Sunscreen — how much is actually enough?
The SPF on the bottle is measured at 2 mg/cm². Studies weighing real use found about a third of that density. What the evidence says about amount, layering and reapplying.
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 what people actually apply comes to 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. SPF is measured after both the test product and the standard are applied at 2 mg per square centimetre of skin (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?
A single everyday application came to a median of 0.60 mg/cm² across all the skin they could reach, about a third of the test density (Heerfordt et al., 2018).
A separate 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?
On a holiday in Tenerife, people using their own sunscreen as usual developed erythema every day, along with a rise in a marker of UV-induced DNA damage. People instructed to apply SPF 15 at 2 mg/cm² showed virtually no erythema (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). 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. Applying sunscreen twice in a row raised the amount from 0.60 to 1.10 mg/cm² and cut the uncovered area from 20% to 9% (Heerfordt et al., 2018). Two passes still fall short of the 2 mg/cm² test density.
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
SPF is the value at 2 mg/cm², so with too little on, sunscreen may not protect as much as the number says. 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). Rather than reaching for a higher SPF, apply the sunscreen you already have in two layers to get enough on. 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. A single real-world application came to a median of 0.60 mg/cm², a third of the 2.00 mg used in testing, and two coats right after each other reached 1.10 mg (Heerfordt et al., 2018). That is still short of the standard, but better than one. Right after applying, check in the mirror that your whole face is evenly covered. If the area itches or small bumps appear, switch products, and if it continues, see a dermatologist.
- On your 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. Across the whole body, one application left a median of 20% uncovered, and two cut that to 9% (Heerfordt et al., 2018). In the evening, check whether those spots alone look redder. If one patch is red, that is where you missed, so do it first the next day. If you see a new spot or one that keeps changing shape, or a scab that won't go away, see a dermatologist.
FAQ
References
- U.S. Food and Drug Administration. 21 CFR §201.327 "Over-the-counter sunscreen drug products; required labeling based on effectiveness testing," (i)(4)(iii).
- Heerfordt, I. M., Torsnes, L. R., Philipsen, P. A., & Wulf, H. C. (2018). "Sunscreen use optimized by two consecutive applications." PLOS ONE, 13(3), e0193916.
- Petersen, B., Datta, P., Philipsen, P. A., & Wulf, H. C. (2013). "Sunscreen use and failures — on site observations on a sun-holiday." Photochemical & Photobiological Sciences, 12(1), 190–196.
- Taylor, S., & Diffey, B. (2002). "Simple dosage guide for suncreams will help users." BMJ, 324(7352), 1526. (letter)
- Narbutt, J., Philipsen, P. A., Harrison, G. I., et al. (2019). "Sunscreen applied at ≥ 2 mg cm−2 during a sunny holiday prevents erythema, a biomarker of ultraviolet radiation-induced DNA damage and suppression of acquired immunity." British Journal of Dermatology, 180(3), 604–614.
- Diffey, B. L. (2001). "When should sunscreen be reapplied?" Journal of the American Academy of Dermatology, 45(6), 882–885.
