UV rays are a type of energy radiated by the sun that sits just beyond the violet end of visible light on the electromagnetic spectrum. They occupy wavelengths between 100 and 400 nanometers, shorter than the light your eyes can detect but longer than X-rays. That shorter wavelength packs more energy per photon than visible light, which is why UV radiation can alter skin cells, trigger sunburns, and cause lasting damage to DNA even though you can’t see or feel it hitting you in the moment.
The Three Types of UV Rays
UV radiation is split into three bands based on wavelength, and each one behaves differently once it leaves the sun.
- UVA (315–400 nm) makes up roughly 95% of the UV radiation that reaches Earth’s surface. The atmosphere barely filters it. UVA penetrates deep into the skin, contributing to premature aging, wrinkles, and long-term DNA damage that can lead to skin cancer.
- UVB (280–315 nm) is mostly filtered by the ozone layer, but enough gets through to cause sunburns and play a direct role in skin cancer development. UVB is also the wavelength your skin needs to produce vitamin D.
- UVC (100–280 nm) carries the most energy and would be the most harmful of the three, but it’s completely absorbed by the atmosphere. It never reaches the ground under normal conditions. Artificial UVC lamps are used in hospitals and water treatment facilities for sterilization.
How UV Rays Damage Your Cells
When UV photons hit your skin, they’re absorbed by the DNA inside your cells. That energy forces neighboring building blocks in the DNA strand to fuse together, creating abnormal bonds called dimers. These fused spots jam the cell’s ability to copy and read its own genetic instructions. Your body has repair systems that snip out the damaged section and patch it, but the process isn’t perfect. Mistakes that slip through become permanent mutations, and over years those mutations can accumulate into skin cancer.
This damage happens well before a sunburn appears. A sunburn is your body’s inflammatory response to UV injury that’s already occurred. By the time your skin turns red, the DNA damage is done.
UV Rays and Vitamin D
Not all UV exposure is harmful. Your skin uses UVB radiation, specifically wavelengths between 290 and 310 nm, to manufacture vitamin D. When UVB hits a cholesterol compound naturally present in skin cells, it breaks open the molecule’s ring structure to form a precursor that your liver and kidneys then convert into active vitamin D. This process is relatively fast, reaching its peak within hours of sun exposure.
The amount of time you need varies by skin tone, latitude, and season. People with darker skin need more UVB exposure to produce the same amount of vitamin D because melanin absorbs some of the UV before it can trigger the conversion. In northern latitudes during winter, the sun sits too low in the sky for enough UVB to get through the atmosphere, which is one reason vitamin D deficiency is more common in colder climates.
What Changes UV Intensity
The UV radiation hitting your skin at any given moment depends on several overlapping factors. Time of day matters most: 40 to 50% of a summer day’s UV arrives in the three-hour window around solar noon, when the sun’s path through the atmosphere is shortest and least filtering occurs. Latitude plays a similar role. Closer to the equator, the sun is more directly overhead year-round, so UV intensity stays high across seasons.
Altitude increases exposure because there’s less atmosphere above you to absorb UV. Cloud cover reduces UV but not as reliably as people assume. Thin or scattered clouds let significant UV through, and temperature alone is a poor guide. You can get substantial UV exposure on a cool, partly cloudy day.
Surface reflection also matters, though not always in the ways people expect. Fresh snow reflects a large share of UV, which is why skiers can burn on their faces and damage their eyes even in cold weather. Water, by contrast, reflects less than many people think. Its UV reflectance tops out at about 5 to 30% depending on the angle. The real reason people burn so easily at the beach has more to do with the lack of shade. Coastal settings tend to have fewer buildings and trees blocking the sky compared to urban environments, so you’re simply exposed to more of the overhead UV dome.
Effects on Your Eyes
Skin gets most of the attention, but your eyes are highly vulnerable to UV radiation. A short burst of intense exposure, from welding arcs, sun reflected off snow, or staring at a solar eclipse, can cause photokeratitis. This is essentially a sunburn on the surface of the eye. Both eyes are usually affected, and symptoms include pain, redness, watery eyes, blurred vision, light sensitivity, and a gritty feeling as if sand is trapped under your eyelids. Some people also see halos around lights or experience temporary changes in color vision. Photokeratitis is usually temporary and heals within a day or two.
Chronic, lower-level exposure over years is a different problem. Cumulative UV damage raises the risk of cataracts, the clouding of the eye’s lens, and macular degeneration, which erodes central vision. It can also cause tissue growths on the white of the eye called pingueculae and pterygia. These effects build silently over decades, which is why regular use of UV-blocking sunglasses matters even when the sun doesn’t feel intense.
Skin Cancer Risk
UV radiation is the primary environmental cause of skin cancer. Melanoma, the most dangerous form, is projected to see an estimated 112,000 new cases in the U.S. in 2026 alone. It’s more common in men than women and disproportionately affects people with fair complexions and those with long-term exposure to natural or artificial UV sources, including tanning beds. Non-melanoma skin cancers, which include basal cell and squamous cell carcinomas, are far more common overall and are also driven by cumulative UV exposure.
Tanning beds are particularly risky because they deliver concentrated UV, often heavy on UVA, directly to the skin. The idea that a “base tan” protects you is misleading. A tan is itself a sign of DNA damage; it’s your skin producing extra pigment in response to injury.
The UV Index
The UV Index is a standardized scale that tells you how strong UV radiation is at a specific place and time. It accounts for sun angle, ozone thickness, and cloud cover. The scale runs from 1 upward, with higher numbers meaning more risk.
- 1–2 (Low): Minimal danger for most people during normal outdoor activities.
- 3–5 (Moderate): Unprotected skin can burn in 30 to 45 minutes. Seek shade during midday.
- 6–7 (High): Sunburn can happen quickly. Sunscreen, hats, and protective clothing become important.
- 8–10 (Very High): Unprotected skin burns fast. Limit midday outdoor time.
- 11+ (Extreme): Maximum risk. Common near the equator and at high altitudes in summer.
Most weather apps display the UV Index in their daily forecasts, making it easy to plan around peak hours.
SPF vs. UPF
Two rating systems measure UV protection, and they cover different things. SPF, or sun protection factor, appears on sunscreen bottles and rates how well the product filters UVB rays specifically. It’s measured by how much longer it takes UVB to produce a sunburn on protected skin compared to bare skin. SPF does not measure UVA protection, which is why broad-spectrum labeling exists as a separate claim.
UPF, or ultraviolet protection factor, rates fabrics rather than creams. It measures how much of both UVA and UVB a piece of clothing blocks. A shirt with a UPF of 50 allows only 1/50th of UV radiation through the weave. Because UPF covers both UV bands and doesn’t wash off or need reapplication, sun-protective clothing is one of the most reliable forms of defense against UV exposure.