Ben is bewildered…I recently came across this new article published on Allure Magazine and it got me a little confused. In an article published on Futurederm, it’s explicated that 2.0 mg/cm2 is about 1/4 teaspoon of sunscreen for the average person. However, in the Allure article, author Jenny Bailly wrote this: “For your face alone, you’re looking at about half a teaspoon.” I thought 1/4 was too much, but 1/2? I’ll never be perfect with sunscreen application then. Can you please address this on your blog?
The Beauty Brains respond:
The answer is there is no single answer that’s right for everyone. But let me walk you through the different numbers that are out there and how they came to be.
Who decided the right sunscreen dose?
The first thing you need to know is where this value of “2.0mg of sunscreen for every square centimeter of skin” came from. The answer is the FDA. Sort of. In the 2007 proposed sunscreen monograph the FDA declared that the “2.0 mg/cm2 with single-phase spreading” is the amount for “application of sunscreen drug product to plate.” The plate they are referring to is part of the device that is used to measure the efficacy of sunscreens. So the 2.0 mg number is designated as part of the testing protocol.
To ensure that the SPF delivered during actual product usage matches (as closely possible) the SPF value derived from testing, the FDA roughly extrapolates the 2.0 number to figure out how much to apply to your total body. That’s where the “use one ounce or a shot glass full for your entire body” notion came from. But, the FDA opted NOT to specify exact amounts to apply because they feel that is is misleading since it can change from product to product and person to person. Rather than specify exact numbers the FDA-required directions are to apply ‘‘generously’’ and ‘’liberally.”
To make things even MORE confusing in the 2011 final monograph the FDA changed the “Application of sunscreen drug product to plate” from 2.0 mg/cm2 with single-phase spreading” to 0.75m/cm2 with two-phase spreading.” In other words, for the purposes of testing they have decided that using a smaller amount applied in two coats gives better results. I haven’t seen anyone reflect this change in actual skin application, however.
Okay, having said all that, which is better for your face – 1/4 tsp or 1/2 tsp?
The case for 1/2 teaspoon
The “original” teaspoon rule by Schneider uses the “Rule of 9’s” which says that your head, face and neck (presumably including the ears but that’s not explicitly stated) comprise approximately 1/9 of the surface area of your body. Schneider then applies this calculation to the “one ounce/shot glass” dose and arrives at an application amount of 1/2 tsp for the face and neck but without explicit mention of head and ears.
Lim’s modified version (the one cited in the paper that Ben asked about from the Allure article) once again uses the rule of 9’s to calculate that the head, face and neck (presumably including the ears but that’s not explicitly stated) are 1/9 of the body’s surface area. That approach gives “One teaspoon to face, head, and neck takes into account that face and head is the most exposed site for most individuals and that for most head is totally or partially covered by hair.” Again, this presumably means 1/2 tsp for the face alone.
The case for 1/4 teaspoon
Rather than cite the “Rule of 9’s” FutureDerm’s approach is to measure the surface area of the face ONLY (no ears, neck and head) which yields a value of 1/4 tsp for the face only (I like that FutureDerm also breaks down ALL the calculations for you.)
The Beauty Brains bottom line
I’m not sure these two numbers are all that different because it’s ambiguous if the proponents of the 1/2 teaspoon dose are including the ears in their calculation. I think if you’re applying sunscreen to your face ONLY then 1/4 tsp is probably enough (depending, of course, on the size of your face.) If you’re applying sunscreen to your face, ears, and neck then 1/2 tsp will probably be needed to get the job done. The key, as the FDA says, is to apply product generously and liberally.
I hope this helps, Ben. If you want a more specific answer you’ll have to measure your own face and do the calculations.
Reference:
PHOTODERMATOLOGY, PHOTOIMMUNOLOGY & PHOTOMEDICINE Volume 29, Issue 1, February 2013, Pages: 55–56, Prescilia Isedeh, Uli Osterwalder and Henry W. Lim