How to choose a sunscreen: check the amount before the SPF
4 min read
SeriesLooking clean, checked8 / 8
Looking clean and put-together shows up not only in your clothes and hair but in your skin. And one of the things that shapes how your skin looks is the UV light you take in, a little at a time, every day.
QUIZTrue or false: put on an SPF 50 sunscreen and you get SPF 50 protection.Show answer
False. It depends on how much you apply.
SPF is measured with 2 mg of sunscreen per square centimeter of skin. People usually apply far less than that, and when the amount drops, protection drops sharply.
When choosing a sunscreen, it's tempting to compare the numbers. But the first question is whether you'll actually apply it in a sufficient amount, every day.
Contents
People who applied it daily showed less skin aging
A randomized controlled trial in Australia split 903 adults under 55 into a group that applied sunscreen every day and a group left to use it at their own discretion, then compared the skin on the backs of their hands four and a half years later [1].
The daily group showed no detectable increase in skin aging over those four and a half years, and their skin aged 24% less than the discretionary group's [1].
POINT
What mattered wasn't "a strong sunscreen, sometimes," but "applying it every day."
What SPF and PA mean
Sunscreen labels in Japan carry two ratings [2]. (Outside Asia you'll often see "broad spectrum" or a UVA logo instead of PA.)
| Label | Protects mainly against | How to read it |
|---|---|---|
| SPF | UVB (the cause of sunburn) | Higher numbers mean more protection (up to "50+" in Japan) |
| PA | UVA (reaches deeper into the skin) | More "+" signs mean more protection (PA+ to PA++++) |
If visible aging is what you care about, check the UVA rating as well as the SPF.
What's usually missing is the amount
SPF is measured with 2 mg of sunscreen per square centimeter of skin. But when a study asked beachgoers to apply sunscreen themselves, they used on average about 0.5 mg per square centimeter, roughly a quarter of the test amount [3].
A study of the relationship between amount and SPF found that protection falls off faster than the amount does [4]. A thin layer of SPF 50 is not SPF 50.
Japan's Ministry of the Environment manual suggests, for the face, taking a pearl-sized amount of cream (or a one-yen-coin-sized pool of liquid), dotting it on your forehead, nose, both cheeks, and chin, spreading it out, and then applying the same amount again. Because sweat and time wear it off, it also recommends reapplying every two to three hours [2].
Guidelines for choosing
The Ministry of the Environment manual gives these rough guidelines by activity [2].
| Activity | Suggested SPF / PA |
|---|---|
| A short walk or shopping | SPF 10–20, PA+ to ++ |
| Short outdoor play or light exercise | SPF 20–40, PA++ to +++ |
| Long hours outdoors, at the beach or in the mountains | SPF 30–50, PA++ to ++++ |
Beyond that, when I pick something for everyday use, I look at three things.
- Is it pleasant enough to put on every day? (Greasiness, white cast, smell.) If you don't keep using it, it does nothing
- Does it have a UVA rating?
- On days you'll sweat, is it sweat- and water-resistant?
If it doesn't agree with your skin (redness, itching), stop using it. If your skin is sensitive, try a small amount on the inside of your arm before using it on your face.
If I were choosing by these guidelines
References
1. Hughes, M. C. B., Williams, G. M., Baker, P., & Green, A. C. (2013). Sunscreen and prevention of skin aging: a randomized trial. Annals of Internal Medicine, 158(11), 781–790.
2. Ministry of the Environment, Japan. UV Environmental Health Manual 2020 (in Japanese).
3. Bech-Thomsen, N., & Wulf, H. C. (1992). Sunbathers' application of sunscreen is probably inadequate to obtain the sun protection factor assigned to the preparation. Photodermatology, Photoimmunology & Photomedicine, 9(6), 242–244.
4. Faurschou, A., & Wulf, H. C. (2007). The relation between sun protection factor and amount of suncreen applied in vivo. British Journal of Dermatology, 156(4), 716–719.
One more True or false