Factors affecting UV intensity affects the amount of time you can spend in the sun

UV IntensityMany factors can affect UV intensity, which subsequently alters the UV Index. This then affects the amount of time you can spend in the sun before starting to burn.

  • 85% increase from snow reflection
  • 48% increase at 3000 metres (9843 feet)
  • 25% increase from white-water reflection
  • 80% of UV rays pass through cloud
  • 15 – 25% from sand & grass reflection (40% when wet)
  • 15% reflection from concrete buildings
  • 50% can be reflected into shaded areas
  • 50% UVB & 80% UVA pass through the upper 50cm of water
  • 50% increase from water reflection

ALTITUDE

Using the same UV Index (UVI = 10) and Skin Type (3) as before (see “Avoiding Sunburn”), you have 20 minutes of safe sun exposure before burning. Altitude increases UV intensity by up to 16% for every 1000 metres (3281 feet) gained (Figure 5). So, for example, at an altitude of 3000 metres (9843 feet) the UV intensity is increased by almost 50%, which then increases the UV Index by 50%. With the UV Index adjusted to 15, this gives you just over 13 minutes before you start to sunburn:
[ 200 (min) / 15 (UVI) = 13.33 minutes ].
By using an SPF30 sunscreen you can extend this time to almost seven hours:
[ 13.33 (min) x 30 (SPF) = 400 minutes (6 hours 40 minutes) ].

SNOW

Altitude = 3000 metres (9843 feet), UV Index = 15, Skin Type = 3
Reflection from snow can increase the UV intensity by up to 85% (Figure 5). This increases the UV Index at 3000 metres from 15 to a UV Index of 27, so you now have only 7.41 minutes until you start to burn:
[ 200 (min) / 27 (UVI) = 7.41 minutes ].
By using an SPF45 sunscreen this time can be increased by 45 times to 333 minutes:
[ 7.41 (min) x 45 (SPF) = 333 minutes (5 hours 33 minutes) ].

WATER

UV Index = 10, Skin Type = 3
When on water, reflection can increase the UV intensity by up to 50% (Figure 5).
This again increases the UV Index by 50%, so the adjusted UV index will now be 15, giving you just over 13 minutes before you start to burn:
[ 200 (min) / 15 (UVI) = 13.33 minutes ].
By using an SPF30 sunscreen you can remain out on the water for almost seven hours:
[ 13.33 (min) x 30 (SPF) = 400 minutes (6 hours 40 minutes)