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UV Index

UV Index on

(Hong Kong Time)

UV Index
UV Index
Exposure Level
0-2
Low
3-5
Moderate
6-7
High
8-10
Very High
>= 11
Extreme
  • Ultraviolet Radiation
    uv1_v3_backgrd


    The Sun emits electromagnetic radiation of different wavelengths. Some of the radiation, such as that making up the colours of the rainbow, is visible which our eyes can see. The part of the radiation beyond the violet end of visible light is called ultraviolet (UV) radiation which our eyes cannot see.

  • UV-A, UV-B and UV-C

    uv2_v2_backgrd_e

    UV radiation can be classified as UV-A, UV-B and UV-C according to its wavelength or energy. UV radiation reaching the Earth's surface consists mostly of UV-A and some UV-B, as all UV-C and most of the UV-B is absorbed by the atmosphere. The main characteristics of UV-A, UV-B and UV-C are shown in the table below.

    UV-A

    UV-B

    UV-C

    Wavelength (nanometres, nm)

    315-400

    280-315

    100-280

    Absorption by the Ozone Layer

    Nil, i.e. it goes through the ozone layer

    Mostly absorbed

    Totally absorbed

    Proportion of the total UV reaching the Earth's surface

    > 98%

    < 2%

    Negligible

  • Meaning of the UV index

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    The UV index is a measure of the effect of solar ultraviolet radiation on human skin. The higher the UV index, the greater the risk and the less time it takes for harm to occur.

    The UV index and the corresponding exposure level, as categorised by the World Health Organization, are shown in the table above.

    In Hong Kong, the UV index can often exceed 10 on a sunny day in the summer.

  • Health Impacts of UV Radiation

    sun_top

    MODERATE EXPOSURE


    to the Sun helps
    our body produce
    Vitamin D, which could improve
    absorption and utilisation of calcium and phosphorus required for healthy bones.

    beach

    EXCESSIVE EXPOSURE


    to UV radiation
    from the Sun may
    cause painful sunburn
    , premature ageing of the skin, and an increase in the risk of skin cancer and cataracts.

    With regard to skin cancer, about 80% of all cases may be attributed to excessive exposure to sunlight. Specifically, studies have suggested that UV-A is responsible for the wrinkling, ageing and immediate tanning effects, as well as the potential development of skin cancer. Over-exposure to UV-B, on the other hand, may cause sunburn and is one of the major causes of skin cancer.