UV Index

A special UV Index, or measuring system, has been developed to indicate the strength of the Sun’s UV radiation, in terms of its likelihood to cause damage to humans. It is thus more than just a simple measure of UV intensity: the different components of solar UV are weighted according to their damaging effects. Different systems were initially developed by several countries around the World, but these have now all been standardized into a single worldwide system by the World Health Organisation.

This system is now used by governments and weather forecasting organisations around the World to warn people of the likely maximum UV levels, and the time at which they are likely to occur, on a given day, so that they can take appropriate precautions to protect themselves. Published UV Index levels, however, do not usually take any account of cloud cover – they indicate UV levels under sunny conditions.

The UV Index consists of an open-ended numerical scale starting at 0 and normally extending to around 10, although higher levels are common in the southern hemisphere due to thinning of the ozone layer, and index levels as high as 17 have been recorded in extreme conditions. The scale is also linear, meaning that an hour of exposure at UVI 8 should be roughly equivalent to 2 hours at UVI 4 (i.e. doubling the index level doubles the rate of exposure, and vice versa).

The general recommendations for sun protection precautions according to UVI value are:

  • UVI 0 to 2 – Low, no protection required.
  • UVI 3 to 5 – Moderate, some protection required. Apply sunscreen, wear protective clothing including hat and sunglasses, seek shade around midday.
  • UVI 6 to 7 – High, protection required. Apply sunscreen, wear protective clothing including hat and sunglasses, seek shade for 2–3 hours either side of midday.
  • UVI 8 to 10 – Very High, extra protection required. Stay indoors for 2–3 hours either side of midday and seek shade otherwise. Apply sunscreen 20 minutes before going out, wear protective clothing including hat and sunglasses.
  • UVI 11 and over – Extreme, caution required. Stay indoors as much as possible and seek shade otherwise. Apply sunscreen 20 minutes before going out, wear protective clothing including hat and sunglasses.

These recommendations apply to an adult with normally pale to light tan natural skin colour and no other factors, such as medical conditions or treatments, that could increase their sensitivity to UV radiation. Adults with paler skin and children should take more precautions at any given UV Index level, and those with darker skin can get away with less.

Midday in these recommendations means local solar noon, when the Sun is at its highest. This is often not the same as 12:00 by the clock, depending on the local time zone, where you are within it, and whether any daylight saving time adjustment is in effect.

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