The Montreal Protocol addresses the depletion of Earth’s ozone layer, caused by widespread use of synthetic chemicals that destroy ozone molecules in the stratosphere. Signed in 1987, it is an international treaty that phases out the production and consumption of these chemicals. It remains one of the most successful environmental agreements ever enacted, with every United Nations member state ratifying it.
What the Ozone Layer Does
The ozone layer sits in the stratosphere, roughly 15 to 35 kilometers above Earth’s surface. It absorbs most of the sun’s ultraviolet-B radiation before it reaches the ground. Without this shield, UV-B levels at the surface rise sharply, increasing the risk of skin cancer, cataracts, and damage to marine and terrestrial ecosystems. By the mid-1980s, scientists had discovered that this protective layer was thinning dramatically, particularly over Antarctica, where a seasonal “ozone hole” had formed.
The Chemicals That Cause the Damage
The protocol targets a family of industrial chemicals that release chlorine or bromine when they break down in the upper atmosphere. The primary culprits are chlorofluorocarbons (CFCs), which were used in refrigerators, air conditioners, aerosol sprays, and foam insulation. Other controlled substances include halons (used in fire extinguishers), carbon tetrachloride, methyl chloroform, methyl bromide (a soil fumigant), and hydrochlorofluorocarbons (HCFCs), which were introduced as early CFC replacements but still damage ozone.
The treaty organizes these substances into annexes and assigns specific phase-out schedules. In total, it controls nearly 100 chemicals across multiple categories, with developing countries given longer timelines to comply.
How These Chemicals Destroy Ozone
Once released at ground level, CFCs and similar gases are stable enough to drift intact into the stratosphere, where intense UV radiation breaks them apart and frees chlorine atoms. A single chlorine atom then triggers a chain reaction: it reacts with an ozone molecule, breaking it into ordinary oxygen, and in the process reforms into a compound that quickly releases the chlorine atom again. That atom goes on to destroy another ozone molecule, and another, and another. One chlorine atom can eliminate thousands of ozone molecules before it is finally neutralized.
Over Antarctica, the process is even more severe. Polar stratospheric clouds that form during the long, dark winter create surfaces where chemical reactions stockpile chlorine compounds. When sunlight returns in spring, these compounds rapidly convert to their reactive forms, and ozone destruction accelerates. Bromine from halons participates in a similar cycle and is, atom for atom, even more destructive than chlorine. The combination of chlorine and bromine reactions drives the deep ozone losses observed over the South Pole each year.
Health Risks of a Thinner Ozone Layer
The ozone layer’s thinning translates directly into higher UV exposure on the ground, which raises the risk of several serious health conditions. Skin cancer is the most significant concern. Basal cell carcinoma is the most common form, followed by squamous cell carcinoma. Melanoma, while less common, is the deadliest, accounting for most skin cancer deaths.
UV radiation also damages the eyes. The World Health Organization estimates that up to 10% of cataract cases worldwide may be caused by overexposure to UV and are therefore preventable. Other eye conditions linked to high UV include growths on the surface of the eye called pterygia, inflammation of the cornea, and uveal melanoma, the most frequent cancer of the eyeball.
U.S. EPA modeling has estimated that the Montreal Protocol, compared with a scenario of no controls at all, will prevent roughly 443 million cases of skin cancer and 63 million cataract cases among people born in the United States between 1890 and 2100. Even compared with the original, weaker 1987 version of the treaty, the strengthened amendments alone account for an additional 230 million fewer skin cancer cases and 33 million fewer cataract cases.
Damage to Ecosystems and Food Production
The risks extend well beyond human health. Marine phytoplankton, the microscopic organisms at the base of the ocean food chain, are highly sensitive to UV-B radiation. These organisms produce roughly as much biomass as all terrestrial ecosystems combined and are the foundation of marine food production for humans. Elevated UV-B impairs their growth, photosynthesis, and ability to incorporate nitrogen, which would shrink their populations and ripple through the entire aquatic food web.
On land, certain bacteria called cyanobacteria act as natural fertilizers by pulling nitrogen from the atmosphere and making it available to plants. They are especially important in rice paddies and other wet agricultural soils. UV-B radiation disrupts their nitrogen-fixing ability, their growth, and their survival, potentially reducing crop yields in regions that depend on these biological processes.
How Well the Protocol Is Working
The treaty has been remarkably effective. Global production of CFCs has dropped by more than 99% since its peak. NASA and NOAA confirmed in 2025 that the ozone layer remains on track for full recovery later this century. The 2025 Antarctic ozone hole was ranked as the fifth smallest since consistent monitoring began in 1992. Projections show the Antarctic ozone hole recovering around the late 2060s, with mid-latitude regions expected to recover sooner.
The recovery has not been without setbacks. In 2018, scientists detected a mysterious rise in atmospheric levels of CFC-11, one of the most potent ozone-depleting substances. Investigations by the Environmental Investigation Agency traced widespread illegal use to China’s foam insulation sector, where over 85% of surveyed companies confirmed they were still using CFC-11. Chinese authorities identified and demolished three confirmed illegal production sites and found 42 enterprises using the banned chemical. The episode exposed gaps in the treaty’s monitoring and enforcement systems, prompting China to deploy portable detectors across its provinces, install online production monitoring at chemical plants, and begin constructing atmospheric monitoring stations.
The Climate Connection: The Kigali Amendment
When CFCs were phased out, the chemicals that replaced them, hydrofluorocarbons (HFCs), solved the ozone problem but created a new one. HFCs do not destroy ozone, but they are potent greenhouse gases, trapping hundreds to thousands of times more heat than carbon dioxide per molecule. Left unchecked, HFC emissions alone were projected to add 0.28 to 0.44°C to global surface temperatures by 2100.
The Kigali Amendment, negotiated in 2016, expanded the Montreal Protocol to phase down HFC production and use. Under this agreement, countries committed to reducing HFC consumption by 80 to 85% over the coming decades. With the amendment fully implemented, HFCs are expected to contribute only about 0.04°C of warming by 2100, avoiding up to 0.4°C of additional heating. This makes the Kigali Amendment one of the single largest concrete contributions to limiting global temperature rise under existing international agreements.