lily turf (Liriope spicata)
Source: Dr. B. C. Wolverton, Eco-friendly house plants, 1996
What is the verdict?
Plants aren’t able to do all the hard work. It would take an indoor forest to clear the air of all chemicals, dust and harmful micro-organisms. Plants need help – your help. Depending on what indoor air pollutant is excessive in your home, follow the recommendations I gave in the appropriate sections in this book. Briefly though, ventilating the room is still important to reduce pollutant levels and the plants will thank you for the fresh air. Regular cleaning and careful use of household chemicals will also improve the indoor air quality.
If you are lucky enough to be part of your new home design, keep the plants in mind and plan ahead, such as adding skylights and window boxes, or even a conservatory. When you select plants for the purpose of cleaning the air, as a general rule, buy a variety of leafy plants. This way you cover all the odds of one plant being better than the other in removing a particular chemical. You never know what kind of chemicals may end up in your indoor air, sometimes in the future. If dry air is a problem, then ask the nursery for plants with high water usage. The water they drink will eventually disappear into ‘thin’ air – literally, and increase the humidity in the process. Don’t overdo the watering, though. Too much water may promote fungal growth and excessive spores in the air. Also, more plants die from misplaced love – overwatering – than from neglect – underwatering.
One final word, by having plants in your home, you create your own microclimate – the indoor weather. In a small way you also contribute to the recovery of our planet. Good luck.