UVA and UVB damage skin in different ways. The aging vs burning split, why broad-spectrum matters, and what UVC is.
Ultraviolet radiation is split into three bands: UVA, UVB, and UVC. Only UVA and UVB reach the surface of the Earth in meaningful amounts — but they affect skin very differently.
UVB — the burning ray
UVB (280–315 nm) is high-energy and stops mostly in the upper layers of the skin (the epidermis). It is the primary cause of sunburn and most directly damages DNA in keratinocytes, contributing to basal and squamous cell carcinoma.
UVB intensity varies a lot with time of day, season, latitude, and altitude. This is the band the UV Index is built around.
UVA — the aging ray
UVA (315–400 nm) penetrates deeper into the dermis. It is the dominant driver of photoaging — wrinkles, leathery texture, pigment changes — and contributes to melanoma. UVA passes through clouds and standard window glass and is roughly constant throughout the day.
UVC and broad-spectrum
UVC (100–280 nm) is the highest-energy band and is filtered out by the ozone layer; you only encounter it from artificial sources like germicidal lamps. The label “broad-spectrum” on a sunscreen means it has been tested to protect against both UVA and UVB at a defined ratio.
Key takeaways
UVB burns; UVA ages and penetrates deeper.
Broad-spectrum sunscreen covers both.
UVA passes through glass — daily facial SPF still helps indoors.
How to read this UVA vs UVB: What Each Wavelength Actually Does page
The UV index is a measure of erythemal (skin-reddening) irradiance, where each whole point equals 25 mW/m² of UV energy reaching the surface. That is why burn-time estimates fall roughly by half when the index doubles.
UV strength on this page follows solar elevation, ozone thickness, cloud cover, altitude, and surface reflection. Snow can bounce back most of the UV that hits it, water and sand far less, but all of them add to the dose skin receives.
Plan outdoor blocks around the peak UV window, usually between 10am and 4pm local time. Shade, UPF fabric, a wide-brim hat, and UV-blocking sunglasses reduce dose more reliably than sunscreen alone.
Cloud cover is not protection. Thin or broken cloud can still pass most incoming UV, and light cloud edges can briefly scatter more UV toward the ground than a clear sky.
Cumulative dose drives photoaging and long-term skin damage more than any single day. Small daily exposures on commutes, walks, and driving add up across a season.
UV scale reference
The WHO scale groups UV 1-2 as low, 3-5 as moderate, 6-7 as high, 8-10 as very high, and 11 or above as extreme. Protection is recommended from UV 3 upward, and shade during the middle of the day becomes the priority from UV 8 upward.
A practical routine at moderate-to-high UV is roughly one ounce of broad-spectrum sunscreen for full-body coverage, applied 15 minutes before going out, then reapplied on schedule. Clothing, hats, and shade carry the rest of the load, especially for children and anyone with a history of easy burning.
Keep exploring DermUV
These pages give more depth around uva vs uvb: what each wavelength actually does, from UV-scale reference material to definitions and city forecasts.