The ozone layer is a region of Earth’s stratosphere, roughly 15 to 35 kilometers above the surface, where ozone gas (O₃) is found in its highest concentrations. Despite the name suggesting a thick blanket, the ozone here is remarkably sparse; if you compressed it all down to sea-level pressure, it would form a band only about three millimeters thick. Yet that thin veil absorbs the majority of the sun’s ultraviolet radiation before it reaches the ground, making it one of the most consequential features of Earth’s atmosphere for life as we know it.
How Ozone Forms in the Stratosphere
Ozone in the stratosphere is created when ultraviolet light from the sun strikes ordinary oxygen molecules (O₂) and splits them into individual oxygen atoms. Those free atoms then collide with other O₂ molecules and stick together, forming O₃. This process was first described in the 1930s and is sometimes called the Chapman mechanism after the scientist who proposed it. The cycle is self-regulating in a sense: the same UV energy that creates ozone also breaks it apart, so concentrations reach a rough equilibrium.
That equilibrium, though, is not as simple as it sounds. The original Chapman reactions alone cannot explain actual ozone levels. Research has shown that those basic reactions and atmospheric air movements account for only about a fifth of the ozone produced below 45 kilometers; additional chemical cycles involving nitrogen, hydrogen, and halogen compounds play major roles in both creating and destroying ozone throughout the stratosphere.1Reviews of Geophysics. Global ozone balance in the natural stratosphere Understanding those additional cycles became critical once scientists realized humans were injecting new chemicals into the mix.
Why the Ozone Layer Matters for Health
The ozone layer’s chief service to life on Earth is filtering ultraviolet radiation, particularly the shorter-wavelength UV-B and UV-C bands. UV-C is almost entirely absorbed before reaching the ground. UV-B gets partially filtered, and the fraction that does arrive is enough to cause sunburns, DNA damage, and long-term health problems. UV-A, the longest wavelength of the three, passes through the ozone layer more easily, which is why you still need sunscreen even when the ozone layer is healthy.
When ozone thins, more UV-B reaches Earth’s surface, and the consequences are serious. Ultraviolet radiation is classified as a “complete carcinogen” because it can both initiate and promote tumor development. It is linked to all three of the most common skin cancers: basal cell carcinoma, squamous cell carcinoma, and melanoma, which together affect more than a million Americans each year.2PubMed Central. UV radiation and the skin Beyond cancer, UV exposure causes premature skin aging, cataracts, and immune suppression. Even brief exposures damage DNA extensively; cells activate repair pathways to cope, but chronic exposure overwhelms those defenses and creates the mutation burden that drives skin cancer, the most common cancer in the United States.3PubMed Central. Deciphering UV-induced DNA Damage Responses to Prevent and Treat Skin Cancer
Effects Beyond Human Health
Humans are not the only organisms that suffer when UV-B increases. Phytoplankton, the tiny photosynthetic organisms that form the base of nearly every ocean food web, are highly sensitive to short-wavelength UV. Because they need sunlight, they live near the surface of the water column, right where UV hits hardest. Research has shown that even ambient levels of UV-B stress many phytoplankton species, reducing their growth, photosynthesis, nitrogen uptake, and enzyme activity.4PubMed. Effects of solar UV-B radiation on aquatic ecosystems Since phytoplankton also generate a large share of the planet’s oxygen and absorb carbon dioxide, damage to these organisms ripples outward through both marine ecosystems and the global carbon cycle.
Terrestrial plants also sustain UV damage, though many have evolved protective pigments. Amphibians, whose eggs often develop in shallow, sun-exposed water, are considered particularly vulnerable. The broader point is that the ozone layer is not just a human health shield; it is an ecological one, and thinning it raises biological stress across a wide range of species.
Good Ozone Versus Bad Ozone
One of the more confusing aspects of ozone is that the same molecule acts as a protector high up and a pollutant down low. In the stratosphere, ozone absorbs UV and keeps life safe. Near the ground, in the troposphere, ozone forms when nitrogen oxides and volatile organic compounds from tailpipes, power plants, and industrial processes react in sunlight. This ground-level ozone is a major component of smog.
