Sunscreen — how much is actually enough?

When sunscreen fails, it is usually the amount, not the product. The SPF printed on the bottle is measured with 2 mg of product per square centimetre of skin, and studies that weighed what people actually apply found about a third of that. So the thing to change next may not be the SPF number on the label, but how much sunscreen goes on.

How is a sunscreen's SPF number set?

In a lab. The US Food and Drug Administration fixes the application density inside the test procedure itself: both the test product and the standard are applied at 2 mg per square centimetre before measurement (21 CFR 201.327). The international test method uses the same density.

So "SPF 50" is not a promise that the product blocks fifty times more. It is a conditional record: at 2 mg/cm², it measured fifty. Change the condition and the number does not follow you.

How much sunscreen do people actually apply?

Danish researchers asked 31 volunteers to apply sunscreen as they normally would, then weighed it. The median for a single application, across all the skin they could reach, was 0.60 mg/cm² (Heerfordt et al., 2018). The amount actually applied is about a third of the test density.

A separate week-long study at a holiday resort found sunscreen use averaging 0.79 mg/cm² (Petersen et al., 2013). Both come from the same research group, which is worth noting, but no study reporting that people reach the standard amount turned up.

The spots sunscreen missed were more striking than the thinness. After one application, a median of 20% of reachable skin was left uncovered entirely (Heerfordt et al., 2018). That study measured whole bodies rather than faces.

Can we say how much is lost by applying less sunscreen?

One study compared two groups applying different amounts of sunscreen. On a holiday in Tenerife, 22 people using their own sunscreen as usual developed erythema every day, along with a rise in a marker of UV-induced DNA damage. Forty people instructed to apply SPF 15 at 2 mg/cm² showed virtually none (Narbutt et al., 2019). Cumulative UV dose did not differ statistically between them (P=0.08).

Hours outdoors did differ, though. The discretionary group spent about an hour more outside each day, a significant gap (P=0.02). So the study does not isolate the amount of sunscreen applied on its own. The sunscreens were formulated and supplied by a retail group, two of whose employees are authors.

So how much sunscreen should go on?

The familiar two-finger rule traces to a short proposal in a 2002 BMJ letter (Taylor & Diffey, 2002), which noted that SPF is measured at 2 mg/cm² while people apply between 0.5 and 1.5. No study confirming that the finger rule actually delivers 2 mg/cm² turned up since. The finger rule is a useful rule of thumb, not a measurement.

One method does have evidence behind it: apply sunscreen, then immediately apply it again. In the same study, two consecutive applications took the amount from 0.60 to 1.10 mg/cm², and cut the uncovered area from 20% to 9% (Heerfordt et al., 2018). No new sunscreen purchase is required. Two passes still fall short of the 2 mg/cm² test density, but the step from one pass to two is the widest gap you can close today.

Reapplication intervals rest on thinner ground. The "every two hours" advice traces to a mathematical model rather than a trial, and that model calculated that reapplying about twenty minutes after going out beats waiting two hours (Diffey, 2001). Getting enough sunscreen on in the first place comes before keeping to a schedule.

Why the amount of sunscreen beats the number

In the Tenerife study, the product that held erythema off was SPF 15. That was with the full amount of sunscreen on (Narbutt et al., 2019). Raising the SPF number is easy and getting enough sunscreen on is tedious, and the research points at the tedious one. When to start with sun protection is covered in when to start thinking about aging.

One step today

• After your morning sunscreen, go straight back over your whole face with a second thin layer. The study behind this used consecutive applications. A single real-world application came to a median of 0.60 mg/cm², a third of the 2.00 mg used in testing (Heerfordt et al., 2018). Two coats reach 1.10 mg — still short of the standard, but better than one.
• On the second pass, start with the sides of the nose, under the eyes, in front of the ears, and the front of the neck, where a hand tends to skate past. In the evening, check whether those spots alone look redder. If one patch has caught the sun, that is where the second coat missed.