UV Index 3+: Your Daily Sunscreen Trigger
The simplest rule for when to bother with SPF.
The UV Index scale runs from 1-2 (low) to 11+ (extreme); dermatologists advise sun protection whenever it reaches 3 or higher.
UC San Diego's Dr. Y. Linda Liou advises sun protection whenever the UV index is 3 or higher.
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.
Do I still need sunscreen on cloudy or cool days?
Often, yes. The UV Index — not temperature or brightness — tells you the actual UV strength, and clouds let through a large share of UV. If the forecast index is 3 or higher, apply sunscreen even when it's overcast or chilly, because UVA that drives aging and DNA damage still reaches your skin.
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.
r/SkincareScience: 'At what UV index do I actually need to put on sunscreen?'