Can houseplants really purify the air?

“Inspirational” Instagram pics of lush-looking peace lilies and mother-in-law’s tongue number in their thousands. Along with the social media hype, you can find plants with big price tags and even bigger claims about their ability to remove nasty toxins from the air.

But despite the “scientifically proven” assertions on the labels, a lot of these clean air claims are overblown.

Coming down to earth

Many claims for the air-clearing properties of houseplants date back to research published by Nasa in 1989. The agency had a problem. It wanted to find a way to clean the air on space stations and landed on houseplants as a potential solution.

Nasa was particularly concerned about volatile organic compounds (VOCs), emitted from the stations’ building materials. It investigated whether indoor plants could reduce or remove three VOCs – benzene, formaldehyde and trichloroethylene – from the air. The study included 19 plants from mother in-law’s tongue to spider plants.

The results showed some houseplants had the potential to remove some VOCs. However, there are reasons why Nasa’s results can’t be replicated at home.

A 2019 study by Professor Michael Waring from Drexel University in Philadelphia pointed out Nasa’s research was done in sealed chambers. These chambers don’t account for the number of times air is replaced in the home, either through an air-con system or by opening doors or windows.

When these factors are included, Prof Waring found the effect of houseplants on air pollution was minimal.

For plants to clear the air better than an air-con system, or opening a window, Prof Waring estimated you’d need about 100 plants per square metre. In a 100sqm house, that’s 10,000 plants. You’d also need a machete to get in the front door.

Carbon claims

Other claims for houseplants tout their ability to remove carbon dioxide (CO2) and improve relative humidity. We exhale CO2 when we breath and it creates stuffy air when lots of people are in the same room.

Houseplants use CO2 for photosynthesis. However, whether adding more plants reduces CO2 and makes air fresher depends on several variables, according to a 2018 study co-written by Dr Tijana Blanusa, principal horticultural scientist at the Royal Horticultural Society in the UK.

These variables include how light your home is, the quality of the potting mix and how often you water your plants. In optimal conditions, you’d need at least 15 plants to remove just 10 percent of the CO2 you exhale.

“Quite large numbers of plants in well-lit conditions would be required to have [a] measurable impact on room-level air quality,” Dr Blanusa said.

The study found fewer plants were needed to improve humidity. However, like Nasa’s research, it was done in the lab and not in a home environment.

Dr Blanusa said there’s more to be learnt about plants’ potential role in removing contaminants from the air and research is continuing.

Indoor air pollutants

With all the hype about houseplants and clean air, what are the facts about the air we breathe indoors?

Research by the Building Research Association of New Zealand (BRANZ) pinpoints moisture as the biggest indoor air pollutant in Kiwi homes. A damp home is prone to mould and dust mites, which can trigger allergies and asthma.

Moisture is created from cooking and showering, as well as from using unvented clothes dryers and unflued gas heaters. Using extractor fans, or opening a window, generally helps lower moisture levels.

VOCs also affect indoor air quality. They’re off-gasses from building materials and furnishings as well as from household products, such as cleaners, paints and solvents. Concentrations of these contaminants exceed acceptable levels in many homes, according to BRANZ.

In a 2017 study, it found spikes in VOC levels in three of seven Wellington homes assessed. These spikes were linked to recent use of cleaning products and glues in the homes.

BRANZ also tested two homes in Auckland – one air-tight and the other not – before and after they were decorated and furnished. VOC levels increased in both houses, tripling in the fully furnished air-tight house. It took five days for levels to fall significantly, after switching on the homes’ ventilation systems.

Concentrations of particulates in these houses were also high before the ventilation systems were used. These particles may aggravate breathing problems, and irritate the eyes and throat. They’re emitted by cooking and wood burners but can also come from outside air. BRANZ is doing further research on indoor air quality.

How to improve air quality

Rather than buying hundreds of houseplants, here are five things you can do to improve the air quality in your home:

• Open a window once or twice a day – as little as 15 minutes a day will help. Opening a door or window at both ends of the house creates a cross-draught moving air through more effectively.

• Use extractor fans in the bathroom and kitchen to help remove moisture.

• Don’t dry washing inside if can you avoid it. Make sure clothes dryers are vented to the outside.

• Use energy-efficient heating if possible, for example, heat pumps, flued gas heaters and clean-burning wood burners.

• Air your home after decorating. This helps reduce VOC concentrations.