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Ultraviolet radiation: global solar UV index

An educational tool to reduce risks of skin cancer and cataract

Everyone is exposed to ultraviolet (UV) radiation from the sun. Small amounts of UV radiation are beneficial to people, and play an essential role in the production of vitamin D. However, overexposure to UV radiation is responsible for two major public health problems: skin cancer and cataract.

UV radiation

Emissions from the sun include light, heat and UV radiation. UV radiation reaching the Earth's surface is largely composed of UVA with a small UVB component. UV radiation levels are influenced by:

  • Sun elevation: the higher the sun in the sky, the higher the UV radiation level.
  • Thus UV radiation levels vary with time of day and time of year.
  • Latitude: the closer to equatorial regions, the higher the UV radiation levels.
  • Cloud cover: UV radiation levels are highest under cloudless skies but even with cloud cover, they can be high.
  • Altitude: at higher altitudes, a thinner atmosphere absorbs less UV radiation.Ozone: ozone absorbs some of the UV radiation that would otherwise reach the Earth's surface.
  • Ground reflection: grass, soil and water reflect less than 10% of UV radiation; fresh snow reflects as much as 80%; dry beach sand about 15% and sea foam about 25%.

Sun elevation: the higher the sun in the sky, the higher the UV radiation level. Thus UV radiation levels vary with time of day and time of year.Latitude: the closer to equatorial regions, the higher the UV radiation levels.Cloud cover: UV radiation levels are highest under cloudless skies but even with cloud cover, they can be high. Altitude: at higher altitudes, a thinner atmosphere absorbs less UV radiation.Ozone: ozone absorbs some of the UV radiation that would otherwise reach the Earth's surface. Ground reflection: grass, soil and water reflect less than 10% of UV radiation; fresh snow reflects as much as 80%; dry beach sand about 15% and sea foam about 25%.

As UV radiation can neither be seen nor felt, it is important to provide a tool to raise awareness of the problem and alert people on a daily basis to take prompt, appropriate, protective action.

The Global Solar UV Index (UVI)

The UVI is a simple measure of the UV radiation level at the Earth's surface. It has been designed to indicate the potential for adverse health effects and to encourage people to protect themselves. The values of the Index range from zero upward and the higher the Index value, the greater the potential for damage to the skin and eye, and the less time it takes for harm to occur.

While the levels of UV radiation vary during the day, they reach a maximum around mid-day. The UVI is usually presented as a forecast of the maximum amount of UV radiation expected to reach the Earth's surface at solar noon. In countries close to the equator, the UVI can reach up to 20. Summer-time values in Northern latitudes rarely exceed 8.

An internationally harmonized UV index

The UVI was developed through an international effort by WHO in collaboration with the United Nations Environment Programme (UNEP), the World Meteorological Organization (WMO), the International Commission on Non-Ionizing Radiation Protection (ICNIRP), and the German Federal Office for Radiation Protection. Since its initial publication in 1995, many countries have been using the UVI to promote sun protection along with the weather forecast in newspapers, TV and radio. However, most people have not fully understood its meaning or usefulness, and UVI reporting and associated protection messages differed greatly between countries, creating confusion.

INTERSUN, WHO's Global UV project, has developed and introduced an internationally agreed communication concept to ensure uniformity of sun protection messages, to facilitate the delivery of a simple and relevant message and to improve its use as an educational tool. The concept is published in a WHO booklet Global Solar UV Index: a Practical Guide. The following harmonized exposure categories and colours are associated with various values of the UVI:

Category UVI range Colour
Low
Moderate
High
Very high
Extreme
0 to 2
3 to 5
6 to 7
8 to 10
≥ 11
Green
Yellow
Orange
Red
Purple

A standard graphic presentation of the UVI promotes consistency in reporting and improves people's understanding of the problem. The graphics package can be downloaded from the web site of INTERSUN, the Global UV Project, at http://www.who.int.uv/.

Summary of major health concerns

Exposure to the sun is known to be associated with different types of skin cancer, accelerated skin ageing, cataract and other eye diseases. There is also evidence that UV radiation reduces the effectiveness of the immune system.

Skin

Between 2 and 3 million non-melanoma skin cancers and over 130,000 malignant melanomas occur globally each year. A changing lifestyle and sun-seeking behaviour are responsible for much of the increase in skin cancers. In particular, frequent sun exposure and sunburn in childhood appear to set the stage for high rates of melanoma later in life. Depletion of the ozone layer, which provides a protective filter against UV radiation, may further aggravate the problem. Other chronic skin changes due to UV radiation include injuries to skin cells, blood vessels and fibrous tissue, better known as skin ageing.

