Ultraviolet (UV) radiation is a type of light energy with wavelengths shorter than visible light, ranging from 100 to 400 nanometers. It works by transferring enough energy to break or rearrange chemical bonds in the molecules it strikes. That single mechanism drives everything UV does: triggering sunburns, killing bacteria, degrading plastics, producing vitamin D in your skin, and making certain materials glow.
The Three Types of UV
UV radiation splits into three bands based on wavelength, and each behaves differently because shorter wavelengths carry more energy per photon.
- UVA (315–400 nm) is the longest-wavelength, lowest-energy UV. It passes through the ozone layer completely unfiltered and penetrates deep into your skin. UVA is the primary driver of skin aging and contributes to cancer risk over time.
- UVB (280–315 nm) carries more energy per photon. The ozone layer absorbs most of it, but enough reaches the surface to cause sunburn and trigger vitamin D production. UVB is the main concern in sun exposure.
- UVC (100–280 nm) is the most energetic and most destructive to biological material. It never reaches the Earth’s surface naturally because ozone and ordinary oxygen absorb it completely. UVC is the type used in germicidal lamps.
How UV Damages DNA
The core mechanism behind UV’s biological effects is straightforward. UV photons are absorbed by specific chemical bonds in DNA, particularly in the bases thymine and cytosine. The added energy forces open a bond in the base, allowing it to react with a neighboring base and form an abnormal connection called a dimer. Two thymine bases, for instance, can become glued together by two new covalent bonds, creating a tight thymine dimer that distorts the DNA strand.
This distortion is the root of the problem. Cells have repair systems that can fix small numbers of dimers, but when the damage overwhelms those systems, the cell either dies or replicates with errors. In microbes, that’s the goal. In your skin cells, it’s the beginning of a sunburn or, with accumulated damage over years, skin cancer.
What Happens When You Get a Sunburn
Sunburn is an inflammatory response triggered primarily by UVB radiation (280–320 nm). When UV photons damage enough skin cells, the process unfolds in overlapping stages. First, blood vessels near the skin’s surface dilate, producing the initial redness and warmth. Then a full inflammatory cascade kicks in as damaged cells release signaling molecules that recruit immune cells to the area, intensifying the redness, swelling, and pain. Finally, the inflammation gradually resolves as anti-inflammatory processes take over.
The redness you see is not the damage itself. It’s your immune system responding to cells that have been injured or killed. The peeling that follows a bad sunburn is your body shedding cells too damaged to repair.
How UV Produces Vitamin D
Your skin contains a cholesterol-related compound that acts as the raw material for vitamin D3. When UVB photons in the 290–310 nm range hit this molecule, they break one of its ring structures, creating an intermediate form. That intermediate slowly converts into vitamin D3 at body temperature, a process that continues for hours after you’ve gone indoors.
This system has a built-in safety limit. With continued UV exposure, the intermediate gets converted into inactive byproducts instead of more vitamin D3. Those byproducts can slowly revert back to the useful intermediate form in the dark. The result is that short sun exposures lead to prolonged, steady vitamin D3 production, while longer exposures don’t keep increasing output. Your body, in effect, self-regulates to prevent vitamin D overdose from sunlight.
How UV Kills Germs
UV disinfection relies on the same DNA-damage mechanism, scaled up deliberately. UVC lamps, typically emitting light around 254 nm, bombard microorganisms with enough energy to create so many thymine dimers that the organism can’t replicate or survive.
Different pathogens require different amounts of UV energy. Common bacteria like E. coli are relatively easy to kill, requiring only about 5–10 millijoules per square centimeter of UV exposure to reduce their numbers by 99.99%. Cryptosporidium, a parasite notoriously resistant to chlorine, is also surprisingly vulnerable to UV, needing less than 10 millijoules per square centimeter for significant inactivation. Viruses vary widely: rotavirus needs around 36 millijoules for a 99.99% reduction, while adenovirus is much tougher, requiring roughly 110 millijoules at the same level.
This is why UV disinfection has become standard in water treatment, hospital air purification, and food processing. It leaves no chemical residue and works against pathogens that resist chemical disinfectants.
How UV Degrades Materials
UV doesn’t just damage living things. The same bond-breaking energy attacks synthetic materials like plastics, rubber, and fabrics. When UV photons hit polymer chains, they can break the carbon-to-carbon bonds that form the backbone of the material. This creates unstable fragments called free radicals, which react with oxygen in the air and trigger a chain reaction of further breakdown.
The result is familiar to anyone who’s left a plastic chair outside for a few years: the material becomes brittle, fades in color, and eventually cracks apart. All commercial organic polymers degrade in air when exposed to sunlight, because the energy in UV radiation is sufficient to break those backbone bonds. Manufacturers add UV-stabilizing compounds to slow this process, but no additive stops it entirely.
How Fluorescence Works Under UV
Some materials glow visibly when exposed to UV light, a phenomenon called fluorescence. The process starts when a UV photon is absorbed by an electron in the material, bumping it to a higher energy state. The electron loses a small amount of that energy as heat while still in the excited state, then releases the rest as a new photon of light. Because some energy was lost to heat, the emitted photon has less energy and a longer wavelength than the one absorbed. That longer wavelength falls in the visible range, so you see a glow.
This gap between the absorbed and emitted wavelengths is called the Stokes shift, and it’s why UV blacklights can make white clothing, certain minerals, and security ink appear to glow. The UV light itself is invisible to your eyes, but the visible light emitted by the fluorescent material is not.
How the Ozone Layer Filters UV
Earth’s ozone layer, concentrated in the stratosphere about 15 to 35 kilometers up, acts as a selective UV filter. It absorbs all UVC radiation before it reaches the ground. It absorbs most UVB, though some gets through. And it doesn’t absorb UVA at all, which is why UVA makes up the vast majority of the UV radiation that reaches you on a sunny day.
This filtering is the reason life can exist on land. Without the ozone layer, the full intensity of solar UVC would sterilize exposed surfaces. The partial filtering of UVB is what makes sun protection necessary but manageable.
How Sunscreen Blocks UV
Sunscreens work by either absorbing UV photons (chemical sunscreens) or reflecting and scattering them (mineral sunscreens containing zinc oxide or titanium dioxide). The SPF number on the label tells you how much UVB protection you’re getting, and the relationship is not linear. SPF 15 blocks 93% of UVB rays. SPF 30 blocks 97%. SPF 50 blocks 98%, and SPF 100 stops 99%.
The jump from SPF 15 to 30 is meaningful, but beyond 30 the gains are incremental. No sunscreen blocks 100% of UV, and SPF ratings don’t measure UVA protection at all. For broad-spectrum coverage, look for products labeled “broad spectrum,” which means they’ve been tested against both UVA and UVB.
UV Damage to Your Eyes
Your eyes are highly vulnerable to UV. Photokeratitis, commonly known as snow blindness or welder’s arc, is an acute corneal injury caused by UVB and UVC exposure. It works by damaging the outermost layer of the cornea in several ways at once: directly harming cell membranes, creating DNA damage, and generating reactive oxygen species that trigger inflammation.
Symptoms typically appear within 6 hours of exposure and include intense pain, tearing, light sensitivity, and a gritty feeling. The condition usually resolves within 48 hours without lasting damage. It occurs in environments with unusually high UV levels: skiing at altitude, spending long hours on reflective sand or water, or working near welding arcs without proper eye protection. Chronic, lower-level UV exposure to the eyes over years contributes to cataract formation.