Dermatology and Skin Care Updates from Waterhoody

All about sunscreen: How it works, how much to use, what to look for

All about sunscreen: How it works, how much to use, what to look for

Four part series on sunscreen in one easy to read blog post.



How does sunscreen work?

Sunscreen is made up of particles that are able to absorb the sun’s rays before they hit your skin.  This prevents most of the sun’s dangerous radiation from causing damage to the DNA of your skin cells—damage that could otherwise lead to skin cancer and skin aging.  When the sunscreen’s molecules absorb the UV light, they are converted to inactive particles that can no longer absorb the radiation.   As there are a finite number of sunscreen particles on your skin, eventually most of them are deactivated by the sun’s radiation and the sunscreen loses it’s ability to protect you.  This is the main reason you should reapply sunscreen after about 2 hours of sun exposure. Sun protective (UPF) clothing is an effective alternative to sunscreen as it does not lose it’s sun protective abilities throughout the day and provides lasting protection. Of course, you would still want to use sunscreen on areas not covered by UPF clothing.


What does SPF mean?

SPF stands for sun protective factor and it represents a relative value of how much a sunscreen can increase the time you can be in the sun before your skin starts to burn.  For example if you have light skin and typically it takes 5 minutes before your skin starts to turn pink, an SPF 15 sunscreen theoretically will allow for 5x15 minutes or 75 minutes before you will start to burn. 


What does broad spectrum mean?

Broad spectrum means that a sunscreen protects against two types of ultraviolet sun rays, UVB and UVA.  UVB is the shorter wavelength of sunlight that leads to sunburn and protection against UVB is represented by the sunscreen's SPF.  When you are exposed to UVB your skin starts to turn pink.  So, it is relatively easy to measure whether a sunscreen is protecting you - it will take longer for your skin to turn pink with the sunscreen when exposed to UVB light than without.  

UVA on the other hand is a longer wavelength of sunlight that penetrates into the skin and causes slower deeper damage like wrinkles, thinning of the mid layer of skin and discoloring of the skin.  Because its effects are over longer periods of time, it is more to difficult to measure the value of UVA protection that a sunscreen provides.  Instead, we rely on the properties of the ingredients in the sunscreen to tell us whether it will protect against UVA.  Zinc and Avobenzone are the most effective ingredients to protect broadly and should be in any daily sunscreen used to help prevent photo aging.  The most effective way to protect your skin from the sun is to wear sun protective clothing, which protects against the broadest spectrum, and apply Zinc or Avobenzone-based sunscreen on parts of the skin that are not covered by the clothing.


Does using an SPF greater than 30 make a difference? 

The answer to this question depends on how much sunscreen you apply. In a prior blog post, I defined the SPF value.  Another way of defining SPF is to think about the fraction of the sun’s light that is filtered out with each SPF value. An SPF of 30 allows 1/30 of the rays to reach your skin which is equivalent to blocking out 29/30 or 97% of the rays and an SPF 50 blocks out 98% of the rays (only a 1% increase from spf 30), so the percent difference between these SPF values is not huge. In theory, using an SPF of 30 would block out about 97% of the sun’s rays and that sounds pretty good, right?


However, in order to achieve the SPF on the bottle, you have to apply about a shot glass full of sunscreen to your body with each application.  Most of us only apply about 1/4 to 1/2 of the recommended amount of sunscreen, so we do not achieve our goal of an SPF of 30.  A study published in the Journal of the American Academy of Dermatology in 2012 found the sunscreens with an spf of 70 or greater provided adequate protection when used in a typical fashion (applying about 1/4 of the amount used in traditional SPF testing).  Another study also published in 2012 showed that consumers who applied 2 coats of sunscreen prior to sun exposure obtained adequate protection from the sun when using an SPF of 30 or greater.  


Key points to remember from this series:

  1. Sun protective clothing is easier to use and more effective than sunscreen at protecting your skin from the sun’s radiation
  2. You should reapply sunscreen every 2 hours when you are out all day to maintain adequate sun protection
  3. Sunscreen used on a daily basis should be broad spectrum (contain zinc or avobenzone) to protect from the sun rays that lead to photo aging even when you are less concerned about sunburn 
  4. To get adequate protection from the sun, use SPF 70+ or apply two coats of SPF 30
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