Do houseplants really purify indoor air?

Houseplants and indoor air purification: what the evidence shows

The question may seem straightforward. Yet, when the measurement science is examined, the answer is more nuanced than either "yes" or "no".

The notion of using houseplants as air purifiers dates back to a 1989 US study undertaken for NASA during its research into closed-loop life-support systems for space stations.

Within sealed, tightly controlled chambers, some plant species lowered levels of volatile organic compounds (VOCs). VOCs are chemicals that readily evaporate into the air at room temperature, including potentially toxic substances such as benzene, trichloroethylene and formaldehyde.

The underlying science was robust. The difficulty lies in applying findings from a sealed NASA chamber to an ordinary living room.

This difference is hugely significant, forming the basis of almost all subsequent exaggerated claims about the air-purifying power of houseplants.

Most research indicating that houseplants remove pollutants has one central design feature in common: small sealed chambers where an artificially high concentration of pollutants is added as one dose.

A plant is then put into the chamber, pollutant levels are tracked over time, and researchers calculate a removal rate. This approach is useful for comparing one plant with another.

It is not useful for forecasting what will occur in your home.

The key omitted factor is what building scientists describe as the air exchange rate: the speed at which outside air naturally replaces indoor air through gaps, walls and ventilation systems.

In an actual building, this ongoing dilution is already responsible for much of the reduction in pollutant concentrations.

A 2019 study modelled plant performance alongside real-world air exchange rates and concluded that between ten and 1,000 plants per square metre would be needed to equal what passive building ventilation already provides.

The scientifically supportable conclusion, therefore, is this: houseplants can remove certain pollutants, but they are not an effective independent solution for cleaning air in homes.

That does not make the earlier studies "wrong". Rather, their findings were frequently extended too far into everyday environments, where indoor-air physics operates very differently.

Why real homes differ from laboratory chambers

More recent reviews draw a distinction between potted plants and more engineered plant-based systems. Certain botanical biofilters, which use fans to force air through plant-root substrates, may offer useful air-cleaning potential. However, this is a different technology from having a handful of ornamental plants on a windowsill.

The claim is also often overstated because genuine indoor environments are not fixed. Unlike many chamber experiments, pollutants are not generally emitted once and then allowed to fall away in a sealed area.

At home, emissions can be continuous or occasional, arising from cooking, cleaning, furnishings, consumer goods, heating and traffic pollution drifting in from outdoors. Temperature, humidity, household occupancy and ventilation also vary during the day.

Each of these elements influences how pollutants are released, diluted or deposited inside. As a result, real exposure conditions are considerably more complicated than the controlled settings used for many plant studies.

Effective ways to improve indoor air quality

For these reasons, the most reliable public-health guidance is still simple.

Start by reducing or eliminating the pollution source. This might mean no longer using fume-emitting products, including aerosol sprays or powerful chemical cleaners, and fixing building problems such as dampness or leaks that encourage mould growth.

Next, improve ventilation and use efficient filtration. Ventilation may be enhanced, for instance, by opening windows and doors and using kitchen and bathroom extractor fans that vent to the outside.

The amount of outdoor air can also be increased through combined heating, ventilation and air-conditioning systems, which can be highly effective for filtering air.

Portable air cleaners using high-efficiency particulate air (HEPA) filtration may help to lower airborne particle levels. Meanwhile, ventilation - including opening windows or operating extractor fans - dilutes indoor pollutants when outdoor air quality is suitable.

Air-cleaner quality does vary, however.

For routine use, choose a model suited to the size of the room and one that explicitly says it has a True HEPA filter. This means it is designed to capture at least 99.97% of very small particles.

A unit with an AHAM Verifide label can also be helpful, as this indicates that its clean air delivery rate (CADR) has been independently tested. As a basic rule, a higher CADR means the cleaner can remove particles from the air more quickly, and the packaging will normally state the room size for which the unit is appropriate.

Most air cleaners are primarily intended to tackle particles, including dust, pollen, pet dander and smoke.

For assistance with gases or odours, including VOCs, choose a model with an activated carbon filter, as HEPA filters alone are mainly designed for particles. Packaging will generally indicate whether a unit is intended for particles, gases or both, although no air cleaner can remove every pollutant.

Plants themselves also need proper care. Overwatering and badly maintained pots may contribute to indoor moisture issues or microbial growth. In this respect, even the advantages of indoor greenery rely on how it is looked after.

The value of houseplants beyond air cleaning

Does this make houseplants pointless indoors? Not at all.

Although their direct air-cleaning impact in real homes may be limited, plants can still provide benefits.

Research suggests that they may improve perceived comfort and psychological well-being, and can sometimes have a slight effect on humidity or the indoor microenvironment.

Keep houseplants because you like them and because they can make indoor spaces feel more attractive and restful. They may make a home more enjoyable, and that has value in its own right.

They should not, however, be promoted as a practical answer to serious indoor-air problems.

Pedram Vousoughi, Post-Doctoral Researcher in Biological Sciences, University of Limerick

This article is republished from The Conversation under a Creative Commons licence. Read the original article.