Ultraviolet (UV) light is a type of energy on the electromagnetic spectrum with wavelengths between 100 and 400 nanometers, sitting just beyond the violet end of visible light. Your eyes can’t detect it, but your skin, your DNA, and many materials respond to it powerfully. The sun is the primary natural source, though UV light is also produced artificially for everything from sterilization to curing nail polish.
Where UV Fits on the Spectrum
Light travels in waves, and the length of those waves determines what kind of energy it carries. Visible light, the rainbow of colors you can see, spans roughly 400 to 700 nanometers. UV light picks up right where violet ends, ranging from 400 nanometers down to 100. Below 100 nanometers, you enter the territory of X-rays. The shorter the wavelength, the more energy each photon carries, which is why UV light can do things visible light cannot: break chemical bonds, damage DNA, and trigger biological reactions in living tissue.
The Three Types: UVA, UVB, and UVC
UV light is divided into three bands based on wavelength, and each behaves very differently.
UVA (315 to 400 nm) is the longest-wavelength, lowest-energy UV. It penetrates deep into the skin, reaching the middle layers where it contributes to premature aging, wrinkles, and some skin cancer risk. About 95% of the UV radiation that reaches Earth’s surface is UVA, because the atmosphere doesn’t filter it effectively.
UVB (280 to 315 nm) carries more energy per photon. It’s the primary cause of sunburn and plays the central role in skin cancer development. It also triggers your body’s production of vitamin D. Most solar UVB is absorbed by the ozone layer, water vapor, and oxygen before it reaches the ground, so it makes up only about 5% of surface UV.
UVC (100 to 280 nm) is the highest-energy type and the most dangerous to living cells. It never reaches Earth’s surface under normal conditions because the atmosphere absorbs it completely. However, it can be produced artificially and is widely used in germicidal lamps for sterilizing water, air, and surfaces.
How UV Light Affects Your DNA
When UVB photons hit a skin cell, they can be absorbed directly by the DNA inside. This energy forces adjacent building blocks in a DNA strand to fuse together, creating defects called pyrimidine dimers. Think of it like two links in a chain getting welded to each other: the chain still exists, but it can’t move or function the way it should. These fused spots distort the DNA’s structure, blocking normal copying and reading of genetic instructions.
Your cells have a built-in repair system that snips out damaged sections and patches them. In humans, though, this repair process is slow and error-prone. When damage accumulates faster than cells can fix it, or when repairs introduce mistakes, mutations build up. Over years, those mutations can push cells toward uncontrolled growth, which is how UV exposure leads to skin cancer.
How UV Light Triggers Tanning
A tan is your body’s defense mechanism, not a sign of health. When UV radiation hits the skin, it triggers a chain of chemical signals in pigment-producing cells called melanocytes. UV exposure activates a signaling molecule that tells these cells to ramp up production of melanin, the dark pigment that absorbs UV energy before it can reach deeper tissue. UVB in particular switches melanocytes toward producing the darker, more protective form of melanin rather than the lighter, reddish form. The result is a gradual darkening of the skin over hours to days, offering modest built-in sun protection. But this natural shield is limited; a tan provides roughly the equivalent of SPF 3 to 4, nowhere near enough to prevent damage from prolonged exposure.
Vitamin D Production
UVB light is the trigger for vitamin D synthesis in your skin. When UVB photons in the 290 to 310 nanometer range strike the skin, they break open a cholesterol-related molecule sitting in the outer skin layers. This broken molecule, called pre-vitamin D3, then rearranges itself due to body heat into vitamin D3, which enters the bloodstream and eventually gets converted by the liver and kidneys into the active hormone your body uses for calcium absorption and immune function.
There’s a natural ceiling on this process. With continued UV exposure, pre-vitamin D3 gets converted into inactive byproducts instead of more vitamin D3. These byproducts can revert back to pre-vitamin D3 once you’re out of the sun, but the system prevents toxic overproduction. This is why you can’t overdose on vitamin D from sunlight alone, though you can certainly burn your skin in the attempt to make more.
UV Damage to the Eyes
Skin isn’t the only tissue at risk. Acute UV exposure to unprotected eyes causes a condition called photokeratitis, sometimes known as snow blindness or welder’s arc. Symptoms include tearing, redness, pain, sensitivity to light, swollen eyelids, blurred vision, and sometimes temporary vision loss. It typically appears within six hours of exposure and resolves within 48 hours without lasting damage. The biological damage potential drops steeply with wavelength: UV at 300 nm is roughly 600 times more damaging to the cornea than UV at 325 nm, which is why even small shifts in wavelength matter enormously for eye safety.
Chronic, repeated UV exposure to the eyes over years increases the risk of cataracts and growths on the surface of the eye. Sunglasses labeled as blocking 99 to 100% of UVA and UVB are the most effective everyday protection.
How UV Is Used in Sterilization
UVC light is lethal to bacteria, viruses, and other pathogens because it destroys their genetic material. The most effective germicidal wavelength peaks around 260 to 265 nanometers, where DNA absorbs UV energy most efficiently. At these wavelengths, photons break bonds in DNA and RNA, preventing microorganisms from replicating. This makes UVC a chemical-free alternative for disinfecting drinking water, hospital surfaces, and food processing equipment. Since the sun’s UVC never reaches the ground, germicidal UVC is produced artificially using specialized lamps and, increasingly, LEDs tuned to precise wavelengths.
The UV Index and Sun Protection
The UV Index is a standardized scale that tells you how strong the sun’s UV radiation is at a given time and place. It runs from 0 upward, with higher numbers meaning faster potential for skin and eye damage.
- 0 to 2: Low risk. No special precautions needed for most people, including those with fair skin.
- 3 to 7: Moderate to high risk. Shade during midday, sun-protective clothing, and sunscreen are recommended.
- 8 and above: Very high to extreme risk. Avoid midday sun if possible, and use full protection: clothing, hat, sunscreen, and sunglasses.
The UV Index is reported in most weather forecasts and weather apps. It peaks around solar noon, not at the hottest part of the day, because it measures radiation intensity rather than air temperature. Cloud cover reduces UV but doesn’t eliminate it; thin clouds can still let through a significant percentage.
How Sunscreen Blocks UV
Sunscreen’s SPF number tells you how much UVB it filters. The relationship isn’t linear: SPF 15 absorbs 93.3% of UVB, SPF 30 absorbs 96.7%, and SPF 50 absorbs 98%. The jump from SPF 30 to 50 sounds large but adds only 1.3 percentage points of protection. SPF also says nothing about UVA. For UVA coverage, look for products labeled “broad spectrum,” which means they’ve been tested against both UVA and UVB.
No sunscreen blocks 100% of UV, and real-world protection depends heavily on how much you apply and how often you reapply. Most people use about a quarter to a half of the recommended amount, which dramatically reduces the effective SPF on their skin.