Tropospheric ozone is a powerful oxidant that damages lung tissue, triggers inflammatory responses in the airways, and has been associated with both increased hospital admissions and higher mortality.5PubMed Central. Ozone Pollution: A Major Health Hazard Worldwide The chemistry that creates ground-level ozone is entirely separate from what happens in the stratosphere, so the two problems cannot cancel each other out. You cannot pipe surplus smog ozone upward to patch the ozone layer, and thinning the stratospheric layer does not reduce smog. They are the same molecule in two different places, doing two very different things.
How Human-Made Chemicals Deplete the Ozone Layer
The ozone layer’s biggest modern threat came from a family of industrial chemicals collectively called ozone-depleting substances, or ODSs. The most notorious are chlorofluorocarbons, better known as CFCs, which were once widely used in refrigerators, aerosol sprays, and foam insulation. CFCs are remarkably stable at ground level, which is exactly the problem: they drift intact into the stratosphere, where intense UV light finally breaks them apart and releases chlorine atoms.
A single chlorine atom can destroy thousands of ozone molecules through a catalytic cycle. It reacts with ozone to form chlorine monoxide and ordinary oxygen, and the chlorine monoxide then reacts with another oxygen atom to regenerate the free chlorine, which goes on to destroy more ozone. Laboratory measurements have confirmed that the key step in this cycle, the production of chlorine atoms from the breakdown of chlorine peroxide (ClOOCl), proceeds efficiently under stratospheric conditions.6PubMed. Chlorine-catalyzed ozone destruction: Cl atom production from ClOOCl photolysis
Bromine plays a similar role. Compounds like halons, once standard in fire extinguishers, release bromine in the stratosphere. Atom for atom, bromine is even more destructive to ozone than chlorine. Future changes in the stratospheric background of these halogen compounds will continue to influence how fast ozone is destroyed, because the rates of both bromine- and chlorine-mediated destruction depend on temperature and on how the reservoir chemicals partition between active and inactive forms.7Atmospheric Chemistry and Physics. Reformulating the bromine alpha factor and equivalent effective stratospheric chlorine (EESC): evolution of ozone destruction rates of bromine and chlorine in future climate scenarios
The Antarctic Ozone Hole and Polar Stratospheric Clouds
The most dramatic manifestation of ozone depletion is the Antarctic ozone hole, a region of severely thinned ozone that forms every southern spring (roughly September through November). The hole is not a literal gap; it is a zone where ozone concentrations drop far below normal. It forms over Antarctica rather than uniformly around the globe because of a specific set of conditions that come together at the poles.
During the long, dark polar winter, the stratosphere over Antarctica gets extraordinarily cold, cold enough for ice and nitric acid crystals to form in midair. These are polar stratospheric clouds, or PSCs, and they are central to the ozone hole’s chemistry. PSC particles provide surfaces where stable chlorine reservoir compounds are converted into reactive forms that destroy ozone catalytically once sunlight returns in spring.8Reviews of Geophysics. Polar Stratospheric Clouds: Satellite Observations, Processes, and Role in Ozone Depletion The reaction of hydrochloric acid with chlorine nitrate on these cloud particles releases molecular chlorine, which sunlight then splits into the destructive chlorine atoms.9PubMed. Chlorine chemistry on polar stratospheric cloud particles in the arctic winter
PSCs do double duty: they also remove gaseous nitric acid through sedimentation, which delays the process by which active chlorine gets locked back into harmless reservoir species. This extends the window of ozone destruction well into spring. The Arctic sees a milder version of this process because its polar vortex is less stable and its stratosphere does not get quite as cold, but significant Arctic ozone loss has been observed in especially cold winters.
Recovery Under the Montreal Protocol
The Montreal Protocol, signed in 1987, is widely regarded as one of the most successful international environmental agreements ever enacted. It phased out the production and consumption of CFCs, halons, and other ODSs. Because these chemicals linger in the atmosphere for decades, the effects were not instant, but the trajectory is encouraging.
