April 22, 2026

5 min read

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Whatever happened to the ozone hole, acid rain and DDT?

The environmental crises of the past often seem to disappear—have problems like acid rain and smog been solved?

By Andrea Thompson edited by Jeanna Bryner

People in the mid-'80s react to the worry posed by acid rain.

Jeff Goode/Toronto Star via Getty Images

Each decade seems to have its signature environmental crisis that dominates headlines. Over time those issues seemingly disappear, leaving us to idly wonder from time to time if problems like the ozone hole or acid rain are still planetary concerns or if we’ve solved them.

On this Earth Day, Scientific American is here to give you a status update on some of the major environmental stories of past decades.

A 3D rendering of the ozone hole evolution in 2025.

CAMS

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Ozone Hole

In 1985 three British scientists announced their discovery of a massive hole in Earth’s protective ozone layer that was opening every spring over Antarctica. The culprits, it turned out, were chlorofluorocarbons (CFCs) and certain other chemicals used in refrigeration and aerosol sprays. (Another trio of scientists had warned the world about CFCs in the 1970s and later won the Nobel Prize in Chemistry for it.) When CFCs break down in the atmosphere, they release chlorine that leads to the destruction of ozone, a molecule that contains three oxygen molecules and blocks harmful ultraviolet rays from reaching the planet’s surface.

As people became alarmed about the possibility of rising skin cancer rates, nations met in 1987 to pass the Montreal Protocol and phase out CFCs. It was the first United Nations treaty to be universally ratified and was possible in part because of ready alternative chemicals.

Threats to the ozone layer continue, from, for example, chlorine-containing molecules emitted by massive wildfires and aluminum oxide produced when satellites and other space objects burn up upon reentry into the atmosphere. Even so, CFC levels have plummeted since the Protocol was enacted, and the ozone layer has begun a long, slow recovery.

Current predictions suggest the ozone layer will recover to its state in 1980 over the tropics and middle latitudes by 2040, and it will reach that point over the Arctic by about 2045 and over Antarctica by the mid-2060s.

“The Montreal Protocol is the best environmental agreement we’ve ever created,” Durwood Zaelke, an environmental policy expert at the University of California, Santa Barbara, and founder of an organization focused on reducing climate pollutants, told Scientific American in 2025.

Acid rain damage to the entrance of the Igreja de Santa Cruz in Coimbra, Portugal.

jacquesvandinteren/Getty Images

Acid Rain

In the 1980s and 1990s one of the top environmental crises grabbing public attention was acid rain, with news reports showing images of the partially dissolved faces of statues and trees bare of leaves.

Acid rain is formed when sulfur dioxide and nitrogen oxides released into the atmosphere react with oxygen, water and other chemicals to form sulfuric and nitric acids, which can dissolve in water vapor. Any rain (or snow, fog or even hail) that then falls has a more acidic pH than normal. That rain not only eats away at human stonework but can also harm animals and ecosystems, especially aquatic ones. And because these chemicals can be carried for hundreds of miles, pollution in one state or country can cause damage in another downwind.

Though sulfur dioxide and nitrogen oxides can come from natural sources, such as volcanoes, the majority comes from burning fossil fuels.

And once again, public alarm helped drive action. In the U.S., updates to the Clean Air Act created a cap-and-trade system for power plants—which at the time were largely coal-burning—that gradually ratcheted down the amount of sulfur dioxide and nitrogen oxides they could release, incentivizing the addition of pollutant-capturing scrubbers to smokestacks.

These pollutants are of growing concern in places such as India, where coal-fired power is generally increasing. But in the U.S., sulfur dioxide emissions have declined by 95 percent and nitrogen oxides by 89 percent, according to the Environmental Protection Agency. And wet sulfate deposition, a common indicator of acid rain, dropped by more than 70 percent between 1989–1991 and 2020–2022.

Beachgoers are sprayed with DDT as a new machine for distributing the insecticide is tested for the first time in 1945.

Bettmann/Getty Images

DDT

With publication of her book Silent Spring in 1962, biologist Rachel Carson helped ignite the modern environmen