Clearing the air: Are houseplants as effective as air purifiers during the haze?

SINGAPORE – Many who take up gardening love bringing the outdoors inside. It is a fantastic way to beautify our living spaces and lift our mood. These benefits of indoor greenery are especially appreciated when we are stuck at home during the haze.

But can our potted plants actually improve indoor air quality? I am asked this question all the time when the haze blankets Singapore.

The popular belief that plants double as natural air purifiers usually stems from the famous 1989 NASA Clean Air Study, which looked into ways to clean the air in sealed environments such as space stations.

Subsequent studies have also shown that plant leaves can trap fine particles from the air.

To understand what plants can actually do, it helps to look at the two main components of air pollution that threaten our health: toxic gaseous compounds called volatile organic compounds (VOCs) and fine particulate matter.

During the haze in Singapore, our primary health hazard is PM2.5 – microscopic particles of soot, ash and dust suspended in the air.

Leaves and soil play a big role

Plants absorb VOCs through their stomata – microscopic pores in leaves that open during photosynthesis for gas exchange.

But much of the heavy lifting is done below the surface, where the soil microbiome plays the dominant role.

Bacteria and fungi in the potting mix ingest gaseous toxins as air naturally diffuses through the substrate.

Once absorbed, the plant and these microbes metabolise the pollutants into harmless by-products.

Meanwhile, the waxy outer cuticle of leaves acts as a passive trap, physically trapping (adsorbing) specific chemical compounds.

When it comes to tackling airborne dust, a plant’s efficacy hinges on its leaf physical architecture: texture, total surface area and transpiration rate. Rough, ribbed or hairy leaves maximise particle retention.

Furthermore, as plants transpire, the moisture they release raises local humidity. This causes tiny airborne particles to aggregate, growing heavier until they fall out of suspension and settle via gravity.

Recognising real-world limitations

While the science behind this is fascinating, we have to look at the limitations in a home setting.

The catch with many of these scientific studies is that they were conducted in sealed enclosures – a far cry from a typical apartment.

Our homes exchange air with the outdoors. Even with the windows shut and air-conditioning running, door gaps and the opening and closing of doors allow outdoor air pollutants to slip into our living spaces.

Furthermore, indoor plants metabolise gaseous compounds at an incredibly slow rate, relying entirely on fine particulate dust to passively settle onto their leaves over time.

The same bottleneck applies to helpful soil microbes. To make a meaningful dent, contaminated air needs to be actively forced through the potting substrate. Because there is little airflow through the soil, the root zone’s filtration capacity is severely restricted.

We also have to consider indoor lighting. Plants in our homes receive a fraction of the light intensity they would receive outdoors.

With less light, photosynthesis slows down, meaning their uptake and metabolism of gaseous compounds drop significantly.

This indoor stagnation stands in stark contrast to how plants behave in nature. Outdoor greenery is vastly more effective at removing particulate pollution from the atmosphere.

On a macro scale, natural and urban forests, along with greenery in our streetscapes, parks and gardens, can act as massive sinks for fine particulate matter. The complex, multi-layered, dense canopy structures of outdoor trees, coupled with wind, maximise physical contact between polluted air and leaf surfaces.

Conversely, indoor plants operate in a confined environment with restricted airflow, severely capping their passive filtration capacity.

Air purifiers to the rescue

While your houseplants will catch a nominal layer of dust, they are simply not effective at filtering the air in a room during the haze.

To protect your well-being, it is best to rely on dedicated air purifiers with high-efficiency particulate air, or HEPA, filters, rather than try to turn your living space into an impassable, high-maintenance indoor jungle.

In the meantime, while staying indoors, you can spend your time washing or wiping down your plants’ leaves with a damp cloth.

Clearing accumulated grime from the foliage surfaces unblocks their pores and helps your plants stay healthy, photosynthesise better and survive comfortably until the air clears up.

Green peace of mind

Just because your plants cannot filter the air does not mean they are useless during the haze. While they might not clear out PM2.5, they help ease the stress of being stuck indoors with gloomy skies.

Caring for your plants gives you a simple, grounding connection to nature. Slowing down to water them or watching a new leaf uncurl provides a comforting break from the haze outside.

To get the most out of your indoor garden, group a few plants together to create a small green corner near your desk or couch.

Select plant species that are low-maintenance and tolerant of the lower light conditions typically found at home.

If needed, you can install a grow light linked to a timer switch to keep them thriving and utilise self-watering pots to streamline your routine.

Houseplants may not replace a HEPA filter, but for your peace of mind, they are entirely irreplaceable.