It killed forests in Europe and wiped out marine ecosystems in North America; it made food turn toxic; where it impacted humans, it led to respiratory illnesses and lung cancer, heart problems, and low birth weight in infants.

The culprit: acid rain – now a byword for the consequences of irresponsible human industrialization. Sulfur dioxide and nitrogen oxides, released into the atmosphere from cars, coal-burning power plants, and factories, had reacted with the water and oxygen naturally present in the air to form sulfuric and nitric acid. Once airborne, these particles could be blown across hundreds of miles, eventually coming down as rain, fog, or even acid snow.

Despite various attempts to deny its existence – some even reaching as high as the White House itself – the fact that Earth’s most precious resource was slowly becoming deadly was eventually accepted. By that point, it was hard to deny: in 1972, scientists had measured the pH of rain deposited on New Hampshire’s White Mountains and found it to be only a smidge above 4 – about as acidic as tomato juice, and well past the point at which all fish die.

Today, of course, we don’t tend to worry about acid rain – at least, not in the West. Thanks to decades of research and political action, the problem has been isolated, legislated for, and largely reversed.

Now, we have a much bigger problem to deal with.

It’s rainin’ PFAS

Our atmosphere may no longer be highly acidic, but it’s still rife with something equally toxic and artificial: PFAS, aka per- and polyfluoroalkyl substances, or “forever chemicals”.

As the name implies, these substances don’t break down naturally – they’re extremely resistant to metabolic and environmental degradation, and some have estimated half-lives of up to 1,700 years. Others have no known half-life at all.

Now, that’s a big problem – because PFAS are also fricking everywhere. We don’t just mean in artificial things like fire-fighting foams or nonstick cooking pans, where they were originally intended to be; since their introduction in the 1940s, these chemicals have made their way “everywhere – [they’re] in air, soil, and water as well as in wildlife, plants and humans,” wrote Ian Cousins, Professor of Contaminant Chemistry at Stockholm University, Bo Sha, Jana H. Johansson and Matthew Salter, all researchers in Stockholm University’s Department of Environmental Science, and Martin Scheringer, a senior scientist at the Swiss Federal Institute of Technology Zurich, in a 2022 article for The Conversation.

“They can be found on the highest mountains, in the deep oceans and on both poles,” they noted, as well as “in rainwater, from the Tibetan Plateau to Antarctica”.

Not only are these chemicals in the rainwater, but they’re there at levels that wildly exceed the health limits set out by the US’s Environmental Protection Agency. While we don’t know exactly what the health effects of exposure are for most of the nearly 15,000 different types of PFAS, the information we do have is hardly reassuring: those PFAS we have research on have been associated with “serious human health harms,” Cousins et al write, “including different forms of cancer, development toxicity, infertility and pregnancy complications, high cholesterol, ulcerative colitis, liver hypertrophy (‘enlargement’), and thyroid disease.”

Overall, this makes rainwater everywhere – including places like sub-Saharan Africa and South and Southeast Asia, where it’s a vital resource for human consumption – unsafe for drinking. That’s true “even in the remotest regions of the Earth,” Cousins and colleagues wrote – and it’s going to remain so for a long, long time. 

“PFAS do not not break down in the environment. Their only route for removal from environments where we produce food is slow dilution into the deep oceans,” they reiterated. “Rainwater levels may take decades to fall below the levels set in health advisories.” 

“The exact recovery time is uncertain,” they warn. But “the situation will […] not improve soon.”

The new acid rain

“Forever chemicals” aren’t the only things pelting down on us every rainy day. Also rife within our water and atmosphere is plastic – hundreds of millions of tonnes of it, cumulatively, leading some scientists to relabel our planet’s natural phenomena as “plastic rainfall” from “plastic clouds”.

Like PFAS, there’s basically no place on Earth that remains free from this modern, semi-synthetic rain. It’s been found in cloud water harvested from the top of Mount Fuji, and collected in buckets in the wilderness of the USA; it’s so abundant that plastic now constitutes fully one part in 25 of the rainwater that’s been surveyed, according to recent measurements.

“We were shocked at the estimated deposition rates,” said Janice Brahney, Assistant Professor in Utah State University’s Department of Watershed Sciences, in 2020. “[We] kept trying to figure out where our calculations went wrong.” 

Alas, there was no mistake. “We […] confirmed through 32 different particle scans that roughly 4 percent of the atmospheric particles analyzed from these remote locations were synthetic polymers,” Brahney said.

As with the majority of PFAS, the effect of these microplastics on the environment and our health is so far an open question. We know they can interfere with the natural behaviors of animals, and they’ve been shown to act as carriers for viruses, even making them hardier and more deadly in the process. 

But any effects in the natural world are likely to cascade, making the overall result difficult to predict: “[They] can not just block up the digestive tract of small animals, like worms,” University of Strathclyde microplastic researcher Steve Allen told Wired in 2020. “But it’s also the chemicals that are on these plastics and in these plastics that can have an effect on the soil.”

“A lot of that is still theoretical – we’re still trying to work it out,” he said.

A global issue

After all these years, acid rain is still an issue – but it’s so far away from us, geographically speaking, that we tend not to notice. With plastic and PFAS rain, we don’t have the luxury of distance. 

“The extreme persistence and continual global cycling of certain PFAS will lead to the continued exceedance of the above-mentioned guidelines,” Scheringer said in a 2022 statement.

“In other words, it makes sense to define a planetary boundary specifically for PFAS,” he added – “and […] this boundary has now been exceeded.”

The bad news? There’s not a whole lot we can do about it. “There are ways to remove PFAS from water, but it is not clear if the levels can be brought below the latest health advisories,” Cousins and the team wrote.

Perhaps more can be done about the presence of microplastics – so long as the infrastructure is available to filter it out. That isn’t always the case; one of the ways in which so many microplastics make it out into the wide world in the first place is because wastewater treatment plants can’t deal with them, and so just release them into the environment.

Of course, it might be that even that is just obscuring, rather than solving, the problem. Microplastics aren’t the end point of the plastic pollution life cycle – they, in turn, break down into nanoplastics, and at that point, they’re even further out of our control.

“I couldn’t see anything smaller than four microns,” when analyzing the plastic particles found across the US’s national parks, Brahney told Wired – “but that doesn’t mean it wasn’t there.” 

“Just because we can’t see them in front of us, doesn’t mean we’re not breathing them in.”