Tuesday, July 17, 2012

Sunscreen SPF and Diminishing Returns

(Graph courtesy of remixyourhealth)

SPF, or Sun Protection Factor, seemed to come out of nowhere in the 1980s as people started to become aware of the need for sunscreen.  (Before that, "suntan oil" appeared to be a derivative of cooking oil that helped broil skin.)

The original explanation I heard for the meaning of SPF was that it allowed you to stay out in the sun longer by a certain factor.  My friends and I initially interpreted this as, if you were going to stay out in the sun an hour, you could stay out two hours with an SPF of 2.  Or 10 hours with an SPF of 10.  An SPF over 24?  That meant you could extend your life by adding hours to your day.

Like MPG, SPF is a hard number to build good intuitions around. The right interpretation of SPF is that, if you want sun damage of X amount, you can now stay N times as long to get the same X sun damage.  You want a an hour of sun damage?  At an SPF of 10, you can stay out10 hours and get the same amount of sun damage as one hour.

But most people don't have in mind a level of sun damage they want. Instead, they have in mind a time that they want to spend in the sun.  As with MPG, SPF is better understood once it is flipped.  Flipping it reveals that there are surprising diminishing returns.  As SPF numbers start to sound very impressive, growing from 30, to 45, to 60, to 100--and the price grows as well--the actual increase in skin protection is negligible.  More after the jump.

What is the most important SPF increase to achieve?  The first one:  The one you get going from an SPF of 1 to an SPF of 2.  Why? Because it cuts the sun that reaches your skin in half.  No additional SPF will ever be as valuable as that step from 1 to 2. Here are some other comparisons:

Going from 1 to 10 gives your skin 90% protection.

Going from 1 to 30 gives your skin 97% protection.

Going from 1 to 60 gives your skin 98% protection.

Thus, although an SPF of 60 may seem "twice as good" as an SPF of 30, it is adding only 1% more protection.

The formula that translates SPF to protection is simply 1 - 1/SPF.  The figure at the top of the post plots the relationship.

The upshot of this for sunscreen users (see this post at Webmed):  Buy 30 SPF sunscreen, because it blocks 97% of the sun, and then take the money you saved by avoiding high priced, high SPF sunscreen to buy more of it so that you can reapply it often.  The biggest problem with people's use of sunscreen is not the SPF number they buy, but the fact that sunscreen does wash off due to sweat, swimming, etc., and is not reapplied often enough.

The FDA last year implemented sunscreen reforms that go into effect in 2012, including:

  • allowing sunscreens that are above SPF 15 and that block both UVA and UVB rays to be labelled "broad spectrum"; those that fall short of these criteria must have a skin cancer warning. 
  • requiring that sunscreen products that have SPF values higher than 50 be labeled as “SPF 50+” because the "FDA does not have adequate data demonstrating that products with SPF values higher than 50 provide additional protection compared to products with SPF values of 50."
This latter change is a way of keeping consumers from being too impressed by large SPF numbers that are essentially meaningless.

An alternative strategy would be to replace SPF with an inverse measure that captures "skin protection."   But this could raise it's own dangers.  Perhaps the FDA was worried that if sunscreen were labelled 90%, 97%, and 98%, people would too easily settle for 90%--it sounds good enough.

Here is how sunscreen bottles will be labelled after the changes

And here is the side panel for sunscreen that fails to meet the "broad spectrum" criteria, including the cancer warning: