Remember when your mother used to scold you for opening your mouth to the deadly sky as it was pouring? "It's acid rain," she'd say. Envisioning raindrops burning through my gums, jaw, and skin, I always followed my mother's advice. But the last I ever heard about acid rain was in the "˜90s. So where did it go? And what was acid rain, anyway?

Technically speaking, acid rain is rain with, well, acid! Anything with a pH level of below 7 is considered acidic. While even "clean" rain has a pH of 5.5 (due to the normal atmospheric carbon dioxide which reacts with the rain to form carbonic acid), precipitation with pH levels as low as 2 were measured in the US and Europe in the 1980s. While mom wasn't exactly right about the holes the scary raindrops could burn through your tongue, she did have something right about the water's harmful effects—particularly on aquatic life, vegetation and agriculture. So, what causes it? And how did Captain Planet save the environment from these evil raindrops?

What causes acid rain?

It all comes down to NOx and SOx. Nitrogen and sulfur emissions from factories and electric power generation facilities combine with water and oxygen in the atmosphere to form an acid deposition. Acid rain was found in 1952 in England, but it's been around since the Industrial Revolution and only became a true public scare in the 1990s when a New York Times article exposed its environmental and health detriments.

While the article prompted acid rain to quickly become one of the most talked about environmental issues in the early 90s, the phenomenon suddenly vanished and people stopped talking about it. So, what exactly caused this ultra-green magic trick?

Who Stopped the Rain

The brunt of it is that acid rain went away because we curbed our sulfur dioxide emissions. Under the United States Environmental Protection Agency's (USEPA) Acid Rain Program, Title IV of the Clean Air Act, the government established a cap regulating the amount of sulfur we could emit, in an effort to reduce emissions to 10M tons below 1980 levels.

Companies themselves could decide how to manage under those restrictions, either by switching fuels or developing new processes that emitted less sulfur. They also had the option to buy pollution allowances from other companies whose emissions were below the regulated cap. The cap placed on each company was lowered over time. As the caps were lowered, the allowances became more and more expensive. This created a strong new market and further enticed companies to switch to less polluting processes and energy sources.

This cap and trade program achieved 100% compliance in reducing sulfur dioxide emissions and was enforced in two phases. Phase I began in 1995 and 445 electricity plants reduced emissions by almost 40% below the required cap. Phase II, which began in 2000, had even more stringent policies. Overall, the companies that have participated in the program have reduced sulfur dioxide emissions 22% below the mandated levels. That's pretty remarkable.

Of course, this cloud isn't all silver lining. While many view the USEPA's program a success and a model for what can be done with other types of emissions, acid rain has yet to be fully obliterated.