May 31, 2004

Updated UV Index adds new category, based on better science, health data

To mark the twelfth anniversary of the creation of the UV Index, Environment Canada last week introduced an updated UV Index incorporating new standards based on improved scientific and health information. Changes include the addition of a new category and year-round UV Index forecasting.

The UV Index will now take altitude into consideration, as UV rays are stronger in the mountains than at sea level, as well as the reflection of UV rays off the snow, which increases UV levels.

In addition, the scheme to determine the UV Index, based on observations from Brewer spectrophotometers, has been refined. These changes will result in a forecast UV Index which will be somewhat higher than in the past, particularly in the spring,. In addition, UV Index forecasts will now be uniformly rounded to the nearest whole number. Decimal fractions will no longer be used

Year-round UV Index forecasting will now be provided as well. In previous years, the UV Index was included in the public weather forecast every day during the peak UV season, from April to September. The UV Index will now be included throughout the year whenever UV levels register 3 or above. This means that under certain conditions, the UV Index could appear in winter. It also means that it may not appear on heavily overcast or rainy days during the summer.

Another important change is a revision in categories: in the past, the UV Index values were grouped into four categories: low, moderate, high and extreme. The thresholds of the categories have been changed and a new category, "very high," has been added before the "extreme" category.

In accordance with international recommendations, the categories of the UV Index are now as follows:

* low: 0 to 2 (formerly 0 to 3.9)

* moderate: 3 to 5 (formerly 4 to 6.9)

* high: 6 and 7 (formerly 7 to 8.9)

* very high: 8 to 10 (new category)

* extreme: 11 and higher (formerly 9 and higher)

Finally, recommended sun protection actions for each UV Index category have been updated by health partners including Health Canada, the Canadian Cancer Society, the Canadian Dermatology Association and the Canadian Association of Optometrists.

Since its creation in 1992, the UV Index has been adopted in 26 countries. The World Health Organization (WHO) and the World Meteorological Organization (WMO) adopted Canada's UV Index in 1994 as an international standard to be used by weather services around world. In the fall of 2002, the WHO and the WMO published Global Solar UV Index: A Practical Guide to provide guidelines for the use of the Index around the world. Environment Canada has revised its UV Index based on these guidelines.

The changes to the Canadian UV Index are being introduced in conjunction with the United States who are also making changes to their UV program. Many other countries around the world have adopted the new guidelines, including several countries in Europe, New Zealand and Australia.

More information is available from Pierre Tourigny at the Meteorological Service of Canada, 416/739-4973, or on the MSC Web site,

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