Tuesday, May 28, 2013

How much is 2 milligrams per centimeter squared?

Sunscreen is tested at 2 mg/cm2, an amount that is incredibly divorced from the typical consumer. Most people use much less than this amount and subsequently get much lower protection. If you use half of this amount, you will receive substantially less than half of its protection.

Makeup Alley has a fabulous chart that helps outline the amount of protection you actually get from your sunscreen if you skimp on the application:

labeled SPF (at 2 mg/cm2)
SPF at 1.5 mg/cm2
SPF at 1 mg/cm2
157.63.9
3012.85.5
5018.87.1

This raises an important and relevant question: how much sunscreen, in real terms, do you actually need to get the recommended SPF? To investigate, I used some paper, some math, and a kitchen scale.

My non-fancy scale. 
FutureDerm has kindly addressed part of this question already. On their blog, the calculated the amount of sunscreen needed to cover your face. They determined that covering your face would require about 0.04oz of product. Although this is very helpful, it doesn't tell the entire story. In the summer, most of us find ourselves showing a lot of skin.

This is how much sunscreen you need for your face alone.
To calculate how much sunscreen I actually need, I first needed to figure out the surface area of my body. Since I very rarely find myself frolicking naked, I did this while wearing a bikini. I took strips of paper and taped them all over myself. It certainly wasn't perfect, but it gave me a good approximation. The skin I had left showing was about 1.508 meters squared (although there is certainly some fuzziness around that number).

If you don't have a patient boyfriend to help you tape paper to your butt, there is an alternative method worth considering-- body surface area is commonly used to calculate chemotherapy drug doses, so some "rules of thumb" have emerged. One is the DuBois and DuBois Body Surface Area Formula. It's hardly perfect, especially for individuals who are not within the "normal" BMI range, but it may be close enough for your purposes. According to the formula, BSA=[weight (kg) x height (cm)/3600](1/2). I plugged in my numbers and found a BSA of 1.467 meters squared. This is almost certainly an underestimation, as my original calculations excluded areas that were already covered, such as my scalp, my boobs, and my butt, however it's definitely in the same ballpark as my experimental data.

The next step, of course, is to calculate the amount of sunscreen needed per square meter. 1 centimeter squared is equivalent to 0.0001 meters squared. Thus, 2 mg/cm2  equates to 20,000 mg per square meter. 20,000mg is about 0.705oz. You can calculate how much sunscreen you need by multiplying 0.705oz by the number you get on the DuBois and DuBois BSA Formula, but I will go ahead and use my experimental data, since I have it.

By these calculations, I need 1.063oz of sunscreen to cover my bikini-clad body.

I am not wearing enough sunscreen, you guys.
HOLY SHIT THAT IS A LOT OF SUNSCREEN.

Anyways, I am pretty sure the moral of this story is never go outside. But if you do go outside, be sure to put on more sunscreen than you could possibly imagine that you might need.

16 comments:

  1. I thought of another one for this topic... try out your powder sunscreen with its recommended mass requirement. And then do a head to head test with powder (physical) vs liquid (chemical) sunscreens. I have to say I'm kind of tempted to do that one myself since I could do it on my arm or something and then spend an afternoon lying around outside and still "doing science." And also too, neither you nor FutureDerm showed us what trying to put that volume of sunscreen on your face looks like. Go on - amuse us, I know you want to look comically white and gross on the internet.

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  2. Something that this and another of your posts has me wondering about is 100 SPF sunscreen. While I know that 100 is very unrealistic, does that mean that at 1mg you'd get about 10 SPF?

    Also, what a difference using just 25% less makes!

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  3. Well, this does explain why I burn easily even when I slather on what seems like a ton of SPF 50 sunscreen. It is so gloppy and sticky and ewwwww. The spray on stuff doesn't seem to offer much protection at all, and I wonder how these measurements translate to the spray, which is so much harder to measure. The quantity of lotion just for the face is daunting. I don't think I could get it to "stick" and not just look like a wet mess.

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  4. That is crazy! I'm not even sure how you could put that much sunscreen on your face and still put makeup on top of it. Or do anything but look like a gloppy white mess. I definitely do not wear anywhere close to enough sunscreen. It's a good thing I like staying indoors, shirts with sleeves, and wearing hats.

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  5. HOLY SHIT THAT'S SO MUCH SUNSCREEN.

    I guess I'll just keep my ghostly self indoors at all times. Or invest in some sunscreen stock...

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  6. Thanks for doing the math and paper-to-butt-taping for us all. I have heard the "shot glass of sunscreen" to cover your body before, and I do try to go by that rule. I buy my sunscreen at Costco for precisely that reason.

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  7. Thanks for mathing this out, love your blog! I am always amazed at the recommended amounts, makes me think this sunscreen business is half a scam. There's no way my skin can soak up that much sunscreen, and certainly not every 2 hours as is usually recommended. I am curious about the cost of your calculated dosages for both face and body. That must work out to a few bottles a week if you use as much as often as they say.

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  8. Dude, the spray, the SPRAY! Oh Robin I totally want to go try out a mass estimate with a spray thing and see what it's really like. I bet there isn't even a full body amount in one spray can. Freaking FDA nonsense irrelevant measurements.

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  9. I just have to say how much I adore this website. You rock the house!

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  10. What the hell. If we used this much sunscreen every day, we'd be lucky to get a little more than half a week out of a bottle of sunscreen. No friggin way.

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    Replies
    1. I suppose it depends on your priorities. It's important to take care of your skin!

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  11. Great work, congratulations.
    Regards

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  12. Dear Robyn,
    I love your blog, and I think yours is a really insightful, valuable voice to the beauty world. Given some of the references you make in your postings, I'm curious as to your profession. If you don't mind me asking, are you a research scientist by day? Simply very well read? Terribly impressive.

    ReplyDelete
    Replies
    1. I sell wine by day! Hopefully I will be doing research in the near future, though. Currently in the job application process. ;)

      Delete
  13. I'm noticing that a bunch of your older articles are missing images D:
    (sorry if this double-posts!)

    ReplyDelete
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