The sun emits radiation of different wavelengths. Some of the radiation with wavelengths of rainbow colours can be visible by our eyes. Beynod these wavelengths are radiation in the ultraviolet (UV) and infrared which our eyes cannot see.

UV radiation is important to us because unprotected exposure to it can cause skin and eye damage. UV radiation can be broadly subdivided into UV-A, UV-B and UV-C.

The UV index is a measure of the solar ultraviolet (UV) intensity at the Earth's surface relevant to the effect on human skin. The skin-damaging UV radiation is governed by the an spectrum. This spectrum has been adopted by the International Commission on Illumination (CIE) to represent the average skin response over the solar UV spectrum.

Strength of UV Radiation

The UV Index indicates the strength of the UV radiation. The higher the number, the stronger the UV radiation and the more likely the damage to skin. During the cloudy and rainy periods, the UV index will be lower. The following table demonstrates the relationship between the UV Index and the Exposure Level. (Source: The World Health Organization)

New UV Index Classification Scheme

A new Ultraviolet (UV) Index classification scheme from the Department of Health (DH) has been adopted since August 8, 2002 (Thursday).

The new scheme was introduced by the World Health Organization (WHO) recently based on a joint recommendation with the World Meteorological Organization (WMO), the United Nations Environment Programme (UNEP), and the International Commission for Non-Ionizing Radiation Protection (ICNIRP). The purpose is to provide a harmonised UV Index classification scheme for adoption by authorities and other organisations in reporting the UV Index and in skin cancer prevention work.

The existing UV Index classification scheme has been used since October 1999.

The UV index is a measure of the potential harm of UV radiation on human health. The higher the UV Index, the more intense and dangerous the UV radiation. When working or playing in the sun, one should always pay attention to the latest UV Index broadcast by the media and take appropriate precautions against over-exposure. Children are especially vulnerable. Therefore, they should not be left in the sun for extended periods of time.

Points to Note

• Cover up with loose clothing, wear a broad brim hat and UV-absorbing sunglasses.
• Stay indoors or in the shade between 10 a.m. and 2 p.m. to avoid the midday sun.
• Use a sunscreen lotion of SPF 15 or above, use it liberally and reapply frequently.

Other Links

• Hong Kong Observatory: Background Information on UV Radiation and UV Index
• UK Health Education Authority: Sun Know How Program
• World Health Organization: Protection Against Exposure to UV Radiation
• USA Today: UV Index - What it means?