A lot of indoor plant sellers will tell you that indoor plants will purify the air. Sometimes, they refer to experiments carried out by NASA to prove the point. However, careful analysis of some of these claims shows that the claims are often exaggerated, or taken out of context.
That doesn’t mean that plants have no impact on indoor air quality – they can. So, how can you get the most of plants’ abilities to affect the indoor environment? I can’t promise miracles, however.
A brief history
In the early 1980s, NASA was investigating the ways that astronauts could maintain their environments whilst on long-term missions. The reaction of photosynthesis is a way of producing fresh oxygen for the astronauts to breathe, and some plants are good at removing other pollutants from the air and water.
The idea was that by having completely sealed living environments, coupled with optimal growing conditions for vast amounts of greenery, humans and plants could live in complete balance and the resources needed for a long-term mission could be reduced. At the time, only a few years after the last manned landing on the moon and with early space stations (Skylab and Mir – the forerunners of the International Space Station) being lived in for months at a time, this research was clearly very important.
The results of the experiments showed that some plants were especially good at removing pollutants such as some volatile organic compounds (VOCs), some of which are quite unpleasant and are associated with harmful indoor air quality. These have become quite well known and are often referred to as the NASA list of air cleaning plants.
Sick buildings – could plants be the answer?
In 1984, the World Health Organization gave a name to a recently discovered phenomenon. People were feeling ill in modern buildings, and analysis of the environments in some of these buildings identified a number of VOCs as being likely causes of the problem. That problem became known as Sick Building Syndrome, and a great deal of effort was expended trying to identify the building components most responsible for the release of these chemicals into the air inside buildings.
It didn’t take long for someone to notice that a lot of the VOCs identified as being associated with sick building syndrome were those that some plants seemed to be good at removing. Indeed, one of the scientists involved with the original NASA experiments – Bill Wolverton – has since made a career writing about how houseplants can create fresh air.
Thinking back to the original NASA experiments, you will notice that they were carried out with a specific purpose in mind, and the plants were grown under conditions that made them actively grow – their metabolism was optimized by controlling the environment with high light levels, good humidity, warm temperatures and precise levels of plant nutrients. There were also vast numbers of plants in the growth chambers.
If you ever visit the hot houses at a botanic garden, such as Kew or the Eden Project, you will experience exactly the type of environment the plants experienced in the NASA experiments. The air inside those spaces is uplifting, fresh and life-supporting. The difficulty lies in recreating those conditions in homes and offices.
Issues of indoor air quality
Fortunately, since the early 1980s, the use of products in buildings associated with sick building syndrome has been significantly reduced. Most homes and offices are not full of nasty VOCs. However, there are some pollutants that have the potential to cause harm, or at least discomfort.
Volatile organic compounds (VOCs)
There are many sources of VOCs in houses. In fact, almost everything you can smell is a VOC of one type or another. Paints and new furnishings often release some compounds, but more mundane products are the biggest source: cleaning products, cosmetics and toiletries. Cooking, too, also produces types of VOC, as does opening a bottle of wine or mixing a cocktail. Most of these VOCs are harmless, although some can be irritating. Other VOCs are actually the result of human physiology: when you breathe out, there will be some VOCs on your breath as well as carbon dioxide and water – these are just the products of digestion and metabolism.
More worryingly are the VOCs that can enter the house from outside. Vehicle emissions, agricultural and industrial activities all contribute to VOCs in the atmosphere that will find their way indoors.
A more pernicious threat to human health comes from ultra-fine particulate matter, usually produced as a result of combustion. These are often classified as PM10 (particles smaller than 10 μm in diameter) and PM2.5 (particles smaller than 2.5 μm in diameter). These particles can be breathed deeply into the respiratory system, where they remain. Fine particulates come from vehicle exhausts, inefficient combustion of gases and even cooking.
