A plastic mist descends from the sky each day. You can't see it. Or feel it. It has no smell or taste. But researchers think we are seriously underestimating the invisible downpour.
New estimates by a team of scientists from the University of Auckland in New Zealand suggest an average of nearly 5000 microplastic particles settles on every square meter of Auckland's urban rooftops on any given day.
That dusting adds up to around 74 metric tons of plastic each year, equivalent to about three million plastic bottles.
That gobsmacking amount is much, much more plastic than was recently calculated to be drifting down on London, Hamburg, or Paris. A study in 2020 estimated an average of just 771 microplastic particles falling onto the same-sized patch in London.
But that doesn't necessarily mean London is six times less polluted by airborne plastics than Auckland. After all, it's a much bigger city and located in a far less remote part of the world.
Instead, it's more likely that previous air quality estimates, like those in London, were simply not measuring the smallest microplastics in circulation.
Today, there is no standard methodology or protocol for identifying microplastics, which means that every study is conducted in a slightly different way.
As our ability to measure the tiniest of microplastics continues to improve, experts are noticing far more offenders hiding in the air around us than ever before.
"Future work needs to quantify exactly how much plastic we are breathing in," says chemist Joel Rindelaub from the University of Auckland.
"It's becoming more and more clear that this is an important route of exposure."
The findings from New Zealand are based on a 9-week study of two sites in Auckland – one on the roof of a university building in the city and another on the fence in a city suburb. Airborne microplastics were caught using a funnel and jar contraption.
At each site, researchers counted the remnants of eight different types of airborne plastics . The most prolific were polyethylene (PE), used in grocery bags and squeeze bottles, polycarbonate (PC), used in protective gear and medical devices, and poly(ethylene terephthalate) (PET), used in food and beverage packaging.
When winds from the coast were blowing particularly strong, microplastics caught in the city tended to increase in number. The results suggest that to a certain extent, the airborne plastics floating around Auckland are kicked up from the coastline's wind and waves.
"The production of airborne microplastics from breaking waves could be a key part of the global transport of microplastics," says Rindelaub.
"And it could help explain how some microplastics get into the atmosphere and are carried to remote places, like here in New Zealand."
This could be another potential reason why airborne plastics seem to be lower in number in Germany and England. But even inland, drifting micropollutants are still very much a problem.
In 2019, a small pilot study in Europe found microplastics in the remote Pyrenees mountains, which were probably blown there from a few small towns nearby.
In 2021, scientists warned that airborne microplastics were now so ubiquitous in the atmosphere, they may already be having an effect on Earth's climate. In the future, if concentrations continue to climb, the particles could exacerbate the greenhouse effect by absorbing and scattering light and heat.
In the Auckland study, researchers found their urban site trapped a greater number of microplastics than their residential site in every week bar one.
The vast majority of particles captured during the experiment were between 10 and 50 micrometers in size. Most of them were plastic fragments, and only 3 percent were larger than 100 micrometers.
The findings are slightly different to what was previously found in London, where plastic fibers, not fragments, seemed to be the bigger source of pollution. The discrepancy could be due to updated sampling techniques or different types of pollution in various parts of the world.
A 2019 survey conducted in Hamburg, for instance, only measured for microplastics larger than 63 micrometers. The count of airborne particles from this city was also 18 times fewer than what was recently found in Auckland.
No one knows yet whether airborne microplastics have effects on human health, but the smaller the fragment or fiber, the more likely it is to cross over into our cells when we breathe it in.
If plastic pollution falling from the sky really does have toxic effects, it's too insidious for scientists to tell just yet.
The study was published in Environmental Science & Technology.