In the 1970s, acid rain was one of the most serious environmental threats in North America and Europe. The air was so laden with pollution from coal power plants and cars at the time that it turned the rain toxic. Downpours killed fish, destroyed forests, eroded statues, and damaged buildings, sparking public outcry.

“Acid rain is a particularly alarming demonstration of the simple adage that what goes up must come down,” former Colorado Senator Gary Hart said in 1979. “With acid rain,” he said, “what comes down is much worse than what went up — worse in its potential damage to trees and crops, worse in its potential damage to fresh‐water lakes and fish and tourism.”

A few decades later, acid rain had largely disappeared.

Beginning around 1990, the US and Europe passed legislation that limited the amount of acid-forming pollutants — such as sulfur dioxide and nitrogen oxides — that power plants could emit. Laws requiring car manufacturers to put catalytic converters into new vehicles, which reduced harmful emissions, were also taking effect. That brings us to today: While precipitation in some regions is still unnaturally acidic, on the whole, acid rain is largely a problem of the past and a major environmental success story.

Now, however, there’s another problem with our rain — and it’s even more alarming.

While precipitation has become less acidic, a growing body of evidence suggests that it’s now full of many other pollutants that pose a risk to public health, including microplastics. And unlike the compounds that cause acid rain, these pollutants are almost impossible to get rid of.

The new pollutants in our rain

As government regulators focused on reigning in air pollution, companies were busy generating new sources of pollution, including plastics and PFAS, the so-called forever chemicals. PFAS, which stands for perfluoroalkyl and polyfluoroalkyl substances, are a large group of compounds used, among other things, to make fabric stain-resistant and pans nonstick.

Over time, these modern-era substances — which famously take decades to millennia to degrade — have leached into the environment, reaching every corner of the planet, no matter how tall or deep. Microplastics, PFAS, and some other compounds, such as pesticides, are now so widespread that they’ve essentially become part of our biome, not unlike bacteria or fungi.

They’re so common, in fact, that they’re even found in the rain.

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