UVB rays are the primary cause of sunburn. These shorter-wavelength ultraviolet rays strike the outer layer of your skin and directly damage the DNA inside your skin cells, triggering the redness, pain, and peeling you recognize as a burn. UVA rays play a supporting role, contributing roughly 20 to 30% of the sun’s burn-inducing energy depending on conditions. A third type, UVC, is the most energetic of all but never reaches the ground because it’s completely absorbed by ozone and oxygen in the atmosphere.
Why UVB Rays Do the Most Damage
UVB radiation has a wavelength between 280 and 315 nanometers, short enough that your DNA directly absorbs the photons. When that happens, the energy warps the structure of your genetic material, creating defects called pyrimidine dimers. These are essentially fused sections of the DNA strand that the cell can’t read correctly. Your body responds to this widespread DNA damage with inflammation: blood vessels dilate, immune cells flood the area, and the skin turns red and tender. That inflammatory response is sunburn.
UVB doesn’t penetrate very deep. It acts mainly on the epidermis, the outermost layer of skin. But it hits hard there, killing enough cells that your skin peels in the days after a bad burn. This same DNA damage is the reason UVB exposure is strongly linked to skin cancer, particularly in genes that normally suppress tumor growth.
UVA’s Contribution Is Bigger Than You’d Think
UVA rays (315 to 400 nanometers) are often described as “tanning rays” and dismissed as harmless, but they do contribute to sunburn. Research published in the Journal of Photochemistry and Photobiology calculated that UVA accounts for about 20% of the sun’s erythemal (burn-causing) energy when the sun is high in the sky, rising to around 30% when the sun is at a lower angle. The reason: at lower sun angles, more UVB gets filtered out by the atmosphere while UVA passes through relatively unchanged, so UVA’s share of the total burn potential increases.
In tanning beds, the balance shifts dramatically. UVA contributes 50 to 80% of the erythemal energy in sunbeds, depending on the type of lamps used. That’s a major reason indoor tanning carries serious skin cancer risk even though users may not burn as visibly as they would outdoors.
UVA also penetrates deeper than UVB, reaching into the dermis, the thicker layer beneath the surface that contains collagen and blood vessels. This deeper penetration is why UVA is the bigger driver of premature aging, wrinkles, and loss of skin elasticity.
Why UVC Rays Don’t Reach You
UVC radiation (100 to 280 nanometers) is actually the most damaging wavelength of ultraviolet light, but it’s irrelevant to sunburn under normal circumstances. The ozone layer, sitting roughly 15 to 40 kilometers above Earth’s surface, absorbs it completely. You’ll encounter UVC only from artificial sources like germicidal lamps and certain welding arcs, where it can cause rapid burns to skin and eyes with very brief exposure.
What Makes UV Intensity Stronger or Weaker
Not all sun exposure carries equal burn risk. Several environmental factors determine how much UVB actually reaches your skin at any given moment.
Time of day is the biggest variable. UV rays are strongest between 10 a.m. and 4 p.m., when the sun is highest and its light travels the shortest path through the atmosphere. In early morning and late afternoon, that path lengthens considerably, and the atmosphere filters out much more UVB before it reaches the ground.
Latitude matters too. Near the equator, the sun sits almost directly overhead, and the ozone layer is naturally thinner, so UV intensity is highest. At higher latitudes, sunlight arrives at a steeper angle and passes through more ozone-rich atmosphere, reducing UVB exposure significantly.
Altitude increases your risk. UV radiation intensifies by about 10 to 12% for every 1,000 meters of elevation gain, because there’s less atmosphere above you to absorb the rays. Reflective surfaces compound the problem: fresh snow can bounce back up to 90% of UV radiation, effectively exposing you from below as well as above. Sand and water reflect smaller but still meaningful amounts.
Why Some People Burn Faster Than Others
Your skin’s melanin content determines how quickly UVB causes visible damage. Dermatologists classify skin into six phototypes. People with type I skin (very pale, often with red or blond hair and blue or green eyes) always burn and never tan. Type II (fair skin, blue eyes) burns easily and tans poorly. Types III and IV burn less readily and tan with increasing ease. Types V and VI (brown to dark brown skin) rarely or never burn visibly, though UV still damages their DNA.
These differences come down to how much melanin your skin produces and how effectively it absorbs UVB photons before they reach the DNA in deeper cells. Less melanin means less natural shielding, which means fewer minutes of sun exposure before the damage threshold is crossed and inflammation kicks in.
How Sunscreen Addresses Both Ray Types
SPF ratings measure protection against UVB specifically, since UVB is the dominant cause of sunburn. But because UVA also contributes to burns, aging, and cancer risk, the FDA requires sunscreens labeled “Broad Spectrum” to protect against both. To earn that label, a product must demonstrate a critical wavelength of at least 370 nanometers, meaning its protective coverage extends well into the UVA range rather than blocking only UVB.
In practical terms, if you’re choosing a sunscreen, look for both a high SPF (for UVB) and the Broad Spectrum label (for UVA). SPF 30 blocks about 97% of UVB rays, and SPF 50 blocks about 98%. The gap narrows quickly above that. Reapplying every two hours, or immediately after swimming or sweating, matters more than chasing a higher number on the bottle.