Iit is also very important to know the UV Index (UVI) for where you will be that day, and how it will be enhanced by the terrain or environment in which you will be practicing your sport.
The equations in the table below can be used to calculate how long you can stay in the Sun without sunscreen before you start to burn (i.e. before minimal erythemal dose, or MED, occurs).
Avoiding Sunburn – at sea level
A person with a Skin Type of 3, in a UV Index of 10, will start to sunburn after just 20 minute of unprotected exposure to the Sun.
[ 200 (min) / 10 (UVI) = 20 minutes ]
By using an SPF30 sunscreen this then becomes 600 minutes, or 10 hours.
[ 20 (min) x 30 (SPF) = 600 minutes ]
Refer to this section on SPF for further explanation.
Note: Many factors affect the UV intensity, which subsequently alters the UV Index. This then affects the amount of time that you can spend in the Sun before starting to burn.
The following information corresponds to the Environmental UV Reflection diagram (left), and demonstrates how your environment or terrain affects the level of UV radiation that you are exposed to:
1. 85% increase from snow reflection
2. 100% increase at 3000m altitude
3. 25% increase from white-water reflection
4. 80% of UV rays pass through cloud
5. 20% from sand and grass reflection – and 40% when wet
6. 15% reflection from concrete buildings
7. 50% can be reflected into shaded areas
8. 50% UVB and 80% UVA passes through the upper 50cm of water
9. 50% increase from water reflection
Avoiding Sunburn – ALTITUDE
Using the same UV Index (10) and Skin Type (3) as for the sea level example above, we can see that there is just 20 minutes of safe Sun exposure beofre burning.
Altitude increases UV intensity by up to 16% for every 1000 meters gained (3281 feet). So, for example, at an altitude of 3000 meters (9843 feet) the UV intensity is increased by almost 50%, which in turn increases the UV Index by 50%.
With the UV Index then adjusted to 15, this gives a time of just over 13 minutes before you start to sunburn (refer to the above table for sunburn time equations).[ 200 (min) / 15 (UVI) = 13.33 minutes ]
By using an SPF30 sunscreen you can extend this time to 400 minutes (almost 7 hours).[ 13.33 (min) x 30 (SPF) = 400 minutes ]
Avoiding Sunburn – SNOW
Using the same conditions as for Altitude above:
Altitude = 3000 meters (9843 feet), UV Index = 15, Skin Type = 3.
Remember that the UV Undex has been increased because of the altitude (calculated above).
Reflection from snow can increase the UV intensity by up to 85%. This would increase our UV Index at 3000 meters (9843 feet) from 15 to an effective UV Index of 27, so there is now only just over 7 and a half minutes until sunburn begins (refer to the above table for sunburn time equations).
By using an SPF45 sunscreen this time can be extended by 45 times to 333 minutes, or 5 and a half hours.[ 7.41 (min) x 45 (SPF) = 333 minutes ]
Avoiding Sunburn – WATER
Using the same varaibles as for teh original example at sea level:
UV Index = 10, Skin Type = 3.
When on water, reflection can increase UV intensity by up to 50%.
This increases the UV Index by up to 50%, making the adjusted UV Index 15. Now there is only just over 13 minutes until before sunburn begins (refer to the above table for sunburn time equations).
By using an SPF30 sunscreen you can stay protected out in the water for over 6 hours.[ 13.33 (min) x 30 (SPF) = 400 minutes ]