Introduction to UV radiation and UV index What is UV radiation? The sun emits electromagnetic radiation of different energy or
wavelengths. Some of the radiation, such as that making up the
colours of rainbow, is visible which our eyes can see. The part of
radiation beyond the violet end of visible light is called ultraviolet
(UV) radiation which our eyes cannot see. UVA, UVB and UVC
Factors affecting the ground­level UV intensity Application of UV radiation in daily life
UVA, UVB and UVC UV radiation can be classified as UVA, UVB and UVC according to the
energy or wavelength. UV radiation reaching the earth's surface
consists mostly of UVA and some UVB, as all UVC and most of the
UVB are absorbed by the atmosphere. The main characteristics of
UVA, UVB and UVC are shown in the following table. Wavelength (nanometre, nm) UVA UVB UVC 315­400 280­315 100­280 Absorption by the
ozone layer Nil, i.e. it goes Mostly Practically all through the absorbed by absorbed by the ozone layer the ozone layer
ozone layer Amount reaching the Earth's surface > 98% of UV radiation is UVA < 2% of UV radiation is UVB Negligible ( 1 nanometre = 0.000000001 metre or 1x10­9 metre) Reference:
"Global Solar UV Index: A Practical Guide" , World Health Organization, 2002 [top]
Factors affecting the ground­level UV intensity Factor Influence on UV intensity at the Earth's surface Varies with time of the year, time of the
Position of the day and the latitude. The higher the sun's
sun position, the higher the UV intensity. Amount of ozone in the atmosphere Ozone absorbs UV radiation. The more
abundant the ozone in the upper
atmosphere, the less UV radiation
reaches the Earth's surface. Clouds, rain, fog and haze UV radiation is both absorbed and
scattered by clouds, rain, fog and haze. Ground reflection Most natural surfaces such as grass, soil
and water reflect less than 10% of UV
radiation. However, fresh snow strongly
reflects around 80% of UV radiation.
S a n d a l s o r e f l e c t s 1 0­2 5 % o f U V
radiation. The higher the altitude, the higher the UV
Altitude above intensity. It is because the atmosphere
the sea level gets thinner as we go up, and absorbs
less UV. Reference:
"Global Solar UV Index ­ An educational tool to reduce risks of skin cancer
and cataract" , Fact Sheet 271, World Health Organization, 2002 [top]
Application of UV radiation in daily life There are many applications of UV radiation in our daily life. The
following presents some examples. Application Principle Type of UV radiation used Disinfection of drinking water and sterilization of apparatus High energy UV radiation destroys the DNA of bacteria and micro­
organism. UVC Checking of counterfeit banknotes Genuine banknotes have fluorescent marks that are visible only under UV light.
UVA Hardening of inks and coatings Certain inks and coatings harden when exposed to UV radiation. UVA or UVB
Pest control Insects are attracted to the UV light and killed by the electric shock in bug zappers. UVA Medical applications such as treatment of psoriasis (a kind of disease causing red and scaly skins) UV radiation can suppress UVA or UVB
the immune system and reduce inflammatory responses. UV lamp for picking out counterfeit banknotes Reference:
A bug zapper "Environmental Health Criteria 160: Ultraviolet radiation", published under
the joint sponsorship of the United Nations International programme, the
International Labour Organization and the World Health Organization, 1994 [top]