02-08-2025
- Health
- San Francisco Chronicle
Should you wear sunscreen when it's cloudy in S.F.? Here's what experts say
How much can the sun damage our skin if we can't even see it?
That's a reasonable question in San Francisco, especially given that it's the city's coldest, foggiest summer in recent memory.
But experts say there's no ambiguity: You still need to wear sunscreen. Weather — even our eternal fog — should not play a role in your decision.
'There's a significant or clinically meaningful amount (of skin-damaging radiation coming through), even if it is foggy,' said Dr. Sungat Grewal, co-director of laser surgery and cosmetic dermatology at UCSF.
On foggier days, clouds scatter and absorb a portion of ultraviolet radiation from the sun, but a lot still gets to the ground — especially UVA rays (which primarily cause skin aging), and to a lesser extent UVB rays (which primarily cause sunburn). Both types can lead to cancer.
Up to 80 to 90 percent of UV rays can penetrate fog and clouds, according to Dr. Estelle Kahn, a dermatologist at Kaiser Permanente.
All UV radiation contributes to your risk of getting skin cancer, and the damage is cumulative — meaning the problem is not just one day, but rather total exposure throughout your lifetime.
Grewal recommends wearing tinted, broad-spectrum sunscreen with an SPF of 30 or higher, every day. 'Broad spectrum' protects against both types of ultraviolet light, whereas SPF mainly refers to UVB protection. The tinted element is more for cosmetic purposes — fighting aging — but still important to consider, both Grewal and Kahn said. Tinted sunscreen blocks visible light, which leads to pigmentation and other skin appearance issues.
'The best sunscreen is the one that you like … and are going to want to use every day,' Grewal said.
The UV index, found at the Environmental Protection Agency's SunWise UV Index app or your preferred weather app, shows considerable variation during the day in San Francisco. On Friday in San Francisco, for example, the index — which runs on a scale of 1 to 11 — showed 2 until around 9:30 a.m., but then soared to 10 around 1 p.m.
While you can forgo sunscreen if the index is at 1 or 2, indices of 3 and upward warrant sun protection, according to the EPA — and a 1 or 2 can very quickly become a higher number, with 8 or more considered 'very high to extreme.' The National Weather Service calculates the UV index using both UVA and UVB rays.
Winter months have a lower UV index. But Kahn said you still need to wear sunscreen then. She reiterated that UV radiation is invisible and doesn't correlate with temperature.
Chronicle meteorologist Greg Porter's advice? 'Don't base your sunscreen choice on the cloud cover, certainly in this city.'
