UVA vs UVB, Explained Like You're Scrolling at 2AM
The single most useful thing to understand about sun damage, in 60 seconds.
About 95% of UV radiation reaching the ground is UVA; only ~5% is UVB.
Dermatologists summarize it simply: UVB causes the burn, UVA drives the aging.
Frequently asked questions
What is the difference between UVA and UVB rays?
UVB rays have shorter wavelengths and cause sunburn, affecting mainly the skin's surface; UVA rays penetrate deeper and drive premature aging, wrinkles and pigmentation. About 95% of the UV reaching the ground is UVA. Both damage DNA and contribute to skin cancer, which is why 'broad-spectrum' protection against both matters.
Does UV come through windows and clouds?
Largely, yes. Standard glass blocks most UVB but lets UVA through, so windows at home, in the office, or a car's side windows still expose skin to aging rays (laminated windshields block most UVA). Clouds stop only a fraction of UV, so protect skin near windows and on overcast days.
r/SkincareAddiction: 'Do I really need SPF if I'm inside all day near a window?'
Sources & citations
- EPA, 'A Guide to the UV Index' (U.S. Environmental Protection Agency, 1994/updated)
- epa.gov ↗