Ozone in the upper stratosphere has been rising since about 1998. Analysis across multiple satellite datasets shows that near-global total ozone (between 60°S and 60°N) is recovering at roughly 0.4% per decade since 1995, compared to a decline of about 1.5% per decade before 1995. That ratio of decline to recovery closely matches the change in stratospheric halogen loading, confirming that the Montreal Protocol is the driving force behind the turnaround.10Atmospheric Chemistry and Physics. Global total ozone recovery trends attributed to ozone-depleting substance (ODS) changes derived from five merged ozone datasets
The picture is not entirely rosy, however. While upper stratospheric ozone is clearly rebounding, satellite data indicate that ozone in the lower stratosphere between 60°S and 60°N has continued to decline since 1998. This ongoing lower-stratospheric loss partially offsets the upper-stratospheric recovery, which is why total column ozone at non-polar latitudes has stopped declining but has not yet shown a clear upward trend.11Atmospheric Chemistry and Physics. Evidence for a continuous decline in lower stratospheric ozone offsetting ozone layer recovery The reasons for this lower-stratospheric decline are still being investigated; possible contributors include changes in atmospheric circulation patterns driven by climate change, and short-lived chemicals not fully regulated by the Montreal Protocol.
How Climate Change and the Ozone Layer Interact
Climate change and ozone depletion are often discussed as separate environmental problems, but they are deeply entangled. Greenhouse gases warm the surface but cool the stratosphere. That cooling has a counterintuitive benefit for ozone: it weakens the catalytic cycles that destroy ozone, because those reactions run slower at lower temperatures. Modeling studies show that greenhouse-gas-induced stratospheric cooling could cause upper-stratospheric ozone to return to pre-1980 levels several decades before the ozone-depleting chemicals themselves have fully washed out of the atmosphere.12Geophysical Research Letters. Impacts of climate change on stratospheric ozone recovery In other words, some of the apparent ozone recovery is being helped along by climate change, not just by declining halogen levels.
The interaction runs the other way too. The ozone hole has influenced climate patterns across the entire Southern Hemisphere. The loss of stratospheric ozone over Antarctica altered temperature gradients in the atmosphere, shifting the Southern Hemisphere’s westerly jet stream poleward and affecting rainfall patterns from the subtropics to the Antarctic coast.13Geophysical Research Letters. Ozone hole and Southern Hemisphere climate change As the ozone layer recovers, climate models project that this jet-stream shift will slow or partially reverse, potentially decelerating some aspects of Southern Hemisphere climate change even as greenhouse gas concentrations continue to rise.14PubMed. The impact of stratospheric ozone recovery on the Southern Hemisphere westerly jet
Wildfires and Volcanic Eruptions as Wild Cards
Even as human-made chemicals decline, natural events can temporarily set back ozone recovery. The massive Australian wildfires of 2019–2020 injected enormous amounts of smoke into the stratosphere. That smoke enhanced ozone depletion at high and mid-latitudes, reducing minimum total ozone near 70°S by up to about 30 Dobson Units in 2020 and 2021, with additional losses from smoke-filled vortices bringing the total reduction to roughly 45 Dobson Units.15Atmospheric Chemistry and Physics. Radiative forcing and stratospheric ozone changes due to major forest fires and recent volcanic eruptions including Hunga Tonga For context, the pre-depletion ozone column over Antarctica is on the order of 300 Dobson Units, so 45 DU is a meaningful dip.
Wildfire smoke appears to damage ozone in two ways. Below the altitude where polar stratospheric clouds form, smoke particle surfaces host the same kinds of heterogeneous reactions that volcanic sulfate aerosols do, directly destroying ozone. Within the PSC zone, smoke particles can slightly increase the number and surface area of PSC particles, amplifying the chlorine-activation process that drives the ozone hole.16Atmospheric Chemistry and Physics. Ozone depletion in the Arctic and Antarctic stratosphere induced by wildfire smoke The 2022 eruption of the Hunga Tonga–Hunga Haʻapai volcano similarly reduced ozone near 70°S by up to about 15 Dobson Units.17Atmospheric Chemistry and Physics. Radiative forcing and stratospheric ozone changes due to major forest fires and recent volcanic eruptions including Hunga Tonga As climate change makes large wildfires more frequent, this smoke-driven ozone loss could become a recurring complication for recovery projections.
Geoengineering and the Ozone Layer
Some proposals to combat global warming involve injecting sulfate aerosols into the stratosphere to reflect sunlight and cool the planet, a concept called solar radiation management (SRM). The approach would deliberately mimic the cooling effect of large volcanic eruptions. But there is a catch: those same sulfate aerosols could damage the ozone layer.
