UV Safety
UVA vs UVB: What Each Wavelength Actually Does
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.