After how long should I reapply my sunscreen?

After how long should I reapply my sunscreen?

By SkinCabin

Before we get to when we should reapply, let's spend some time understanding what SPF number is and how it is calculated.

SPF number calculation

Sun Protection Factor, commonly called SPF, tells you how long a sunscreen protects your skin from sunburn. Sunburn is caused by Sun's UVB radiation.

How is this number calculated?

  1. Human volunteers are exposed to the light from the UV lamp with no sunscreen applied to their skin.  Time taken for their bare skin to burn is noted.
  2. Human volunteers are now exposed to light from a UV lamp with 2mg of sunscreen applied to a square centimetre of their skin. Time taken for their sunscreen covered skin to burn is noted.

SPF number = Time taken for sunscreen covered skin to burn / Time taken for bare skin to burn

Example:
If it takes 10 minutes to burn without sunscreen and 200 minutes to burn with sunscreen then SPF number is = 200 / 10 = 20


So after how long should I reapply my sunscreen...Is the reapplication time same as the SPF number?

Assume the following:
Lets say it takes 10 minutes for your skin to burn without sunscreen. You have SPF 15 sunscreen on.
Question: How much longer can you be out in Sun before your skin starts to burn?

We know the formula for SPF number is = Time taken for sunscreen covered skin to burn / Time taken for bare skin to burn...And so...
SPF 15 = time for sunscreen covered skin to burn / time for bare skin to burn
15 = time for sunscreen covered skin to burn / 10
time for sunscreen covered skin to burn = 15 * 10 = 150 minutes

So based on the above calulations, would it be correct to say you have to reapply sunscreen every 150 minutes?
Not entirely correct.

Sun's UV intensity changes throughout the day. The intensity is indicated by a number called UV index. You need to adjust that 150 minutes to take UV index into account. UV index changes throughout the day.

Let's say, UV index at 10 am is 8
time for sunscreen covered skin to burn = 150 / 8 = 18.75 minutes

Let's say, UV index at 12 pm is 11
time for sunscreen covered skin to burn = 150 / 11 = 13.63 minutes

So for the example above, at 10 am you have to reapply every 18.75 minutes. At 12 pm you have to reapply every 13.63 minutes.

Please note that the time for sunscreen covered skin to burn is the same as the time to reapply.

How often should I reapply my sunscreen = SPF number (x) time for your bare skin to burn / UV index

How long does it take for my bare skin to burn?
Everyone is different - but it usually takes around 15 minutes for bare skin to burn.

Quick recap with another example: If UV index is 9 at midday and if it takes 15 minutes for your bare skin to burn, you need to reapply SPF 30 every = 30 * 15 / 9 = 50 minutes.



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