Larger particulates, such as dust, can irritate the respiratory system and contribute to asthma and allergies. These are either produced inside buildings (and are usually composed of dead skin cells and pet dander), or can be blown indoors through doors and windows (such as fine dust from roads and fields or construction, or pollen from trees and grass). Since most homes are not airtight (and most people wouldn’t want them to be – opening a window is a great way to refresh the air and create cooling breezes indoors), there is little that can be done to prevent dust from getting in from the outdoors. Remember, also, that good ventilation is recommended as a way of reducing the risk of Covid.
Elevated levels of carbon dioxide are more of a problem in offices than in homes. Small meeting rooms with lots of people, will result in CO2 levels rising fast and getting to concentrations high enough to cause drowsiness and impair cognitive function. In the home, this is less of a problem, although in the winter, when everyone is indoors and windows remain firmly shut, CO2 levels might rise above comfortable levels.
How can plants help
Despite the fact that most homes are unable to house enough houseplants to actively purify the air, nor are they able to provide the conditions for them to be physiologically active enough to achieve the sorts of effects seen under laboratory conditions (that would be extremely uncomfortable for people), there are ways that plants can be used indoors to improve air quality – and some plants are better than others.
The key is to match the plants well to their environment. The more closely matched they are, the more physiologically active they will be, and that is when the effects will be greatest.
When you search for indoor plants online (whether for home or office), you will often see that retailers often include details about the conditions that they do best under. If you choose plants that suit the different conditions found in the various spaces in your home or office, then you are more likely to notice an effect.
Plants affect the indoor environment in three main ways.
First, the bacteria in the soil that live amongst the roots are able to break down some VOCs, and convert them into substances useful to the plants. This is an entirely natural phenomenon, although only relatively recently properly understood in horticulture. Plants with healthy roots and good soil will have the biggest impact, and those that are the fastest growing will also be the most effective.
Second, plants that are actively photosynthesizing will be removing some carbon dioxide from the air. Plants that originate in dark tropical conditions (such as rainforest floors) are able to photosynthesize extremely efficiently – they have evolved ways of making photosynthesis work even in very low light conditions, so that means more carbon dioxide is used by the plants.
Third, plants with hairy or slightly sticky leaves are able to trap particulates on the leaf surfaces, including fine particulates. In fact, plants such as ivy and Cotoneaster are used outdoors to mitigate the effects of pollution in urban areas. Some indoor plants can do that too (although they will need to be cleaned – there is no rain indoors to wash that pollution away). In fact, research carried out at Washington State University some years back showed that many different types of foliage plant attracted dust to their leaf surfaces – possibly as a result of an electrostatic effect – so almost any leafy plant will be useful.
Which plants work best
Plants that are adapted to low light conditions will be the best to improve indoor air quality, especially reducing VOCs and carbon dioxide. Plants in the aroid family, such as Spathiphyllum, Philodendron species, Aglaonema species or Monstera species will be good, as will other jungle-floor plants such as Calathea species, Ctenanthe and even small palms, such as Dypsis lutescens.
If light levels are slightly higher, Dracaena species have been shown to be effective at reducing levels of carbon dioxide (experiments carried out in Australia by Margaret Burchett, Fraser Torpy and colleagues, in real office conditions have shown that relatively few plants are needed to have a measurable effect).
Plants such as Ficus benjamina and varieties of ivy (Hedera helix) and some ferns that do well indoors are good at removing particulates.
Over recent years, the plant/microbe interactions in the soil have led to a number of innovations that use plants to actively clean the air. These systems were originally designed for large commercial spaces, but domestic-scale systems are becoming available.
In commercial buildings, green walls can have a dramatic effect on indoor air quality – especially when set up with good lighting systems.
In the home, small green walls can now be purchased for relatively low cost, and can be installed by a competent DIYer. Not only do they take up little in the way of floor space, the large volume of compact plants in a good root environment means that they are going to be very effective – especially if you invest in some plant lights to illuminate them (and these are also getting much cheaper).
More recently, active air systems have been developed that use fans to pass air through the foliage and the roots to increase the size of the effect. Domestic-scale active air green walls are being developed and table-top systems, such as Vitesy’s Natede planter, are now already on the market.