UVB rays are a type of ultraviolet radiation from the sun with wavelengths between 280 and 315 nanometers, shorter and more energetic than UVA rays but longer than UVC. They’re the primary cause of sunburn, the trigger for your skin’s vitamin D production, and a major driver of skin cancer. While the ozone layer absorbs much of the UVB radiation heading toward Earth, enough reaches the surface to have significant effects on your skin, eyes, and health.
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 gets almost entirely absorbed by the atmosphere before reaching the ground. UVA (315 to 400 nm) is the least energetic of the three but makes up the vast majority of UV radiation at Earth’s surface. UVB sits in the middle at 280 to 315 nm, carrying enough energy to directly damage DNA in skin cells.
The ozone layer in the stratosphere absorbs most UVB radiation, particularly at the shorter wavelengths closer to 280 nm. As wavelengths approach 315 nm, this absorption weakens, which is why some UVB still reaches the ground. The amount that gets through varies depending on time of day, season, cloud cover, and your location on the planet.
How UVB Affects Your Skin
UVB rays don’t penetrate deeply. They’re mostly absorbed by the epidermis, the outermost layer of skin, which is why their effects are concentrated at the surface. Within the epidermis, UVB disrupts the structural “glue” holding skin cells together. It damages the fats and protein bonds between cells in the outermost barrier (the stratum corneum), weakening the skin’s ability to retain moisture and protect against the environment.
The more consequential damage happens at the DNA level. UVB radiation directly causes chemical cross-links between adjacent DNA bases in skin cells, creating defects called cyclobutane pyrimidine dimers. These are essentially kinks in the DNA strand that force the cell’s repair machinery into action. When repair fails or errors accumulate over time, mutations can develop that lead to uncontrolled cell growth. UV radiation is the environmental carcinogen responsible for more human malignancies than any other, driving both squamous cell and basal cell carcinoma formation. More than 90% of melanoma cases in the United States are attributable to UV exposure.
UVB and Sunburn
Sunburn is primarily a UVB-driven response. The redness, pain, and peeling you experience after too much sun exposure result from inflammation triggered by DNA damage in epidermal cells. The dose of UV needed to produce visible redness varies enormously from person to person, depending on skin pigmentation, previous sun exposure, the body site exposed, and the intensity of the radiation source.
Wavelength matters just as much as total dose. A given amount of energy delivered at UVB wavelengths can cause severe reddening of the skin, while the same amount of energy from a UVA source would produce no visible reaction in most people. That difference spans roughly four orders of magnitude, meaning UVB is thousands of times more potent at causing sunburn than UVA, watt for watt. This is why SPF ratings on sunscreen are specifically a measure of UVB protection.
UVB and Vitamin D Production
UVB rays are essential for vitamin D synthesis in the skin. When UVB photons (most effectively in the 290 to 310 nm range) strike exposed skin, they break apart a cholesterol-related molecule already present in your epidermis, converting it into a precursor form of vitamin D3. That precursor then slowly rearranges through body heat into active vitamin D3, which enters the bloodstream.
This process has a built-in ceiling. With continued sun exposure, the precursor molecule gets converted into inactive byproducts instead of more vitamin D. Those byproducts can revert back to the precursor form once you’re out of the sun, which means even a short period of exposure leads to prolonged vitamin D production in the hours afterward. People with darker skin need more time in the sun to produce the same amount of vitamin D as lighter-skinned individuals, but both reach the same maximum level. The key finding: vitamin D production peaks at exposure levels below those that cause sunburn.
Effects on the Eyes
Your skin isn’t the only tissue vulnerable to UVB. Acute UVB exposure to the eyes causes photokeratitis, a painful condition of the cornea sometimes called snow blindness or welder’s arc. It feels like having sand in your eyes, with tearing, light sensitivity, and blurred vision that typically resolves within a day or two as the superficial corneal cells regenerate.
Chronic, repeated UV exposure to the eyes carries more serious risks. Over years, it contributes to cataracts, growths on the white of the eye (pterygium and pinguecula), and has been linked to age-related macular degeneration and cancers of the eyelid and eye surface. Sunglasses that block UV radiation protect against both acute and long-term damage.
What Changes UVB Intensity
The amount of UVB reaching your skin depends on several environmental factors. Altitude is one of the strongest: UV levels increase by 10% to 12% for every 1,000 meters of elevation gain, because there’s simply less atmosphere overhead to absorb the radiation. This is why sunburn happens faster in the mountains, even when the air feels cool.
Latitude plays a major role too. Near the equator, sunlight travels a more direct, shorter path through the atmosphere and encounters less of the ozone and air molecules that would otherwise filter UVB. At higher latitudes, the sun sits lower in the sky, its rays travel through more atmosphere at an angle, and more UVB gets absorbed before reaching the ground. Time of day follows the same principle: UVB peaks when the sun is highest, roughly between 10 a.m. and 4 p.m. Season, cloud cover, and reflection off snow, water, or sand also shift the effective dose.
How Sunscreen Blocks UVB
SPF, or sun protection factor, is a direct measure of how well a sunscreen shields against UVB. The numbers translate to specific percentages of UVB blocked:
- SPF 15 blocks 93% of UVB rays
- SPF 30 blocks 97% of UVB rays
- SPF 50 blocks 98% of UVB rays
The difference between SPF 30 and SPF 50 is only one percentage point, which is why dermatologists generally recommend SPF 30 as a practical minimum. The bigger variable is application: most people apply far less sunscreen than the amount used in testing, which means real-world protection is lower than the number on the bottle. Reapplying every two hours and after swimming or sweating matters more than chasing a higher SPF number.
SPF only measures UVB protection. If you want coverage against UVA as well, look for “broad spectrum” on the label. UVA penetrates deeper into the skin and contributes to aging and cancer through different mechanisms, so blocking both types offers more complete protection.
Skin Cancer Trends Linked to UV Exposure
The relationship between UVB and skin cancer shows up clearly in population data. Melanoma rates among men younger than 50 have been declining by about 1% per year since the early 2000s, likely reflecting increased sun protection awareness in younger generations. But rates in men over 50 are still climbing, by about 0.8% per year from 2016 to 2022, reflecting decades of accumulated UV damage earlier in life. In women, melanoma incidence is rising by nearly 2% per year overall, though rates in women under 50 have stabilized.
The encouraging news is on the treatment side. Melanoma death rates dropped by nearly 6% per year between 2013 and 2017 thanks to advances in treating advanced disease, though that decline has since slowed. Basal cell and squamous cell carcinomas, the most common UV-related skin cancers, are so frequent that most cancer registries don’t even track them, making true incidence impossible to estimate.