Here’s the SPF You Really Get When You Apply Sunscreen Sparingly [updated]
SPF 15 sunscreen will allow you to stay in the sun for about 15 times longer before you burn yourself. Pretty cool, right? Unfortunately, most of us do not get this level of protection because we are not applying enough. What you probably get: A simple SPF of 3-5.
This is because sunscreens are tested with two milligrams per square centimeter of skin. At this level, it takes a full ounce of sunscreen or about a shot glass to cover your entire body. But in real life nobody uses that much, so it’s good to know what kind of protection you really get from this bottle.
Studies that ask people to wear sunscreen have shown that most of us apply between half and a quarter of that amount . A 1997 study measured the SPF of three sunscreens on seven skin samples, and this is what they found at an average of 0.65 mg / cm 2 , a fairly typical application thickness:
- SPF 4.8 from a bottle labeled SPF 25
- SPF 3.5 from a bottle labeled SPF 15
- SPF 2.7 from a bottle labeled SPF 8
When they applied sunscreen at 1.3 mg / cm 2 the numbers were slightly better:
- SPF 9.6 from a bottle labeled SPF 25
- SPF 7.2 from a bottle labeled SPF 15
- SPF 4.9 from a bottle labeled SPF 8
They concluded that we should assume that we are getting 20 to 50 percent SPF per bottle. More recent research tends to be consistent with these numbers. For example, when comparing sunscreens with SPF 50+ and SPF 100+, people used 1.1 mg / cm 2 on a sunny day at a ski resort. These people probably got somewhere around half of the SPF listed. But if you have a habit of smearing it with the thinnest layer, well, now you know why you always come home burnt.
Updated 06.20.2018 – We have previously cited data from the EPWG , which argued that even SPF 100 sunscreen will give you an effective SPF 3.2. This is inconsistent with most of the scientific literature, so we’ve updated our post with more consistent numbers.