First the Washington Post summarizes what scientists believed in the 1970s. Chlorofluorocarbons, or (CFCs, "could float up into the stratosphere and break down a protective layer of Ozone, allowing more ultraviolet light to enter the atmosphere and harm humans, crops, and entire ecosystems. In fact, this had already happened: There was a hole in the ozone layer over the South Pole." Experts view the subsequent treaty to cut down on the use of CFCs — the 1987 Montreal Protocol — as a landmark environmental achievement. Scientists estimate that the pact has prevented millions of cases of skin cancer. Today, the ozone hole is recovering well. But a provocative scientific paper published Friday in the journal AGU Advances suggests that the link between the ozone layer and human health is more complicated than it seems. Under certain circumstances, the researchers write, small decreases in the ozone layer could now save lives... The researchers initially were examining something else: what would happen to the chemistry of the atmosphere if humans injected sulfates into the stratosphere, a controversial strategy to cool the planet. But in the process, they found that the chemicals would alter the atmosphere's ozone content — with consequences for human health. Sulfate chemicals are known to deplete ozone high in the atmosphere, but, the paper shows, they could also decrease ground-level air pollution. Ozone, or O3, occurs in two forms in the atmosphere: what scientists call "good ozone" in the stratosphere, the layer of the atmosphere that sits 6 to 31 miles above the surface, and "bad ozone" in the troposphere, the atmospheric layer that reaches to the ground... an air pollutant in the troposphere that comes from power plants, cars, and industrial sites. It can be deadly, exacerbating respiratory diseases. According to one study, over 400,000 people died from long-term exposure to ozone in 2019 alone. The new paper shows that "good ozone" and "bad ozone" can interact in unexpected ways. When good ozone is depleted, more UV light reaches the troposphere, which increases the rate of skin cancer. But UV light also catalyzes chemical reactions in the troposphere, including one in which hydroxide, or OH — which some scientists call the "Pac-Man of the atmosphere" — swallows up pollutants. The more UV light, the more OH eats up dangerous pollutants. This decrease in ground-level air pollution, according to the study, could actually outweigh the rise in skin cancer. A small decrease in stratospheric ozone, according to their study, could save between 33,000 and 86,000 lives every year. Only a few papers have made this connection, including one in 2018 that similarly found that a small decrease in the ozone layer could save lives from air pollution... One way to read the study is as another warning of how dangerous ground-level air pollution is and how far the world still needs to go to clean it up. (Outdoor air pollution writ large is associated with an estimated 4.2 million premature deaths every year.)

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