San Francisco clients ask me this more than anything else: do I really need SPF when it's foggy?
Yes. Here's why, and here's how to pick one you'll actually use.
What the fog doesn't block
There are two types of UV radiation that reach the Earth's surface:
UVB (burning rays): These cause sunburn and are partially blocked by clouds. On overcast SF days, UVB is reduced — sometimes significantly.
UVA (aging rays): These penetrate clouds and glass almost entirely. UVA is responsible for hyperpigmentation, collagen breakdown, and a significant portion of skin cancer risk. The fog does almost nothing to stop it.
If you're sitting by a window at work, you're getting UVA exposure all day. If you're walking outside in the fog, you're getting UVA exposure.
Why most people in SF skip SPF
It doesn't feel sunny. The sky is gray, it's cool, there's no obvious UV threat. This is exactly what makes SF a tricky climate — the usual cues (bright sun, heat, visible light) that remind people to apply SPF aren't there.
The damage accumulates silently. Photodamage from years of "it's just fog" shows up as uneven tone, brown patches, and accelerated aging.
How to pick an SPF you'll actually use
The best SPF is the one you'll apply every morning. Here's what to look for:
Minimum SPF 30, broad-spectrum. Broad-spectrum means it protects against both UVA and UVB. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%. The difference is marginal — consistency matters more than the number.
Texture for your skin type. If SPF breaks you out or feels heavy, you'll stop using it. Oily and acne-prone skin does well with lightweight fluid or gel formulas. Dry skin benefits from SPF built into a moisturizer.
Chemical vs. mineral. Mineral SPF (zinc oxide, titanium dioxide) sits on top of the skin and reflects UV. Chemical SPF absorbs into the skin and converts UV to heat. For acne-prone and sensitive skin, we often recommend mineral formulas.
Application
Apply SPF as the last step of your morning routine, after moisturizer. Use about a quarter teaspoon for your face and neck — most people apply far less than they should.
Reapply every two hours if you're outside for extended periods.
If you're only commuting and working indoors, morning application is typically sufficient.