Human-made plastic isn't just flooding the world's oceans, it's also piling up on the land and in the soil.
For years now, the synthetic microfibres woven into our clothing have been leaching into the environment. Even when we don't throw away our clothes or when we buy them secondhand, wastewater from our washing machines can break up these tiny pollutants.
New estimates reveal nearly as many synthetic microfibres are now amassing on land as those leaking into waterways.
Since the mass production of synthetic microfibres - like polyester and nylon - began in the 1950s, scientists predict at least 5.6 million metric tons of synthetic microfibres have been released from clothes washing.
Even more gobsmacking, half of those pollutants were produced in the last decade alone.
"The numbers in the research are staggering," says public health scientist Paul Harvey, who was not involved in the study, "but most likely an underestimate."
Scientists have known about these tiny terrestrial microfibres for a while now, but we've never actually known the extent of the problem.
Analysing the global extent of synthetic microfibre release is near impossible, especially since many regions do not have detailed data on wastewater treatment.
Still, if we want to understand the true scope of this issue, we need some sort of ballpark figure, and these results are some of the best estimates to date.
Using income as a proxy for the level of a nation's water and sewage treatment where data were unavailable, researchers compared the effects of hand washing and machine washing on microfibre pollution in wastewater.
Many people don't realise that when they put their clothes in the washing machine, tiny synthetic fibres of plastic can leach out into the water, especially if you set your machine to wash delicates.
When these tiny plastics are caught in a city's wastewater treatment facility, they are usually turned into biosolids, used as soil or fertiliser. The rest either goes to landfill, is incinerated, or otherwise ends up dumped in the ocean.
While plastic pollution in oceans has received a lot of attention in recent years, and for good reason (it poses a serious threat to marine mammals and ocean ecosystem), waterways are not the only place plastic accumulates.
The new global analysis reveals just under half of all synthetic microfibres end up on land, either on the surface (1.9 million metric tons) or in landfills (0.6 million metric tons). Meanwhile, bodies of water receive nearly 2.9 million metric tons.
The authors acknowledge the limitations of their figures. They admit their models are based on multiple assumptions and simplifications, such as global washing frequency, the percentage of clothing in use on an annual basis, and washing machine ownership worldwide.
That said, they examined several alternative scenarios and still found a range of total microfibre emissions somewhere between 4.3 million metric tons and 7.0 million metric tons.
Each year, that's at least 176,500 metric tonnes of microfibre plastics going to cropland or landfill. Donated and recycled clothing was also not taken into account, which means this is probably an underestimation.
As clothing stock grows worldwide and more people acquire washing machines, these numbers are only set to increase. With wastewater treatment sites becoming ever more popular, the authors think microfibres once destined for the sea could soon be redirected to land.
It's not clear what this will do to the soil, to our crops or our health, let alone the many terrestrial animals that live here with us. Studies have shown synthetic microfibres can retain their fibrous forms in the terrestrial environment for more than 15 years, so the choices we make now will have consequences for the future, whether we know the risks now or not.
"Large-scale removal of microfibers from the environment is unlikely to be technically feasible or economically viable, so the focus needs to be on emission prevention," says study lead Jenna Gavigan from the University of California at Santa Barbara.
"Since wastewater treatment plants don't necessarily reduce emissions to the environment, our focus needs to be reducing emissions before they enter the wastewater stream."
In other words, removing these microplastics from the sludge of treatment facilities is probably not going to happen. Instead, the authors suggest we focus on switching to more environmental fabrics and redesigning our washing machines to filter microfibres better.
Ian Rae, an environmental chemicals expert not involved in the study, thinks those solutions are wishful thinking.
"Good luck with that!" Rae wrote in a comment.
"Another way is by ensuring more efficient filtering of the water as it leaves the machine, about which [the authors] observe that collected material would need to be disposed of by incineration or landfill. And here's a puzzle: if landfill is acceptable as a disposal method, you'd have to ask why there is such a fuss, in the abstract of their paper and the publicity surrounding the publication of their work, about microfibres going to 'terrestrial environments'."
He's got a point, too. The truth is, we don't really know what to do with the ubiquitous use of microfibres already in our clothing. How do we stop them from leaching into the environment? If we do catch them in a filter, where should we put them next? Once they leach into the land and the water, can we ever hope to recover them?
"There are huge unknowns," ecologist Sangwon Suh told the USCB publication, The Current. "The amount of microplastics and microfibres that are generated is quite massive and continuing to rise, and if it continues there will be big changes, the consequences of which we are not yet sure. That's what makes it concerning."
The study was published in PLOS One.