UVB light is a type of ultraviolet radiation with wavelengths between 280 and 315 nanometers, sitting between the higher-energy UVC and the lower-energy UVA bands. It makes up a small fraction of the sunlight that reaches Earth’s surface, but it plays an outsized role in human health: UVB is the specific wavelength responsible for both vitamin D production in your skin and the sunburns that increase skin cancer risk.
Where UVB Fits in the UV Spectrum
The World Health Organization divides ultraviolet radiation into three bands based on wavelength. UVC (100 to 280 nm) is the most energetic but is almost entirely absorbed by the atmosphere before it reaches the ground. UVA (315 to 400 nm) is the least energetic of the three and makes up the vast majority of UV light you’re exposed to daily. UVB sits in the middle, with enough energy to interact directly with DNA in skin cells but not so much that the atmosphere blocks it completely.
The ozone layer absorbs 97% to 99% of incoming UVB radiation. That small percentage that gets through is still powerful enough to cause sunburn, trigger vitamin D synthesis, and damage skin cells over time. Standard window glass and car windshields block most UVB, which is why you won’t get a sunburn sitting by a closed window. On cloudy days, however, up to 80% of UV radiation still reaches the ground, so overcast skies don’t eliminate your exposure.
How UVB Produces Vitamin D
When UVB hits your skin, it converts a cholesterol-related compound already present in skin cells into a precursor form of vitamin D3. Your body’s heat then transforms that precursor into usable vitamin D3, a process influenced partly by skin temperature and partly by binding proteins in the surrounding tissue. In lab experiments with human skin, a single UVB exposure session converted up to 35% of the available precursor into pre-vitamin D3.
This is the only way your body manufactures vitamin D on its own. Dietary sources and supplements provide vitamin D too, but the skin pathway is the one your body evolved to rely on. That’s why UVB exposure, in moderate amounts, is considered essential for maintaining healthy vitamin D levels, particularly for people living at higher latitudes where winter sunlight contains very little UVB.
How UVB Damages Skin
The same energy that makes UVB useful for vitamin D also makes it dangerous. UVB photons are absorbed directly by DNA in skin cells, causing neighboring building blocks in the DNA strand to bond together abnormally. About 75% of this damage takes the form of structures called cyclobutane pyrimidine dimers, with the remaining 25% forming a related type of lesion. Both types block normal DNA copying and gene expression, disrupting how skin cells function and reproduce.
Your cells have built-in repair systems that fix most of this damage, but repeated or intense UVB exposure can overwhelm those defenses. Unrepaired DNA errors accumulate over time and can eventually lead to mutations that cause skin cancer. This is why UVB is classified as a carcinogen, and it’s the primary reason sunscreen exists. SPF ratings specifically measure UVB protection: SPF 15 blocks 93% of UVB rays, SPF 30 blocks 97%, and SPF 50 blocks 98%. The gains shrink quickly after that, with SPF 100 reaching 99%.
Artificial UVB Sources
UVB doesn’t only come from the sun. Artificial UVB lamps are manufactured for medical, agricultural, and pet care purposes. The two main technologies are fluorescent tubes and LEDs. Fluorescent UVB bulbs, which use mercury vapor to generate UV wavelengths, remain the dominant technology because they produce significantly more UVB output per dollar. UVB LEDs exist and are improving, but they currently deliver far less total UVB compared to fluorescent tubes of similar wattage. A typical fluorescent UVB lamp emits a spectrum ranging from 280 nm up through UVA and into visible light, while LED options use narrower-wavelength diodes centered around 310 nm.
Medical Uses of UVB
Dermatologists use controlled UVB exposure as a treatment for several chronic skin conditions. This therapy, called phototherapy, comes in two forms: broadband UVB, which covers the full 280 to 320 nm range, and narrowband UVB, which concentrates output around 311 to 313 nm. Narrowband UVB has become the more widely used option because it delivers therapeutic wavelengths with less of the burn-causing radiation outside the target range.
The conditions most commonly treated with UVB phototherapy include moderate to severe psoriasis, eczema (atopic dermatitis), vitiligo, and certain types of cutaneous lymphoma. Treatment typically involves standing in a light booth for a carefully timed session, starting with short exposures and gradually increasing. Phototherapy for psoriasis has been used for decades and remains one of the standard options when creams and topical treatments aren’t enough on their own.
UVB Lighting for Reptiles
Reptiles kept as pets need UVB light to survive. Unlike mammals, most reptiles can’t absorb enough vitamin D from food alone. They depend on UVB exposure to synthesize it through their skin, just as humans do. Without adequate UVB, captive reptiles develop metabolic bone disease, a painful condition where bones become soft and deformed due to calcium deficiency.
For UVB bulbs to be effective in a reptile enclosure, they need to be positioned 9 to 15 inches from where the animal basks. Farther than that, the UVB intensity drops too much to trigger vitamin D production. The bulbs also degrade over time, losing UVB output months before the visible light dims noticeably, so they need regular replacement even when they still appear to be working. Most reptile keepers replace UVB bulbs every 6 to 12 months depending on the type.