Humans are protected from harmful UV-B rays by a combination of natural and artificial shields, starting with the ozone layer high in the atmosphere and extending down to the melanin in your skin cells. No single defense is complete on its own. The ozone layer filters out most UV-B before it reaches the ground, your body has built-in biological defenses that absorb and repair UV-B damage, and external tools like sunscreen and clothing block what gets through.
The Ozone Layer: Earth’s Primary UV-B Filter
The most important line of defense sits between 7 and 31 miles above the Earth’s surface in a region called the stratosphere. Here, a concentrated band of ozone gas acts as what NASA describes as a “planetary sunscreen,” absorbing a large portion of the sun’s UV-B radiation (wavelengths between 280 and 315 nanometers) before it ever reaches the ground. Without this layer, life on land would be exposed to levels of UV-B radiation far beyond what biological systems can handle.
The ozone layer doesn’t block all UV-B. According to the EPA, UV-B is only partially absorbed, which is why you can still get sunburned on a clear day. UV-A radiation (the longer-wavelength ultraviolet light responsible for skin aging) passes through even more freely, since ozone absorbs it only weakly. This partial filtration means the remaining defenses, both biological and artificial, matter a great deal.
The ozone layer took serious damage in the late 20th century from industrial chemicals, most famously chlorofluorocarbons. An international agreement in 1992 began phasing those chemicals out, and the recovery is measurable: levels of ozone-depleting substances over Antarctica have declined by about a third since peaking around the year 2000. The 2025 Antarctic ozone hole was the fifth smallest since 1992, roughly 30% smaller than the largest hole ever observed in 2006. Full recovery over Antarctica is projected around the late 2060s.
Melanin: Your Skin’s Built-In Absorber
The pigment that gives skin its color also functions as a UV shield. Melanin is produced by specialized cells in the outer layer of your skin and absorbs UV-B photons before they can penetrate deeper into tissue and damage DNA. When your skin tans after sun exposure, it’s producing more melanin as a protective response. People with darker skin naturally carry more melanin and have somewhat greater built-in UV-B protection, though no amount of melanin makes anyone immune to UV damage.
Melanin doesn’t just absorb UV energy. It converts that energy into small amounts of heat, which dissipates harmlessly. This makes it an unusually effective biological sunscreen. However, melanin’s protection is limited, especially against intense or prolonged exposure, which is why the body also relies on a second biological system: DNA repair.
A Natural UV Absorber in Your Skin
Your skin contains another molecule that absorbs UV-B directly. Trans-urocanic acid is a naturally occurring compound found in the outermost layer of the epidermis. It has strong absorption in the UV-B range and was once considered an ideal natural sunscreen ingredient because of how efficiently it soaks up UV-B energy and converts it to heat rather than triggering chemical reactions.
There’s a catch, though. When trans-urocanic acid absorbs UV-B, it changes shape, flipping into a different molecular form called the cis isomer. Researchers discovered in 1983 that this cis form suppresses the immune system, and subsequent studies confirmed the finding. This is one reason why heavy UV-B exposure can temporarily weaken your skin’s local immune defenses. Urocanic acid was gradually removed from cosmetic products because of this effect, but it remains a natural part of your skin’s chemistry.
DNA Repair: Fixing What Gets Through
Even with all these filters, some UV-B photons reach the DNA inside your skin cells and cause damage, creating distortions in the DNA strand that can interfere with normal cell function. Your cells have a dedicated repair system for exactly this type of injury, called nucleotide excision repair.
The process works like a molecular search-and-rescue operation. Specialized sensor proteins scan your DNA for UV-induced distortions. Once damage is found, these proteins recruit a team of enzymes that cut out the damaged section of the DNA strand, remove it, and rebuild the missing piece using the undamaged opposite strand as a template. The whole system is remarkably precise: one protein complex identifies the damage, hands it off to a second protein that verifies the lesion, and then a cascade of cutting and rebuilding enzymes restore the original sequence.
This repair system handles the vast majority of everyday UV-B damage without any issues. Problems arise when UV-B exposure overwhelms the system’s capacity, either through intense sunburns or years of cumulative exposure. Unrepaired DNA damage can accumulate and eventually lead to mutations that drive skin cancer. Some people are born with defects in these repair genes, a condition called xeroderma pigmentosum, which makes them extraordinarily sensitive to sunlight.
Sunscreen and the SPF Scale
Sunscreen is specifically designed to block or absorb UV-B radiation before it reaches your skin cells. The SPF number on the bottle tells you how much UV-B protection it provides, and the scale is not linear. SPF 15 blocks about 93% of UV-B rays. SPF 30 blocks 97%. SPF 50 blocks 98%, and SPF 100 blocks 99%. The jump from SPF 15 to SPF 30 is far more meaningful than the jump from SPF 50 to SPF 100.
Most dermatologists recommend SPF 30 as the practical minimum for everyday use. The diminishing returns above SPF 50 are small in percentage terms, but they can matter for people who burn easily or spend long hours outdoors. Reapplication matters more than the number on the bottle: sunscreen breaks down with sun exposure and wears off with sweat and water, so even high-SPF products lose effectiveness over a few hours. It’s also worth noting that standard SPF ratings measure UV-B protection specifically. If you want UV-A protection as well, look for products labeled “broad spectrum.”
Clothing and UPF Ratings
Fabric is one of the most effective and underappreciated UV-B barriers. Clothing is rated on the UPF (Ultraviolet Protection Factor) scale, which measures how much UV radiation, both UV-B and UV-A, can pass through the material. A UPF 50 fabric blocks 98% of the sun’s UV rays, allowing only 1/50th to reach your skin. The Skin Cancer Foundation requires a minimum UPF of 50 to earn its Seal of Recommendation, while UPF 30 to 49 is considered very good protection.
Not all clothing offers equal protection. Tightly woven, dark-colored fabrics block more UV than loose weaves or light colors. A white cotton T-shirt typically has a UPF of about 5 to 7, meaning it lets a surprising amount of UV through. Specialized sun-protective clothing is woven or treated to achieve UPF 50 or higher, and unlike sunscreen, it doesn’t need reapplication or wear off with sweat. For prolonged outdoor activity, clothing with a rated UPF offers more reliable protection than sunscreen alone, particularly on areas like shoulders and the back of the neck where sunscreen is easy to miss or forget to reapply.