The ozone layer primarily blocks ultraviolet (UV) radiation from the sun, not visible light. It absorbs 100% of the most dangerous UV-C rays, roughly 95% of UV-B rays, and only about 5% of UV-A rays. Visible light, the spectrum you can actually see, passes through almost entirely.
Three Types of UV Radiation
The sun emits ultraviolet light across a wide range of wavelengths, and scientists divide it into three bands based on energy level. UV-C (wavelengths below 280 nanometers) is the most energetic and the most dangerous to living things. UV-B (280 to 320 nanometers) is moderately energetic and responsible for sunburns. UV-A (320 to 400 nanometers) is the least energetic of the three but still contributes to skin aging and some skin cancers.
The ozone layer treats each of these bands very differently. UV-C never reaches the Earth’s surface. It is completely absorbed in the upper atmosphere, first by ordinary oxygen molecules and then by ozone in the stratosphere. UV-B is mostly absorbed, with ozone filtering out about 95% of it. The remaining 5% that slips through is what causes sunburn and increases the risk of skin cancer and eye damage. UV-A is barely filtered at all. Ozone removes only about 5% of UV-A, so the vast majority reaches the ground.
How Ozone Actually Absorbs UV Light
The process starts with ordinary oxygen. High in the atmosphere, UV-C radiation is energetic enough to split an oxygen molecule (two oxygen atoms bonded together) into two separate atoms. Each of those free atoms then bonds with another oxygen molecule to form ozone, which is three oxygen atoms linked together. This ozone sits in a thin layer between about 10 and 50 kilometers above the Earth’s surface. If you compressed it all down to ground-level pressure, it would only be about 3 millimeters thick.
That thin layer is remarkably effective. When UV-B photons hit an ozone molecule, they break it apart, releasing energy as heat. The freed oxygen atoms then recombine with other oxygen molecules to form ozone again, restarting the cycle. This constant breaking and reforming is what converts harmful UV energy into harmless heat, preventing it from reaching the surface. The absorption is strongest at shorter UV-B wavelengths (closer to 280 nm) and weakens as wavelengths approach 320 nm, which is why some UV-B still gets through.
What About Visible Light and Infrared?
Ozone does interact with light outside the UV spectrum, but the effect is negligible for everyday purposes. It has a weak absorption band in visible light around 610 nanometers (orange-red wavelengths), known as the Chappuis bands, and it absorbs some infrared radiation in the 5 and 10 micrometer ranges. These absorptions matter to atmospheric scientists who use them to measure ozone concentrations from satellites, but they don’t meaningfully reduce the visible light or heat energy reaching the ground. The sunlight you see and the warmth you feel pass through the ozone layer essentially unfiltered.
Why UV-B Matters Most for Health
UV-C is more dangerous than UV-B in theory, but since the ozone layer blocks it completely, it poses no natural threat at ground level. UV-B is the critical band because the small percentage that gets through is potent enough to directly damage DNA in skin and eye cells. When UV-B photons hit DNA, they cause specific types of lesions that distort the DNA strand. The body has repair mechanisms to fix this damage, but those systems sometimes fail, which is how repeated UV-B exposure leads to mutations, sunburns, and eventually skin cancer.
The eyes are particularly vulnerable. UV-B is absorbed by the cornea, damaging the outermost cell layer. Over time, this contributes to conditions like pterygium (a growth on the eye’s surface) and corneal clouding. An estimated 5 to 10% of all skin cancers occur on the eyelids, one of the areas most consistently exposed to sunlight.
Location and Season Change Your Exposure
The ozone layer is not uniformly thick everywhere. Its concentration varies with latitude, season, and even the solar cycle. The most dramatic thinning occurs over Antarctica each spring (September through October), where a seasonal “hole” forms. In 2024, this depleted zone averaged nearly 20 million square kilometers, roughly three times the size of the contiguous United States. During its peak on September 28, it stretched to 22.4 million square kilometers.
Ozone depletion is not limited to Antarctica. Measurable thinning has been documented over latitudes covering North America, Europe, Asia, Australia, and much of Africa and South America. Thinner ozone means more UV-B reaches the surface, which is why UV index readings can vary so much by location and time of year. Large volcanic eruptions also play a role: the tiny particles they inject into the stratosphere create surfaces where ozone-destroying chemicals work more efficiently.
The Ozone Layer’s Recovery Timeline
The Montreal Protocol, which took effect in 1992, banned the chemicals most responsible for destroying ozone. Since then, the ozone hole has been slowly shrinking. The 2024 hole ranked as the seventh smallest since recovery began. NASA and NOAA scientists project the ozone layer could fully recover by 2066, though the current state is still far from healthy. In October 2024, ozone concentration over Antarctica dropped to 109 Dobson units, well below the 225 Dobson units that was typical in 1979 before significant depletion began. The lowest value ever recorded was 92 Dobson units in October 2006.
Until full recovery, the amount of UV-B reaching the surface in affected regions remains elevated compared to pre-depletion levels, particularly during spring and summer months at higher latitudes.