Do Houseplants Really Purify Indoor Air?

At first glance, the question appears straightforward. Yet, when the measurement science is considered, the answer is more nuanced than simply “yes” or “no”.

The notion of the houseplant as an air purifier originates with a 1989 US study undertaken for NASA during its work on closed-loop life-support systems for space stations.

In airtight, carefully controlled chambers, some plant species lowered the levels of volatile organic compounds (VOCs). VOCs are chemicals that readily evaporate at room temperature, including potentially harmful substances such as benzene, trichloroethylene and formaldehyde.

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

That difference is hugely important, forming the basis of much of the exaggerated reporting on the air-purifying powers of houseplants that has appeared since.

Why NASA chamber studies differ from homes

Most research finding that houseplants can remove pollutants has one key design feature in common: researchers use small sealed chambers containing artificially elevated levels of pollutants, added as one large dose.

A plant is put into the chamber, pollutant levels are monitored over time and a removal rate is worked out. This approach is useful for comparing one plant with another.

However, it is not a reliable way to forecast what will occur in a home.

The crucial factor left out is what building scientists refer to as the air exchange rate. This describes how rapidly outside air replaces indoor air through cracks, walls and ventilation systems.

In an actual building, this ongoing dilution already does most of the work of lowering pollutant concentrations.

When a 2019 study modelled plant performance using real-world air exchange rates, it concluded that between ten and 1,000 plants per square metre would be required to equal the results already achieved by a building’s passive ventilation.

The answer supported by the science is therefore this: houseplants can remove certain pollutants, but they are not an effective independent air-cleaning solution for homes.

This does not make the earlier studies “wrong”. Rather, their conclusions have frequently been extended too far into everyday environments, where indoor-air physics is very different.

Recent reviews also separate potted plants from more purpose-built plant-based systems. Certain botanical biofilters, which use fans to push air through plant-root substrates, may offer worthwhile air-cleaning potential. But this is a separate technology from having several ornamental plants on a windowsill.

Indoor pollution is constantly changing

The claim is also often overstated because real indoor settings 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 ongoing or occasional, arising from cooking, cleaning, furniture, consumer goods, heating and traffic pollution drifting in from outdoors. Temperature, humidity, household occupancy and ventilation levels vary during the day as well.

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

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

First, cut down or eliminate the pollution source. This could mean no longer using fume-producing items, including aerosol sprays and harsh chemical cleaners, and fixing building issues such as dampness or leaks that encourage mould.

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

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

Portable air cleaners using high-efficiency particulate air (HEPA) filtration may help to reduce particles in the air. Meanwhile, ventilation - including open windows or extractor fans - dilutes indoor pollutants when outside air quality is suitable.

Choosing air cleaners for indoor air

Not all air cleaners are of the same standard.

For routine use, choose a unit sized appropriately for the room that clearly says it has a True HEPA filter. This indicates it is made to capture at least 99.97% of very small particles.

An AHAM Verifide label is also useful, as it confirms that the clean air delivery rate (CADR) has been independently assessed. As a basic rule, a higher CADR means the cleaner can remove particles from the air more quickly, while packaging will normally state the room size for which the unit is intended.

Most air cleaners are primarily designed to deal with particles, including dust, pollen, pet dander and smoke.

For assistance with gases or odours, including VOCs, select a model with an activated carbon filter, since HEPA filters on their own are mainly intended for particles. Packaging will generally state whether a device is designed for particles, gases or both, although no air cleaner can remove every pollutant.

It is worth bearing in mind that plants require maintenance too. Excessive watering and neglected pots may cause moisture issues or encourage microbial growth indoors. In this respect, the benefits of indoor greenery also rely on how it is cared for.

Does this make houseplants pointless indoors? Certainly not.

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

Research indicates that they may improve perceived comfort and psychological wellbeing and, in some circumstances, have a small effect on humidity or the indoor microenvironment.

Keep houseplants if you like them, and because they can make interior spaces calmer and more appealing. They may make a home feel more enjoyable, which has value in its own right.

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

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.