The UV Index, sometimes referred to as UVI, was developed in the late 1980’s as a measure of the UV radiation at the Earth’s surface and an indicator of the potential for skin damage from overexposure to the Sun. It is a unitless quantity that predicts UV intensity levels for the following day on a scale from 0 – 10+ (risk increasing with number value), calculated for sea-level elevation with all local weather variables, including factors such as cloud cover.
Since UV exposure can cause both immediate and long-term detrimental effects on the body (from sunburn to skin cancer), the UV Index provides vital information for assessing the extent of solar risk (WHO, 2002).
So how can I use it?
The UV Index is used in conjunction with Skin Type information to calculate how long you can stay out in the Sun before the effects of sunburn begin (i.e. before minimal erythemal dose, or MED, occures).
Calculating further with the SPF value of a sunscreen you can then figure out how long you can extend your time in the Sun.
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 ]