Eye

Acute effects of UV radiation on the eye include photokeratitis, an inflammation of the cornea and iris, and photoconjunctivitis, an inflammation of the conjunctiva, the membrane that lines the inside of the eyelids. Long-term effects of UV radiation exposure of the eye may include the development of pterygium (white or creamy opaque growth attached to the cornea), and squamous cell cancer of the conjunctiva. Some 16 million people worldwide are currently blind as a result of cataracts; of these, WHO estimates that as many as 20% may be due to UV radiation exposure.

Immune system

The immune system is vulnerable to modification by environmental agents such as UV radiation, which appears to diminish the effectiveness of the immune system by changing the activity and distribution of the cells responsible for triggering immune responses. A number of studies indicate that environmental levels of UV radiation can suppress immune responses in both rodents and humans. In rodents, this immune suppression results in enhanced susceptibility to certain infectious diseases. It is therefore reasonable to suspect that exposure to UV radiation may enhance the risk of infection and decrease the effectiveness of vaccines in humans. However, additional research is necessary to substantiate this.

Vulnerable groups

Vulnerable groups Children are particularly sensitive to UV radiation and require special protection. More than 90% of non-melanoma skin cancers occur in fair skinned people who tend to burn. However, even though the incidence of skin cancer is lower in dark skinned people they are nevertheless susceptible to the damaging effects of UV radiation, especially to the effects on the eye and immune system.

Protective measures

Simple precautions will prevent both short-term and long-term damage of UV radiation exposure, while still making the time spent outdoors enjoyable. Sun protection is important in all settings, in particular at all outdoor recreation sites such as beaches and sports centres.

The basic sun protection messages

  • Limit exposure during midday hours.
  • Seek shade.
  • Wear protective clothing.Wear a broad brimmed hat to protect the eyes, face and neck.
  • Protect the eyes with wrap-around-design sunglasses or sunglasses with side panels.
  • Use and reapply broad-spectrum sunscreen of sun protection factor (SPF) 15+ liberally.
  • Protect babies and young children.

Staying out of the sun, either indoors or in shaded areas, during the four hour period around solar noon when UV radiation levels are highest is an effective means of protection. It should be combined with using clothing, hats and sunglasses. Sunscreen should be applied to parts of the body that remain exposed, like the face and hands. However, sunscreen represents a last line of defence, and should never be used to prolong the duration of sun exposure.

Some common myths:

False True
A sun tan is healthy. A tan results from your body defending itself against further damage from UV radiation.
A tan protects you from the sun. A dark tan on white skin only offers an SPF of about 4.
You can’t get sunburnt on a cloudy day. Up to 80% of solar UV radiation can penetrate light cloud cover. Haze in the atmosphere can even increase UV radiation exposure.
You can’t get sunburnt while in the water. Water offers only minimal protection from UV radiation, and reflections from water can enhance your UV radiation exposure.
UV radiation during the winter is not dangerous. UV radiation is generally lower during the winter months, but snow reflection can double your overall exposure, especially at high altitude.
Sunscreens protect me so I can sunbathe much longer. Sunscreens are not intended to increase sun exposure time but to increase protection during unavoidable exposure. The protection they afford depends critically on their correct application.
If you take regular breaks during sunbathing you won’t get sunburnt. UV radiation exposure is cumulative during the day.
If you don’t feel the hot rays of the sun you won’t get sunburnt. Sunburn is caused by UV radiation which cannot be felt. Most of the heating is caused by the sun’s visible and infrared radiation and not by UV radiation.

INTERSUN

INTERSUN, the Global UV project, is a collaborative project between WHO, UNEP, WMO, the International Agency on Cancer Research (IARC) and ICNIRP that aims to reduce the burden of disease resulting from exposure to UV radiation. The project assesses and quantifies health risks, and develops an appropriate response through guidelines, recommendations and information dissemination. Beyond its scientific objectives, INTERSUN provides guidance to national authorities and other agencies about effective sun awareness programmes. These address different target audiences such as occupationally exposed people, tourists, school children and the general public. For further information on INTERSUN refer to our web site: www.who.int/peh-uv


Sources: US Department of Health; The World Health Organization


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