UVA, UVB, and UVC are three bands of ultraviolet radiation defined by their wavelengths, and they differ in how deeply they penetrate your skin, whether they reach you at all, and the type of damage they cause. UVA (315–400 nm) has the longest wavelength and penetrates deepest into skin. UVB (280–315 nm) is shorter and more energetic, responsible for sunburns and vitamin D production. UVC (100–280 nm) is the most energetic but is completely absorbed by the ozone layer, so it never reaches you naturally.
Wavelength Ranges and Energy
The ultraviolet spectrum spans 100 to 400 nanometers, sitting just beyond visible violet light. The three bands break down like this: UVA occupies the longest wavelengths (315–400 nm), UVB sits in the middle (280–315 nm), and UVC covers the shortest (100–280 nm). This matters because shorter wavelengths carry more energy per photon. UVC is the most energetic, UVB is next, and UVA is the least energetic of the three.
That energy difference shapes everything else about how each type behaves, from how the atmosphere filters it to how it interacts with your DNA.
What Reaches the Earth’s Surface
The ozone layer in the stratosphere acts as a selective filter. It absorbs UVC completely, meaning none of it reaches the ground under normal conditions. UVB is partially absorbed, with shorter UVB wavelengths blocked more effectively than longer ones near 320 nm. UVA passes through almost untouched.
The practical result: roughly 95% of the UV radiation hitting your skin on a sunny day is UVA, with UVB making up the remaining fraction. UVC is absent entirely from natural sunlight. You can only encounter UVC from artificial sources like germicidal lamps.
How Each Type Affects Your Skin
UVA penetrates through the outer skin layer (epidermis), through the deeper layer (dermis), and can even reach subcutaneous tissue beneath that. Because it reaches so deep, UVA is the primary driver of photoaging, the kind of premature wrinkling and leathery texture caused by years of sun exposure. It also triggers immediate skin darkening and persistent pigmentation. UVA damages DNA indirectly by generating reactive oxygen species, unstable molecules that harm cells as a byproduct of the radiation being absorbed.
UVB, despite being less penetrating, is more destructive to the outer skin. It reaches the epidermis and the uppermost dermis but goes no further. Its higher energy means it is directly absorbed by DNA, causing mutations without needing an intermediary. UVB is the main cause of sunburn. That redness you see after too much sun is largely a UVB effect, driven by increased blood flow to damaged skin. Acute UVB overexposure causes sunburn, and repeated exposure over time contributes to skin cancer.
UVC would be the most damaging of all if you encountered it naturally. Even brief artificial UVC exposure, as little as 30 to 70 seconds from certain germicidal lamps, can cause burns, severe eye inflammation, and irreversible changes in skin and retinal cells.
Skin Cancer Risk
All three types of UV radiation are classified as Group 1 carcinogens by the International Agency for Research on Cancer, meaning there is sufficient evidence that they cause cancer in humans. But they contribute to different types.
A nationwide U.S. cohort study published in the Journal of the National Cancer Institute found that melanoma risk was more closely tied to UVA exposure than UVB. After statistically adjusting for UVB levels, participants living in areas with high UVA still showed elevated melanoma risk. The reverse was not true: there was little evidence of increased melanoma risk from ambient UVB once UVA was accounted for. Scandinavian research supports this pattern, finding melanoma incidence tracks more closely with UVA levels by latitude. UVB, on the other hand, is more strongly associated with squamous cell carcinoma, a non-melanoma skin cancer that develops in the outermost skin cells UVB directly damages.
UVB and Vitamin D
UVB is the only type of UV radiation that triggers vitamin D production in your body. When UVB wavelengths between 290 and 315 nm hit your skin, they convert a cholesterol compound in the epidermis into previtamin D3, which your body then processes into active vitamin D. UVA does not do this. Neither does UVC. This is why sitting behind a window on a sunny day, which blocks most UVB but lets UVA through, does not help your vitamin D levels.
The trade-off is real: the same UVB exposure that produces vitamin D also causes sunburn and DNA damage. Brief, moderate sun exposure (typically 10 to 30 minutes depending on skin tone and latitude) can generate meaningful vitamin D before burning occurs.
Eye Damage
UV radiation affects more than skin. Chronic UV exposure to the eyes is linked to cataracts, growths on the eye surface called pterygium, and age-related macular degeneration. The acute version of UV eye damage is photokeratitis, essentially a sunburn of the cornea. Photokeratitis is caused specifically by UVB and UVC wavelengths, not UVA. Snow blindness and welder’s flash are common forms. Sunglasses labeled as blocking 99–100% of UV radiation protect against both UVA and UVB reaching the eyes.
UVC in Germicidal Lamps
Because UVC is so effective at destroying the DNA of viruses, bacteria, spores, and fungi, it is widely used in sanitization. Germicidal UV lamps and LED devices use UVC wavelengths to disinfect air, surfaces, and objects. Hospitals have used this technology for decades, and consumer-grade UVC products became popular during the COVID-19 pandemic.
These devices are not without risk. Direct UVC exposure causes irritation, burns, and a form of photokeratitis that can damage the cornea even with brief contact. Retinal cells are especially vulnerable. Consumer germicidal lamps vary widely in safety design, and some tested models caused harmful changes in skin and eye cells after exposures as short as 30 seconds. UVC sanitization devices should only be used in unoccupied spaces or with proper shielding.
Sunscreen and Protection
SPF numbers on sunscreen measure protection against UVB specifically, since UVB causes the visible sunburn that the test is based on. To get UVA protection, you need to look for the “Broad Spectrum” label. The FDA requires broad-spectrum sunscreens to pass an in vitro test showing protection across UVA wavelengths, with a critical wavelength of at least 370 nm. This means the product must absorb meaningfully across most of the UVA range, not just UVB.
A sunscreen with SPF 30 but no broad-spectrum label would prevent sunburn while still allowing deep-penetrating UVA to reach your dermis, contributing to photoaging and melanoma risk over time. Broad-spectrum SPF 30 or higher covers both threats. UVC protection is not a consideration for sunscreen since the ozone layer already handles it.