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.
“There can be these strange, unexpected ripple effects that have substantial consequences for human health,” said Jonathan Buonocore, a professor of environmental health at Boston University and one of the paper’s authors. “This is a surprise.”
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.
“Good ozone,” also known as the stratospheric ozone layer, protects the planet from too much UV radiation. Excessive UV radiation can alter DNA, cause skin cancer and eye problems, and hurt animals and plants. This is why the world acted swiftly to protect the ozone layer in the 1980s.
“That was the wisdom of the protocol,” said Sebastian Eastham, a research scientist at the Massachusetts Institute of Technology’s IT Laboratory for Aviation and the Environment and one of the paper’s authors. “It recognized that we were doing something to the stratosphere that we didn’t intend.”
“Bad ozone,” on the other hand, is 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.
In interviews, the authors of the study emphasized that the Montreal Protocol is still valuable because it has helped avert a catastrophic loss of the ozone layer. “The Montreal Protocol has been an incredible achievement in avoiding one very unfortunate outcome,” Eastham said.
But smaller-level changes, Eastham said, create complex trade-offs. For example, those most likely to die from skin cancer are largely in the developed world, while those who would benefit from less air pollution live mainly in the developing world. “You can’t automatically balance one person’s loss against another person’s gain,” he said. “Those are really hard policy decisions.”
Atmospheric scientists who did not work on the study had mixed reactions. David Fahey, director of the chemical sciences laboratory at the National Oceanic and Atmospheric Administration, praised the scientific records of the authors of the paper. When scientists of this caliber “band together like this — well then, you should stop and pay attention to them,” said Fahey, who co-chairs the Montreal Protocol’s scientific assessment panel. “It’s a quality paper.”
But Fahey warned that the paper should be read as an early attempt to quantify the impacts of geoengineering, rather than the last word.
“This is a shot over the bow,” he said, adding that any attempt to factor in such trade-offs into the Montreal Protocol would be challenging. “It’s a very difficult business to get 197 nations to agree,” he said. “There’s really no discussion that I’m aware of that says, ‘Well, let’s only heal the ozone layer so much.’”
Ross Salawitch, a professor of atmospheric science and chemistry at the University of Maryland, questioned the study’s findings.
“If we take their paper literally, one might conclude that the Montreal Protocol — which has saved the world from a massive ozone depletion — maybe that’s had a deleterious effect on human health,” he said. “And that would be a very provocative statement.”
Salawitch critiqued the researchers for not accounting for how sulfate particles might change the meteorology of the atmosphere, which could in turn change some of the chemistry they described. The paper, he argued, also ignores the way that other human actions — like industrial activities, driving cars and more — influence ground-level ozone pollution much more.
“We know that human activities can have such a greater effect on surface ozone,” he said. “We’re working on both problems in tandem; to kind of isolate one and look at the effects on the other — it’s a strange way of looking at the world, in my opinion.”
Both Eastham and Buonocore, two of the paper’s authors, emphasized that their intention is not to criticize the Montreal Protocol, but to provide more detailed information about its impacts. They also pointed out that the paper looks specifically at sulfate, rather than CFCs. While CFCs in the atmosphere might have similar effects on ground-level air pollution, the paper didn’t explicitly study them.
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.)
But it also shows that changing even one small component of the global atmosphere — whether through climate change itself, or attempts to curb it — can cause a complex web of chain reactions. And, even after so many years of earth system science, researchers still have a limited understanding of what all of those changes will be.
“We start breaking stuff, and then applying fixes that break something else,” Buonocore said.