Ozone is the gas most responsible for absorbing ultraviolet radiation in Earth’s atmosphere. It filters out nearly all of the most dangerous short-wavelength UV before it reaches the ground. Molecular oxygen also absorbs UV at even shorter wavelengths higher in the atmosphere, and nitrogen handles a small slice at the extreme end of the spectrum. Together, these gases act as a layered shield that removes about 95% of incoming UV-B and all UV-C from sunlight.
Ozone: The Primary UV Shield
Ozone, a molecule made of three oxygen atoms, is concentrated in the stratosphere roughly 15 to 35 kilometers above Earth’s surface. It has a strong absorption band between 200 and 360 nanometers, which covers both UV-C (100 to 280 nm) and UV-B (280 to 315 nm). This single gas is the reason UV-C radiation never reaches the ground and why only about 5% of the UV in midday sunlight is UV-B. The remaining 95% of surface-level UV is the longer-wavelength UV-A, which ozone absorbs far less effectively.
Ozone forms in the first place because of UV. High-energy solar radiation with wavelengths below 240 nm splits ordinary oxygen molecules apart in the tropical stratosphere. The freed oxygen atoms then collide with intact oxygen molecules and bond to form ozone. This cycle of creation and destruction is continuous, maintained by sunlight itself, and it keeps the stratospheric ozone layer in a rough equilibrium.
Molecular Oxygen and Nitrogen
Before sunlight even reaches the ozone layer, molecular oxygen (O₂) absorbs the shortest and most energetic ultraviolet wavelengths higher up in the atmosphere. Oxygen absorbs strongly in what physicists call the vacuum ultraviolet range, below about 200 nm, with peak absorption around 145 nm. At these wavelengths, the photon carries enough energy to break the oxygen molecule apart entirely. This process, called photodissociation, is what supplies the free oxygen atoms that go on to build ozone in the stratosphere. So oxygen does double duty: it directly blocks the most extreme UV and provides the raw material for the ozone that blocks the rest.
Nitrogen makes up 78% of the atmosphere but is essentially transparent to visible light and ordinary ultraviolet. Its absorption kicks in only at extreme ultraviolet wavelengths between 80 and 100 nm, deep in the range already handled by oxygen. Nitrogen’s contribution to UV shielding is real but limited to these very short wavelengths, which carry enormous energy per photon but represent a tiny fraction of total solar output.
Why Other Greenhouse Gases Don’t Matter for UV
Water vapor and carbon dioxide are powerful absorbers of radiation, but not ultraviolet. They absorb in the infrared range, at wavelengths between roughly 4,000 and 80,000 nm. That’s the part of the spectrum associated with heat, not the high-energy UV that causes sunburn and DNA damage. Ozone does absorb a narrow band of infrared as well (around 9,000 to 10,000 nm), making it a minor greenhouse gas, but its importance to life on Earth comes almost entirely from its UV absorption in the stratosphere.
How Photon Absorption Actually Works
When an ozone or oxygen molecule absorbs a UV photon, the energy doesn’t just disappear. If the photon carries enough energy to exceed the strength of the chemical bond holding the molecule together, that bond breaks. This is photodissociation. For oxygen, the molecule splits into two oxygen atoms. For ozone, it splits into one oxygen molecule and one free oxygen atom. In both cases the photon’s energy is converted into the kinetic energy of the fragments, which means it becomes heat in the surrounding atmosphere rather than dangerous radiation reaching the surface.
Not every absorbed photon causes a bond to break. Some molecules re-emit the energy as light (fluorescence) or transfer it to neighboring molecules through collisions. But for the electronic states involved in UV absorption by oxygen and ozone, direct dissociation dominates, with quantum yields close to 100%. This makes the process extremely efficient as a radiation shield.
What Gets Through to the Surface
About 5% of the solar radiation reaching Earth’s surface is ultraviolet. Of that, roughly 95% is UV-A (315 to 400 nm) and 5% is UV-B (280 to 315 nm). Zero UV-C makes it through. UV-A penetrates the upper layers of skin and contributes to premature aging and some skin cancers. UV-B is more energetic and is the primary cause of sunburn, cataracts, and immune suppression. UV-C, if it ever reached the surface, would be the most biologically destructive of all: it directly damages DNA and RNA by causing structural changes that prevent cells from replicating normally. This is why UV-C lamps are used to sterilize air and surfaces in hospitals.
The Ozone Layer Today
Synthetic chemicals, particularly chlorofluorocarbons, thinned the ozone layer significantly in the late 20th century, creating the annual “ozone hole” over Antarctica. Scientists define the hole as the area where ozone concentrations drop below 220 Dobson units. For perspective, 100 Dobson units corresponds to a layer of pure ozone just 1 millimeter thick, about the thickness of a dime. In 2006, concentrations over the South Pole hit a record low of 92 Dobson units.
The Montreal Protocol, which banned the worst ozone-depleting chemicals, has put recovery on track. NASA and NOAA ranked the 2025 ozone hole as the fifth smallest since 1992, with a lowest reading of 147 Dobson units on October 6. Projections show the Antarctic ozone hole recovering fully around the late 2060s. The stratospheric ozone layer elsewhere is recovering faster, which means the gas shield that absorbs ultraviolet radiation is gradually returning to its pre-industrial strength.