All rain is acidic.

No, really. It works like this; As rain falls to the earth it picks up carbon dioxide in the atmosphere which creates carbonic acid. According to the Environmental Protection Agency (EPA), “normal, clean rain has a pH value of between 5.0 and 5.5.”

An object’s pH value measures how acidic it is based on its number of hydrogen atoms. On a pH scale, the smaller a value an object has, the more acidic it is. The higher the value, the more alkaline or basic the object is. Zero is the most acidic, while 14 is the most basic and pure water is neutral with a pH value of seven.

Which means clean rain water is acidic, but only slightly. Does that make sense? Great, because here’s where it gets dicey.

If all rain is slightly acidic, then what distinguishes it from acid rain? The simple answer is that it has an even lower pH value. That might not seem like much, but each move up or down the pH scale is actually by ten. So, if pure water is a 7 and, according to the EPA, acid rain is usually around a 4, then acid rain is actually a thousand times more acidic than pure water.

This is caused when we burn fossil fuels, like coal, and the emissions enter the atmosphere. Specifically, sulfur dioxide and nitrogen oxide. In 2017, the EPA estimated that two-thirds of the sulfur dioxide and one-fourth of nitrogen oxide in the atmosphere come from burning fossil fuels to generate electricity.

So the next thing we have to ask is so what? Why should we care?

Besides the fact that just the term “acid rain” sounds terrifying, when rain becomes acid rain, it falls to the ground and seeps into the earth. This can destroy the soil and create a harsh environment for trees and plant life. The ground water then returns to bodies of water and disrupts the ecosystem. It kills plants and marine life, which trickles upwards to affect the ecosystems that depend on those plants and marine life for food sources.

The good news is that in 1990, Title IV of the Clean Air Act Amendments established the Acid Rain Program (ARP).

The ARP required major emission reductions of sulfur dioxide and nitrogen oxides and set a permanent cap on the total amount of sulfur dioxide that could be emitted by electric generating units in the United States. Plants have also installed scrubbers or, according to the EPA, “equipment which remove the sulfur dioxide from gases leaving the smokestack.”

Those regulations must have worked. In May of 2018, biogeochemist Douglas A. Burns told the New York Times that since the 1970s, “rainwater in the Northeast has seen about a tenfold decrease in acidity.”

But that’s just the United States. Acid rain continues to be a problem in other countries. Dr. Burns said China is only just beginning to bring their acid rain problem under control. China’s emissions have caused acid rain in neighboring countries Japan and South Korea. It’s also still a problem in India, where, according to Dr. Burns, emissions are increasing.