Sunscreen
Sunscreen Science: SPF, PA+++, Mineral vs Chemical
What SPF really measures, what PA+++ means, the difference between mineral and chemical sunscreens, and how much you actually need to apply.
Sunscreen labeling is one of the most confusing parts of skin care. Two sunscreens with the same SPF can offer very different protection — and most people apply only a fraction of the amount used in lab testing.
This article unpacks what the labels actually mean and how to choose.
What SPF measures
SPF — Sun Protection Factor — measures protection against UVB, the radiation that causes sunburn. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%. The jump from 30 to 50 is small, but the absorbed dose is roughly halved.
Importantly, SPF does not measure UVA protection — the longer-wavelength UV that drives photoaging and contributes to skin cancer. For UVA, look for the term ‘broad-spectrum.’
PA+ ratings
Common in Asian markets, the PA system rates UVA protection on a + scale. PA+ is some protection; PA++++ is the highest. If you live somewhere PA isn't on the label, ‘broad-spectrum’ on a US/EU sunscreen serves a similar role.
Mineral vs chemical
Mineral sunscreens use zinc oxide or titanium dioxide to physically scatter and absorb UV. They start working immediately, are very stable, and are gentle on sensitive skin.
Chemical sunscreens (avobenzone, octocrylene, etc.) absorb UV via organic molecules. They tend to feel lighter, but some are less photostable and a few are being reformulated due to absorption concerns.
The best sunscreen is the one you'll actually wear consistently. Both categories, used correctly, prevent sunburn and reduce skin cancer risk.
How much to apply
Lab SPF tests use 2 mg of sunscreen per cm² of skin. That's roughly a shot glass (1 oz) for the whole body and a nickel-sized dollop for the face. Most people apply about half that, so real-world SPF is often 30–50% of the label.
Reapply every 2 hours, and immediately after swimming, sweating, or toweling off.
Key takeaways
- Choose broad-spectrum SPF 30+ for daily use.
- Reapply every 2 hours when outdoors.
- Both mineral and chemical sunscreens work — pick what you'll wear daily.