What the UV Index Actually Means (and When to Care)
That little number in your weather app is more useful than you think.
UV intensity rises roughly 4-5% for every 1,000 feet of elevation gain.
UC San Diego's Dr. Y. Linda Liou advises sun protection whenever the UV index is 3 or higher.
Frequently asked questions
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.
How does UV light age and damage skin?
UVB is absorbed by DNA in surface cells, causing direct mutations that can lead to skin cancer. UVA penetrates deeper, generating free radicals that damage DNA, collagen and elastin — producing wrinkles, sagging and dark spots. The damage accumulates over a lifetime, so daily protection pays off.
What does the UV index mean and when should I protect my skin?
The UV Index, from the EPA and WHO, rates the strength of midday UV on a scale from 1–2 (low) to 11+ (extreme). Dermatologists generally advise sun protection whenever it reaches 3 or higher. It appears in most weather apps and rises with sun elevation, altitude and proximity to the equator.
r/SkincareScience: 'Can someone explain UVA vs UVB like I'm five?'
Sources & citations
- Sklar LR et al., 'Effects of ultraviolet and visible radiation on the skin,' Photochem Photobiol Sci, 2013
- skincancer.org ↗