The ozone layer is primarily damaged by human-made chemicals that release chlorine and bromine atoms into the upper atmosphere. These atoms act as catalysts, breaking apart ozone molecules in a chain reaction where a single chlorine atom can destroy thousands of ozone molecules before it’s deactivated. While the most notorious offenders, chlorofluorocarbons (CFCs), have been largely phased out since the late 1980s, several other sources of damage persist, and some new threats are emerging.
How Ozone Gets Destroyed
Ozone-depleting chemicals are remarkably stable in the lower atmosphere. They can drift upward for years without breaking down. Once they reach the stratosphere, roughly 10 to 30 miles above Earth’s surface, intense ultraviolet light splits them apart, releasing chlorine or bromine atoms. These atoms then attack ozone in a repeating cycle: a chlorine atom strips one oxygen atom from an ozone molecule, forming ordinary oxygen and chlorine monoxide. The chlorine monoxide then reacts with another oxygen atom, releasing the chlorine to go destroy yet another ozone molecule. The chlorine is never consumed in the process, which is what makes it so destructive.
Over Antarctica, where extreme cold creates special ice clouds in the stratosphere, the chemistry is even more aggressive. Two chlorine monoxide molecules can react with each other, and sunlight splits the resulting compound to release chlorine atoms that attack ozone directly. A similar cycle involves both chlorine and bromine working together. These polar reactions are why the ozone “hole” forms over Antarctica each spring.
CFCs: The Biggest Historical Culprit
Chlorofluorocarbons were once used everywhere. They served as refrigerants in air conditioners and refrigerators, propellants in aerosol spray cans, blowing agents for foam insulation and packaging, and industrial solvents. The most common varieties, CFC-11 and CFC-12, were considered miracle chemicals when introduced because they were nontoxic, nonflammable, and chemically inert at ground level. That very stability turned out to be the problem: nothing in the lower atmosphere broke them down, so they accumulated and eventually reached the stratosphere in enormous quantities.
The Montreal Protocol, signed in 1987, banned CFC production in most countries. But because these chemicals last 50 to 100 years in the atmosphere, the ones already released are still up there doing damage. And compliance hasn’t been perfect. In 2018, NOAA scientists detected an unexpected 25 percent jump in global CFC-11 emissions, eventually tracing 40 to 60 percent of the increase to factories in eastern China. After international pressure and renewed enforcement by the Chinese government, emissions from the region dropped by a third, returning to pre-2013 levels by 2019. NOAA scientist Stephen Montzka called it “a major test of the Montreal Protocol,” one that the treaty appears to have passed.
Halons and Fire Suppression
Halons are bromine-containing compounds that were widely used in fire extinguishers, particularly in aircraft, computer rooms, and military equipment. Bromine is roughly 40 to 60 times more effective at destroying ozone than chlorine on a per-atom basis. Like CFCs, halons are classified as Class I ozone-depleting substances and have been phased out under the Montreal Protocol, though existing stockpiles can still be used in some critical applications.
Methyl Bromide in Agriculture
Methyl bromide was a go-to pesticide for decades. Farmers injected it into soil before planting to kill insects, fungi, and weeds, and it was also used to fumigate grain storage facilities and shipping containers. As a Class I ozone-depleting substance, it was phased out for most uses by January 2005. However, exemptions remain. Growers can still obtain “critical use” exemptions when no viable alternative exists, and methyl bromide is still permitted for quarantine and preshipment treatments, such as fumigating imported goods to prevent pest spread across borders.
Nitrous Oxide: The Overlooked Threat
Nitrous oxide doesn’t get the same attention as CFCs, but it is now the single largest ozone-depleting substance being emitted by human activity. Its ozone-depleting potential per molecule is lower than that of CFCs, but the sheer volume of emissions makes it significant. Most of it comes from agriculture, specifically from nitrogen-based fertilizers and livestock manure, with additional contributions from industrial processes and fossil fuel combustion.
Because nitrous oxide is not regulated under the Montreal Protocol, its emissions have continued to climb even as CFC emissions have fallen. Research published in the Philosophical Transactions of the Royal Society found that eliminating human-caused nitrous oxide emissions would have a larger protective effect on the ozone layer than eliminating all remaining unregulated halocarbon emissions combined. This makes agricultural practices and fertilizer management an underappreciated factor in ozone recovery.
