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.

Related DermUV pages

Using Sunscreen Science: SPF, PA+++, Mineral vs Chemical in practice

UV strength on this page follows solar elevation, ozone thickness, cloud cover, altitude, and surface reflection. Snow can bounce back most of the UV that hits it, water and sand far less, but all of them add to the dose skin receives.

Burn-time figures come from minimal erythemal dose (MED) values published for each Fitzpatrick skin type, divided by the erythemal irradiance at a given UV index. Personal MED varies by roughly 20 percent, so treat every number as a planning range.

Reapplication matters more than the SPF number. Reapply roughly every two hours outdoors and immediately after swimming, heavy sweating, or towelling, regardless of what the bottle claims.

Cumulative dose drives photoaging and long-term skin damage more than any single day. Small daily exposures on commutes, walks, and driving add up across a season.

Plan outdoor blocks around the peak UV window, usually between 10am and 4pm local time. Shade, UPF fabric, a wide-brim hat, and UV-blocking sunglasses reduce dose more reliably than sunscreen alone.

UV scale reference

The WHO scale groups UV 1-2 as low, 3-5 as moderate, 6-7 as high, 8-10 as very high, and 11 or above as extreme. Protection is recommended from UV 3 upward, and shade during the middle of the day becomes the priority from UV 8 upward.

A practical routine at moderate-to-high UV is roughly one ounce of broad-spectrum sunscreen for full-body coverage, applied 15 minutes before going out, then reapplied on schedule. Clothing, hats, and shade carry the rest of the load, especially for children and anyone with a history of easy burning.

Keep exploring DermUV

These pages give more depth around sunscreen science: spf, pa+++, mineral vs chemical, from UV-scale reference material to definitions and city forecasts.

Numbers are modelled, not measured at your exact location, so treat them as guidance for planning rather than exact thresholds.