Blog » UV Index And SPF Explained – What You Need To Know To Be Sun Safe


UV Index And SPF Explained – What You Need To Know To Be Sun Safe

We all know that the sun can be harmful to our skin and increase our risk of skin cancer. As well, we have it drilled into us that by wearing sunscreen we can minimize and even eliminate those harmful effects.  But did you know that altitude, snow and water greatly increase the strength of UV rays, the presence of snow alone can double the level of sun exposure! When you’re planning a day out in the mountains, looking up the UV index and grabbing the handy SPF 15 probably won’t be enough. How do you know what strength of sunscreen you should be wearing? What does SPF mean? What does the UV index on the weather report mean? Well, let me explain the UV index and SPF and how it pertains to backcountry beauties.

1. Two definitions you need to know before we do anything

SPF = Sun Protection Factor.  It is the factor by which your safe sun exposure time (no burning) can be multiplied.

UV Index = Is a measure of the Earth’s UV radiation levels at sea level, taking into account the areas weather conditions. Measured daily on a scale from 1-10, it indicates the potential for skin damage due to over exposure. Find your local UV index here.

Why are these two things important to you? The daily UV index combined with your skin type and environmental conditions will tell you how long you can be exposed in the sun, without sunscreen, before you start to burn. Then you can calculate your safe sun exposure time with the SPF value of your sunscreen. Thus ensuring you are adequately protected for the entirety of your outdoors adventure! Now how to calculate said things.

2. Know your skin type

Before you can calculate anything, you need to know your skin type. Skin types are broken up into six categories based on Fitzpatrick Phototype Scale.  Your skin type is based on the amount of natural UV protection from the melanin content in your skin.  Himyaya Sports Sun Protection breaks down the different skin types pretty well. Figure out which category you fall into.

 3. Calculate your safe sun exposure time without sunscreen

Now that you know your skin type and the daily UV index for your area, you can calculate how long you can be exposed to the sun, before you start to burn. Follow the corresponding equation for each skin type.


For example, a person with a skin type of 3 and a UV index of 10, will start to burn in 20 minutes with no sunscreen [200min/10=20min].

4. Calculate how altitude, snow and water effect your safe sun exposure time

Altitude:  UV intensity increases by 16% for every 1000m above sea level. For example, if you are planning to play above 3000m, that’s a 48% increase to the daily UV index. Therefore, a UV index of 10 now becomes 14.8 [10 x 1.48=14.8]. Recalculating the maximum time a person of type 3 skin can safely stay in the sun gives a time of 13.5 minutes [200min/14.8=13.5min].

Snow: UV intensity increases by 85% with the presence of snow. If the snow is found at a higher elevation then sea level (most probable) then remember to adjust the UV index that takes into account that elevation.  So for example if we’re hiking through snow above 3000m, the UV index becomes 27.4 [14.8 x 1.85=27.4] and the maximum time before your skin starts to burn is 7.3 minutes [200min/27.4=7.3min].

Water: UV intensity increases by 50% when on water.  Follow the same calculation process as above for recalculating the UV index to take into account the reflectivity of the water.

5. Calculate the amount of safe sun exposure time with the use of sunscreen

Now that you have your UV index adjusted for the environmental conditions for where your activity is occurring, you can apply the effects of SPF to your sun exposure time.  As mentioned above, the SPF of your sunscreen is the factor by which you can multiply your safe exposure time by.

For example if you are hiking in snow above 3000m, your safe sun exposure time is 7.3 minutes. If you’re wearing sunscreen with an SPF of 15, your safe sun exposure time becomes 109.5 minutes or nearly 2 hours [7.3min x 15spf=109.5min]. Therefore, if you’re going to be out all day, you should be wearing a sunscreen much stronger then SPF 15. A sunscreen of SPF 45 would increase your safe sun exposure time to 328.5min or 5.5 hrs, something much more adequate!


I understand that you’re not likely to whip out your handy dandy calculator (unless you’re an engineer) every time you want to go outside, but this puts the power of the sun and your environmental factors into perspective. The unpleasant physical effects of sunburn (i.e. painful, redness and peeling) and the risk of skin cancer should be enough incentive for people to protect themselves properly against the sun.  As well, something all backcountry beauties should be aware of; sun damage, whether it is a tan or a burn, speeds up the onset of wrinkles. So next time you’re planning an adventure outdoors, remember the power of the sun and choose a strong enough SPF.


  1. Eric says:

    Hmm lets see…

    Skin Type 2
    UV index of 10 (Tioman Island)
    Increase UV index by 50% for water…. gives safe sun exposure time of 6 minutes 40 seconds.

    One half-ass application of SPF 50 for a day long snorkeling trip = the worst sunburn of my life!

    Wish you would have wrote this before your trip lol

    • Morgan says:

      Hahaha yeah…I (we) learnt the lesson the hard way! Just passing on the knowledge so others can avoid the disgusting pain!

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