UV Index 3+: Your Daily Sunscreen Trigger
The simplest rule for when to bother with SPF.
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
How does sunscreen actually work?
Sunscreens use UV filters in two groups. Organic ('chemical') filters such as avobenzone absorb UV and release it as a small amount of heat. Mineral ('physical') filters — zinc oxide and titanium dioxide — mainly absorb UV and also scatter some. Both cut how much UV reaches living skin cells.
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/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 ↗