Unregulated Short-Lived Chemicals
A newer concern involves chemicals called very short-lived substances, or VSLS. These compounds break down in the lower atmosphere within weeks or months, which is why they were initially left out of the Montreal Protocol. But research now shows they can reach the stratosphere fast enough to cause real damage, particularly in tropical regions where strong storms loft air upward quickly.
Dichloromethane, an industrial solvent used in paint strippers, adhesives, and pharmaceutical manufacturing, is the most abundant of these. Its emissions have been rising rapidly, especially from Asia. Chloroform emissions have also grown. A study in Nature Climate Change estimated that these short-lived halogenated chemicals account for roughly a quarter of the observed ozone decline in the tropical lower stratosphere between 1998 and 2018. By 2017, they were injecting an estimated 111 parts per trillion of chlorine into the stratosphere. That’s a small number compared to peak CFC levels, but it’s growing at a time when the ozone layer is supposed to be healing.
Wildfires as an Emerging Factor
Australia’s “Black Summer” megafires of 2019 to 2020 revealed a surprising new mechanism for ozone loss. The fires burned tens of millions of acres and pumped more than one million tons of smoke into the atmosphere, some of it reaching the stratosphere. Researchers found that the surface of smoke particles provided a platform for chlorine-containing compounds, remnants of past CFC emissions, to undergo chemical reactions that produced chlorine monoxide, the same molecule responsible for ozone destruction over Antarctica.
The impact was measurable. The fires likely caused a 3 to 5 percent depletion of total ozone at mid-latitudes across the Southern Hemisphere, affecting skies over Australia, New Zealand, and parts of Africa and South America. By late 2020, the smoke had widened the Antarctic ozone hole by 2.5 million square kilometers, a 10 percent increase in area compared to the previous year. As wildfire seasons grow more intense with climate change, this pathway could become an increasingly important source of ozone damage.
Volcanic Eruptions and Stratospheric Water
The January 2022 eruption of Hunga Tonga, an underwater volcano in the Pacific, was unlike most volcanic events. Instead of mainly ejecting sulfur and ash, it blasted an unprecedented amount of water vapor into the stratosphere, roughly 10 percent of the total global stratospheric water supply, reaching altitudes up to 55 kilometers. That water vapor cooled the stratosphere and increased humidity, enabling chemical reactions on volcanic aerosol particles that normally only happen in the frigid polar regions.
These reactions converted inactive chlorine compounds into chlorine monoxide, which then destroyed ozone. Scientists measured a clear chemical signature: hydrogen chloride dropped by 0.4 parts per billion while chlorine monoxide increased by the same amount, direct evidence of chlorine activation. The finding raised a broader concern: if climate change increases the amount of water vapor reaching the stratosphere through stronger storms and convection, similar ozone-depleting chemistry could become more common even without volcanic eruptions.
How Climate Change Complicates Recovery
The relationship between climate change and the ozone layer runs in both directions. Greenhouse gases warm the lower atmosphere but cool the stratosphere. That cooling matters because colder stratospheric temperatures promote the formation of polar stratospheric clouds, the icy surfaces where the most aggressive ozone-destroying reactions take place. Arctic stratospheric temperatures already hover close to the threshold for forming these clouds, so even modest additional cooling could trigger more frequent and severe ozone losses over the Arctic.
There’s also a feedback loop at work. Less ozone means a colder stratosphere, and a colder stratosphere means more ozone destruction. NASA researchers have explored whether this cooling could be fast enough to delay ozone recovery beyond what current projections suggest.
Where Recovery Stands
Despite these complicating factors, the ozone layer is healing. NASA and NOAA project a full recovery by around 2066, with mid-latitude regions expected to recover sooner than the polar regions. The Montreal Protocol has successfully eliminated most production of CFCs and halons, and the atmospheric concentrations of these chemicals are slowly declining. But the recovery timeline depends on continued compliance with the treaty, the trajectory of unregulated chemicals like dichloromethane and nitrous oxide, and how much climate change reshapes stratospheric conditions in the decades ahead.