The ozone hole is a region over Antarctica where the concentration of ozone in the upper atmosphere drops dramatically each spring, thinning the protective layer that shields life on Earth from the sun’s most damaging ultraviolet radiation. Scientists define it as an area where ozone falls to around 100 Dobson Units, roughly a third of what’s considered normal. The hole forms, peaks, and shrinks in a seasonal cycle driven by extreme cold, unique atmospheric conditions, and human-made chemicals that have been accumulating in the stratosphere for decades.
What the Ozone Layer Actually Does
The ozone layer sits in the stratosphere, roughly 10 to 30 miles above the Earth’s surface. It’s not a distinct shell of pure ozone. It’s a region where ozone molecules are more concentrated than at other altitudes. These molecules absorb most of the sun’s UVB radiation before it reaches the ground. Without this absorption, UVB levels at the surface would be high enough to cause widespread damage to human health, ecosystems, and agriculture.
Ozone concentration is measured in Dobson Units across the entire atmospheric column from surface to space. A healthy reading over most of the planet falls between 200 and 500 Dobson Units. Over Antarctica during the worst weeks of ozone depletion, that number can drop below 150. In 2025, balloon measurements recorded a low of 147 Dobson Units on October 6.
Why It Forms Over Antarctica
The ozone hole is an Antarctic phenomenon because of a combination of geography, temperature, and atmospheric circulation that doesn’t exist anywhere else on Earth. During the Southern Hemisphere’s winter, a powerful ring of winds called the polar vortex forms over Antarctica, isolating the air above the continent from warmer air at lower latitudes. The Southern Hemisphere lacks the large mountain ranges and landmasses that disrupt atmospheric flow in the north. This means the southern polar vortex is stronger, more symmetrical, and far more persistent than its northern counterpart.
Inside that vortex, temperatures plunge below minus 78°C (195 Kelvin). At these extreme temperatures, polar stratospheric clouds form. These wispy, high-altitude clouds provide surfaces where chemical reactions convert stored chlorine compounds from inactive forms into highly reactive ones. The chlorine sits waiting through the dark polar winter. Then, as sunlight returns to Antarctica in August and September, it triggers a burst of chemical destruction that tears through the ozone layer in a matter of weeks.
How Chlorine and Bromine Destroy Ozone
The chemicals responsible for ozone destruction are chlorofluorocarbons (CFCs) and related compounds once widely used in refrigerators, air conditioners, spray cans, and foam insulation. These chemicals are extremely stable in the lower atmosphere, which is precisely the problem. They drift upward for years until they reach the stratosphere, where intense UV radiation breaks them apart and releases chlorine and bromine atoms.
A single chlorine atom can destroy thousands of ozone molecules through a repeating chain reaction. The chlorine atom strips one oxygen atom from an ozone molecule, forming chlorine monoxide and leaving behind ordinary oxygen. When that chlorine monoxide encounters a free oxygen atom, it releases the chlorine again, ready to attack another ozone molecule. Bromine atoms from other industrial chemicals work the same way, and chlorine and bromine can also team up in combined cycles that are especially efficient at destroying ozone. The net result of each cycle is the conversion of ozone into ordinary oxygen, which does nothing to block UV radiation.
How Big the Hole Gets
The ozone hole typically reaches its maximum size in September or early October, then gradually closes as the polar vortex breaks down and ozone-rich air from lower latitudes mixes back in. In 2025, it peaked on September 9 at 8.83 million square miles (22.86 million square kilometers), making it the fifth-smallest hole recorded since 1992. The 2024 season was similarly moderate, ranking as the seventh-smallest since recovery tracking began.
Year-to-year size varies considerably. Unusual weather patterns, volcanic eruptions, and temperature fluctuations in the stratosphere all influence how much ozone is destroyed in a given season. A single small year doesn’t mean recovery is ahead of schedule, just as a single large year doesn’t mean things are getting worse. Scientists look at long-term trends spanning decades.
The Montreal Protocol and Recovery
The international response to ozone depletion is one of the most successful environmental agreements in history. The Montreal Protocol, adopted on September 16, 1987, regulates the production and consumption of nearly 100 ozone-depleting chemicals. It is the only United Nations treaty to achieve universal ratification, meaning every country on Earth has signed on.
The results have been dramatic. Parties to the treaty have eliminated 98% of ozone-depleting substances globally compared to 1990 levels. The protocol has also evolved over time. In 2007, countries accelerated the phaseout of a transitional class of chemicals called HCFCs. In 2016, the Kigali Amendment extended the treaty to cover HFCs, chemicals that don’t harm ozone but are potent greenhouse gases.
Despite these reductions, ozone-depleting chemicals linger in the stratosphere for decades. NOAA and NASA scientists project the ozone layer could fully recover by 2066, with ozone levels returning to where they were in 1980. The holes over the poles are expected to disappear in the second half of this century.
Health Effects of Ozone Depletion
When the ozone layer thins, more UVB radiation reaches the surface. UVB is the wavelength responsible for sunburn, and its effects go well beyond reddened skin. It damages DNA directly, suppresses immune function, and is classified as carcinogenic to humans. In 2020, excessive UV exposure caused an estimated 1.2 million new cases of non-melanoma skin cancer, 325,000 melanomas, and roughly 121,000 premature deaths from skin cancers worldwide.
Eyes are also vulnerable. Chronic UV exposure contributes to cataracts, a clouding of the lens that leads to impaired vision and eventual blindness. Of the estimated 15 million people worldwide who are blind from cataracts, about 10% of those cases may be linked to UV radiation. Sunburns during childhood carry an outsized risk, raising the likelihood of skin cancer later in life regardless of skin tone. Fair-skinned people face the highest risk, but darker-skinned individuals develop UV-related cancers too.
Effects on Ecosystems
Increased UV radiation ripples through ecosystems in ways that go far beyond sunburn. In the ocean, UV affects the tiny photosynthetic organisms called phytoplankton that form the base of the marine food web. Different species of plankton vary in their UV sensitivity, so elevated radiation can shift the composition of these communities even when total biomass appears stable. Those shifts alter how carbon and nutrients cycle through the ocean, with consequences that propagate up through fish, seabirds, and marine mammals.
On land, UV radiation can slow plant growth, reduce crop yields, and degrade materials like plastics and paints. The regions most directly affected by the Antarctic ozone hole, including southern Chile, Argentina, New Zealand, and Australia, experience noticeably elevated UV levels during peak depletion months.
Ozone Depletion Is Not the Same as Climate Change
The ozone hole and global warming are different problems with different causes, though they share some overlap. Ozone depletion is caused by specific industrial chemicals that break down ozone molecules. Climate change is driven primarily by greenhouse gases like carbon dioxide and methane that trap heat in the lower atmosphere. The extra UV radiation that slips through the ozone hole actually cools the stratosphere more than it warms the surface, so the hole itself is not a meaningful driver of global warming.
The connection runs in the other direction too. Some of the same CFCs that destroy ozone are also greenhouse gases, so phasing them out under the Montreal Protocol has provided a modest climate benefit. The Kigali Amendment’s focus on HFCs targets chemicals chosen specifically because they warm the climate, even though they don’t touch ozone. In this way, the treaty originally designed to fix the ozone hole has quietly become one of the more effective climate agreements as well.