Sulfate geoengineering would introduce particle surfaces in the stratosphere that promote heterogeneous chemical reactions, much the way polar stratospheric clouds do. This could increase ozone depletion at mid- and high latitudes, reducing the net transport of stratospheric ozone into the troposphere.18Atmospheric Chemistry and Physics. Impacts of stratospheric sulfate geoengineering on tropospheric ozone It would also heat the lower tropical stratosphere, increasing water vapor concentrations there, which causes further ozone loss.19PubMed Central. Stratospheric solar geoengineering without ozone loss Researchers have explored alternative aerosol materials, such as calcite or alumina, that might achieve the desired cooling without triggering as much ozone chemistry, but these remain theoretical.
One modeling study found that under a specific SRM scenario, stratospheric ozone actually increased overall, which sounds like a benefit until you follow the chain of consequences: higher stratospheric ozone meant less UV-B reaching the surface (potentially cutting into vitamin D production) and also meant decreased ozone breakdown in the troposphere, leading to higher ground-level ozone concentrations, the “bad” ozone that harms lungs.20Atmospheric Chemistry and Physics. Stratospheric ozone changes under solar geoengineering: implications for UV exposure and air quality The takeaway is that deliberately manipulating the stratosphere carries ozone-related side effects that range from straightforwardly harmful to unexpectedly complicated.
The CFC Replacements Have Their Own Problems
When CFCs were phased out, industry switched first to hydrochlorofluorocarbons (HCFCs), which are less damaging to ozone but still contribute some, and then to hydrofluorocarbons (HFCs), which do not harm ozone at all. The problem is that HFCs are potent greenhouse gases. The cumulative global-warming-potential-weighted emissions of HCFCs and HFCs from 2022 through 2060 are projected to be roughly 6.4 and 14.8 billion tons of COâ‚‚-equivalent, respectively, under a business-as-usual scenario.21Nature Communications. Rethinking time-lagged emissions and abatement potential of fluorocarbons in the post-Kigali Amendment era The 2016 Kigali Amendment to the Montreal Protocol targets HFC reductions, and analysis suggests that adopting best environmental practices in developed economies could cut those cumulative emissions by up to 45% compared to business as usual.22Nature Communications. Rethinking time-lagged emissions and abatement potential of fluorocarbons in the post-Kigali Amendment era Still, a large portion of the problem comes from “banks,” meaning refrigerant and foam products already manufactured and sitting in buildings, vehicles, and warehouses, slowly leaking their contents into the atmosphere over the coming decades.
Ozone as a Sign of Life on Other Planets
Beyond its importance to Earth, ozone has become a key target in the search for life on planets orbiting other stars. Oxygen and ozone are considered leading biosignature gases for rocky exoplanets around sun-like stars, because on Earth, a stable ozone layer is maintained by biologically produced oxygen. Detecting ozone in an exoplanet’s atmosphere, especially at low concentrations corresponding to levels thought to have existed during Earth’s early history, would be a strong hint that biological activity is present. Researchers have shown that ultraviolet spectroscopy is a promising way to detect ozone at these low levels, making the UV band a crucial observation window for future space telescopes.23The Astronomical Journal. Reflected Spectroscopy of Small Exoplanets. III. Probing the UV Band to Measure Biosignature Gases
Earth’s own ozone layer may not have always been as robust as it is today. Modeling work suggests that higher concentrations of marine iodide in ancient oceans could have severely suppressed ozone even when oxygen levels were moderately high. At ten percent of modern oxygen levels, a tenfold increase in marine iodide would collapse the ozone column from about 242 Dobson Units to around 5 DU, boosting surface UV-B by roughly fivefold and increasing the most damaging short-wavelength UV exponentially.24PubMed Central. Evolution of the iodine cycle and the late stabilization of the Earth’s ozone layer The implication is that Earth’s ozone shield may have stabilized late in planetary history, and its emergence could have been a prerequisite for complex life colonizing land. The iodine cycle, something rarely discussed outside geochemistry circles, turns out to be a hidden variable in the story of why life stayed in the oceans for so long.