How Long Does It Take to Burn at UV Index 10?
The terrifyingly short answer for peak-summer days.
Visible light is ~45% of sunlight; UV is ~5-7%; infrared makes up the rest.
The Skin Cancer Foundation notes UVA passes through clouds and ordinary window glass.
Questions we get a lot
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/30PlusSkinCare: 'Is the UV index app on my phone actually accurate?'