How Much Microplastic Is Falling From the Sky?

The challenge is that there still isn’t one standard way to measure how much microplastic falls from the atmosphere, which helps explain why estimates vary so widely. A lot of the instruments researchers use to measure particles in the atmosphere weren’t designed specifically to collect microplastics.

“They’re aerodynamically designed for spherical particles of a certain density and to filter out size based on those properties,” Brahney says. But plastics come in different shapes and have lower densities. “They’re not calibrated for microplastics. So we don’t really know how effective they are at pulling in what’s exactly in the atmosphere.”

Brahney used a deposition sampler in her research, which relies on gravity to collect particles as they fall. But that method has limitations too. Wind can create what she calls a “sampler shadow effect”, where some plastic particles are pushed away from the sampler. “So it usually underestimates what’s being deposited,” she says.

Researchers also measure different things. The concentration of plastic floating in the air isn’t the same as the amount deposited on the ground, and Brahney says the two can’t easily be converted.

How Do Microplastics Get Into the Air?

In the ocean, waves can eject tiny droplets of water containing microplastics into the air. Once the water evaporates, the plastic particle is left behind. “Globally, that’s probably one of the most important mechanisms,” Brahney says.

On land, roads can also send existing plastic particles back into the atmosphere. “Cars travelling on roads, especially where those roads intersect with major urban centres, can move all the particles on the surface of the road into the atmosphere from the friction of the tyres on the road surface,” Brahney says.

Agricultural land is another source. Wastewater treatment plants can remove microplastics from wastewater, but some of those particles end up concentrated in soil that is later used as fertilizer. In agricultural settings, the combination of wind, tractors and tools can then kick those particles into the air.

It’s no secret that all of these particles originate from human-made plastic. Researchers like Brahney say the vast majority of what’s collected is usually fibres, or broken-down clothing from waste and littering, largely due to the rise of fast fashion. “Textiles are a major contributor to microplastics in the environment,” Brahney says.