UVA vs. UVB: The Two Rays That Behave Completely Differently
Understand the two main types of UV and every sunscreen label and UV-index forecast suddenly makes sense.
According to the World Health Organization, UVA makes up roughly 95% of the UV radiation reaching Earth's surface, penetrates deeper, and is fairly constant year-round, while shorter-wavelength UVB is the main driver of sunburn and varies by season.
The World Health Organization explains that the two UV wavelength bands affect skin differently, which is why protection against both, not just the sunburn-causing UVB, is recommended.
Questions we get a lot
What's the difference between UVA and UVB?
UVA has a longer wavelength, penetrates deeper into the skin, and stays relatively constant year-round, making up about 95% of the UV that reaches Earth's surface. UVB has a shorter wavelength, is the main cause of sunburn, and varies more by season and time of day. The WHO outlines both bands.
If UVB causes sunburn, why worry about UVA?
Because UVA penetrates deeper and is present consistently year-round, contributing to longer-term skin damage even when you aren't visibly burning. That's why the WHO and dermatologists emphasize protection against both bands.
Does broad-spectrum sunscreen cover both UVA and UVB?
Yes, that is exactly what 'broad spectrum' means: protection against both UVA and UVB. Since the two bands behave so differently, broad-spectrum coverage is the goal, consistent with WHO guidance on UV exposure.
r/SkincareAddiction: 'I never burn through my car window so I figured I was fine. Then I learned UVA goes through glass. What's the actual difference between UVA and UVB?'