If you haven’t heard yet, SPF is the measurement of how much UV protection sunscreen offers you. An SPF of 30, for example, only lets through to your skin 1 bit of UV for every 30 it blocks. That sounds great, right? This is roughly 96% protection! But do you know about the way scientists gather this information? If you did, you’d know why it’s so important that you apply sunscreen generously. Otherwise you aren’t getting the SPF that’s stated on the bottle.
The method scientists use to get the SPF number
Volunteer participants are sat under a high-intensity UV lamp, and the sunscreen in question is applied to a portion of their body. This is then compared to the unprotected skin, and the time it takes for them to burn naturally vs. with the sunscreen is measured. This gives us a rough idea of how much UV light is getting through. If it takes 30x longer for the protected skin to burn, then it’s being exposed to around 1/30th of the UV light, making the formula an SPF of 30. A higher number like 50 means the fraction of light being let through is less (1/50th). A lower number like SPF 15 only extends burn time by a factor of 15, meaning 1/15th of UV light is blocked by the formula.
The important part here is the amount of sunscreen they apply.
How much SPF you should be applying
To get the amount of protection it says on the bottle, we need to apply as much as the scientists did when they tested it. It’s logical that less sunscreen = less protection.
The industry standard for SPF measurements is 2.0 mg on every cm2 of skin (that’s ~0.0655 oz/1 ft2 for the non-metric readers). Every time a sunscreen is tested, this amount is carefully measured out so as to ensure consistency between tests and brands.
‘2.0 mg/cm2’ doesn’t mean much to the everyday person. We don’t weigh our sunscreen out and measure our skin. But with some simple maths we can make this number easier to remember and go by.
The average adult face is ~557.4 cm2 in size. That means to get 2mg on every centimetre of our face, we’d need 1115 milligrams (557.4 x 2), better put as 1.15 grams. That’s about a quarter of a teaspoon for just the face. Surprisingly large! Now think about how much you’d need to use on your arms and legs! It’s estimated to be around a teaspoon for each each arm and each leg. If you wanted to cover every inch of your body, you’d need 7 teaspoons of sunscreen!
Without this amount, not only might we miss spots, but there simply won’t be enough chemical or physical blockers to effectively protect our skin from the sun, potentially leading to photodamage and skin cancer.
John Su wrote a great post illustrating exactly how much he sunscreen he needs for his own unique facial measurements. If you’re interested in the maths, check it out!
The implications of having to use a lot of sunscreen
The problems begin when people rely on moisturiser or especially foundation as their sole SPF protection. Not only is it unlikely that they’re evenly covering all of their face, but I’m almost certain that nobody uses anywhere near that amount of foundation! You then won’t be getting the SPF on the bottle. I think additional SPF in makeup is great as an extra precaution or for touch ups during the day. But you should always use a dedicated sunscreen first and apply it generously to exposed areas.
Remember to apply your 1/4 teaspoon of high SPF this morning!