A hidden chemical leak may be quietly delaying the ozone layer’s comeback.
The Montreal Protocol, signed in 1987, is widely considered one of the most effective environmental agreements ever implemented. It has driven a global phaseout of chemicals that once damaged the ozone layer and increased risks such as skin cancer and other health problems.
Follow-up research led by MIT has shown that reducing these ozone-depleting substances is helping the ozone layer slowly rebuild. (It could return to 1980 levels by as early as 2040, according to some estimates.) However, the agreement includes an exception. Certain ozone-depleting chemicals are still allowed as feedstocks for manufacturing other materials. This exemption was based on the belief that only a very small fraction, about 0.5 percent, would escape into the atmosphere.
Unexpected Rise in Ozone-Depleting Emissions
Recent measurements suggest that assumption no longer holds. Scientists have detected higher levels of ozone-depleting substances in the atmosphere than expected, prompting a reassessment of how much is leaking from feedstock use.
A new study by an international team that includes MIT researchers examines how these higher leakage rates affect the ozone layer’s recovery. Their results indicate that if current emissions continue, the return of the ozone layer could be delayed by roughly seven years.
“We’ve realized in the last few years that these feedstock chemicals are a bug in the system,” says author Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry, who helped identify the original cause of the ozone hole. “Production of ozone-depleting substances has pretty much ceased around the world except for this one use, which is when you have a chemical you convert into something else.”
Industrial Feedstocks and Their Impact
The research, published in Nature Communications, is the first to fully quantify the effect of emissions from feedstocks. These substances are widely used to produce plastics, nonstick coatings, and replacement chemicals for those already restricted under the Montreal Protocol.
According to the researchers, limiting both the use of these feedstocks and the amount that escapes into the atmosphere will be increasingly important, especially as demand for products like plastics continues to rise.
“We’ve gotten to the point where, if we want the protocol to be as successful in the future as it has been in the past, the parties really need to think about how to tighten up the emissions of these industrial processes,” says first author Stefan Reimann of the Swiss Federal Laboratories for Materials Science and Technology.
“To me, it’s only fair, because so many other things have already been completely discontinued. So why should this exemption exist if it’s going to be damaging?” says Solomon.
The study involved researchers from multiple institutions, including MIT, NASA, NOAA, and universities and research centers across Europe, the United States, and Asia.
From Discovery of the Ozone Hole to Global Action
Concern about ozone depletion began in 1985, when scientists identified a growing hole in the ozone layer above Antarctica. This thinning allowed more of the sun’s harmful ultraviolet radiation to reach Earth’s surface.
In 1986, Solomon and other researchers traveled to Antarctica and confirmed the cause. The damage was linked to chlorofluorocarbons, or CFCs, which were commonly used in refrigeration, air conditioning, and aerosol products.
These findings led to the creation of the Montreal Protocol, an international agreement involving 197 countries and the European Union to restrict CFC use. The decision to allow feedstock use was partly based on industry estimates that leakage would remain minimal.
“It was thought that the emissions of these substances as a feedstock were minor compared to things like refrigerants and foams,” Western says. “It was also believed that leakage from these sources was minor — around half a percent of what went in — because people would essentially be leaking their profits if their feedstocks were released into the atmosphere.”
New Data Reveals Higher Leakage Rates
That assumption is now outdated. Western and Reimann are part of the Advanced Global Atmospheric Gases Experiment (AGAGE), a worldwide monitoring network that tracks emissions of ozone-depleting substances.
Recent observations suggest that leakage rates are closer to 3.6 percent, with some chemicals showing even higher losses.
In their analysis, researchers used 3.6 percent as a baseline and compared it with scenarios where leakage is reduced to 0.5 percent or eliminated entirely. They also reviewed production data from 2014 to 2024 to estimate how much of these chemicals will be used as feedstocks through 2100.
Their findings show that overall emissions decline in all scenarios until about 2050, largely due to existing restrictions under the Montreal Protocol. However, with continued leakage at current levels, emissions stabilize around 2045 and only fall by about half by the end of the century.
Ozone Recovery Could Be Delayed
The team then assessed how these emissions affect the ozone layer itself. If leakage is reduced to 0.5 percent, the ozone layer would recover to its 1980 condition by 2066. If feedstock emissions were eliminated, recovery would occur by 2065.
Under current conditions, however, recovery would be pushed back to 2073, delaying progress by about seven years.
“This paper sends an important message that these emissions are too high and we have to find a way to reduce them,” Reimann says. “Either that means no longer using these substances as feedstocks, swapping out chemicals, or reducing the leakage emissions when they are used.”
Industry Innovation and Policy Response
Despite the challenge, researchers believe solutions are within reach. Solomon notes that the chemical industry has a strong track record of innovation and adaptability.
“There are a lot of innovators in the chemical industry,” Solomon says. “They make new chemicals and improve chemicals for a living. It’s true they can perhaps get too entrenched with certain chemicals, but it doesn’t happen that often. Actually, they’re usually quite willing to consider alternatives. There are thousands of other chemicals that could be used instead, so why not switch? That’s been the attitude.”
She also points out that the ability of networks like AGAGE to detect these emissions highlights how much progress has already been made in reducing other sources of ozone damage.
“This isn’t the first time that the AGAGE Network has made measurements that have allowed the world to see we need to do a little better here or there,” Western says. “Often, it’s just a mistake. Sometimes all it takes is making people more aware of these things to tighten up some processes.”
Global Efforts to Close the Gap
Countries participating in the Montreal Protocol meet annually to address emerging issues. Feedstock emissions are already under discussion, and future actions may focus on reducing or eliminating these leaks.
“We wanted to raise the warning flag that something is wrong here,” Reimann says. “We could reduce the period of ozone depletion by years. It might not sound like a long time, but if you could count the skin cancer cases you’d avoid in that time, it would seem quite significant.”
Reference: “Continuing industrial emissions are delaying the recovery of the stratospheric ozone layer” by Stefan Reimann, Luke M. Western, Megan J. Lickley, David Sherry, John S. Daniel, Lambert J. M. Kuijpers, Stephen A. Montzka, Matthew Rigby, Guus J. M. Velders, Martin K. Vollmer, Lukas Emmenegger, Qing Liang, Sunyoung Park and Susan Solomon, 16 April 2026, Nature Communications.
DOI: 10.1038/s41467-026-70533-w
The work was supported in part by the National Science Foundation, NASA, the Swiss Federal Office for the Environment, the VoLo Foundation, the United Kingdom Natural Environment Research Council, and the Korea Meteorological Administration Research and Development Program.
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