What the UV Index Actually Means (and When to Care)
That little number in your weather app is more useful than you think.
The UV Index runs from 1-2 (low) to 11+ (extreme); health authorities advise sun protection once it reaches 3 or higher.
Dermatologists frame it simply: treat a UV Index of 3 or higher as your cue to protect skin, since that is when UV is strong enough to cause damage.
Questions we get a lot
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.
Why does the UV index change through the day and across seasons?
The index tracks how strong UV is at a given place and time, so it peaks around solar noon and is higher in summer, at lower latitudes and at altitude — all situations where the sun is more directly overhead and less atmosphere filters UV. That is why the same city can read 2 in winter and 9 in summer.
Does UV come through windows and clouds even when the index seems low?
Largely, yes. Standard glass blocks most UVB but lets UVA through, and clouds stop only a fraction of UV, so a modest index reading still carries aging UVA. Use the index as a guide for burn risk, but keep daily broad-spectrum protection as a baseline near windows and on overcast days.
r/SkincareScience: 'What UV index number actually means I need sunscreen?'