Fact Sheet No 133
GLOBAL SOLAR UV INDEX
The Global Solar UV Index estimates the average maximum solar ultraviolet radiation (UV) at the Earth's surface. While the intensity of UV reaching the ground varies during the day, it reaches a maximum, when there is no cloud cover, around mid-day. The calculation for the UV Index allows for the different skin damaging effect from each UV wavelength and averages out variations over a 10-30 minute timespan. The Index is generally presented as a forecast of the maximum amount of skin-damaging UV expected to reach the Earth's surface at solar noon. The values of the Index range from zero upward and the higher the Index number, the greater the likelihood of skin and eye damaging exposure to UV, and the less time it takes for damage to occur.
In many countries close to the equator, summer-time values can range up to 20. During the European summer the Index is generally not more than about 8, but can be higher, especially at beach resorts. The following descriptions are usually associated with various values of the Index: Low UV exposure - 1 and 2; Moderate exposure - 3 and 4; High exposure - 5 and 6; Very high exposure - 7 and 8; Extreme exposure - greater than 9.
Recommendations on the calculation, expression and dissemination of the Global Solar UV Index were made in 1995 by WHO, the World Meteorological Organization (WMO), the United Nations Environment Programme (UNEP) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP). National authorities throughout the world now use the Global Solar UV Index. These international organizations recommended that:
Multiple UV Indices
There has been a proliferation of indices in various countries, especially in Europe, that provide different measures of the levels of UV to which people are exposed from the sun. These UV indices may be promoted by some sun-screen or cosmetic manufacturers for commercial purposes or by local authorities unaware that there has been widespread acceptance of an international agreement on the use of the Global Solar UV Index. Using the standardized Global Solar UV Index at the same time as other UV indices may, however, lead to public confusion about the important health messages related to different Global Solar UV Index values. The purpose of the Global Solar UV Index is to provide uniform information to the public about daily UV exposure levels so that consistent messages can be provided on what protective measures are necessary with various index values.
Summary of major health concerns
Exposure to the sun is known to be associated with various skin cancers, accelerated skin ageing, cataract and other eye diseases. There is laboratory evidence to suggest that UV exposure may adversely affect people's ability to resist infectious diseases and to compromise the effectiveness of vaccination programmes.
Chronic skin changes due to UV include skin cancer (both melanoma and non-melanocytic), benign abnormalities of melanocytes, and a range of other chronic injuries resulting from UV exposure to keratinocytes, blood vessels and fibrous tissue, often described as "photo ageing" (degeneration of the elastic connective tissue of the skin). The much-increased rates of skin cancer in patients with xeroderma pigmentosum (a rare pigmentary disease that reduces a person's ability to repair DNA damage caused by UV exposure) suggests that direct UV damage of the DNA is a factor in the cause of these cancers.
UNEP has estimated that more than 2 million non-melanoma skin cancers and 200,000 malignant melanomas occur globally each year. In the event of a 10% decrease in stratospheric ozone, an additional 300,000 non-melanoma and 4,500 melanoma skin cancers could be expected worldwide. Dark-skin populations have a lower risk of skin cancer than Caucasians because of the protection offered by skin pigmentation.
The worldwide incidence of malignant melanoma continues to increase and is strongly related to frequency of recreational exposure to the sun and to history of sunburn. There is evidence that risk of melanoma is also related to intermittent exposure to UV, especially in childhood, and to exposure to sunlamps, although studies conducted so far on the latter have not consistently controlled for other factors that could influence the results.
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 exposure. Experts believe that each 1% sustained decrease in stratospheric ozone would result in an increase of 0.5% in the number of cataracts caused by solar UV. UV exposure of the eye depends on many factors: ground reflection, the degree of brightness in the sky leading to activation of the squint reflex, the amount of atmospheric reflection, and the use of eyewear.
The acute effects of UV on the eye include the development of photo keratitis (inflammation of the cornea and iris) and photo conjunctivitis (inflammation of the conjunctiva, the membrane that lines the inside of the eyelids), which are painful but reversible and easily prevented by protective eyewear. Chronic effects include the possible development of pterygium (white or creamy opaque growth attached to the cornea), squamous cell cancer (scaly or plate-like malignant tumour) of the conjunctiva, and cataracts. There is sufficient evidence to link acute ocular exposure to photo keratitis, but knowledge of the effects of chronic exposure is less certain.
The immune system is vulnerable to modification by environmental agents such as UV, which appears to alter immune response by changing the activity and distribution of the cells responsible for triggering these responses. A number of studies indicate that UV exposures at environmental levels suppress immune responses in both rodents and humans. In rodents, this immune suppression results in enhanced susceptibility to certain infectious diseases with skin involvement, and some systemic infections. Mechanisms associated with UV-induced immunosuppression and host defence mechanisms that provide for protection against infectious agents are similar in rodents and humans. It is therefore reasonable to assume that exposure to UV may enhance the risk of infection and decrease the effectiveness of vaccines in humans, but additional research is necessary to substantiate this.
Effective UV protection programmes could realise substantial savings in health care costs. The US Government alone spends US$3.4 billion per year for 1.35 million cataract operations, while India borrowed US$120 million from the World Bank to reduce its backlog of cataract surgeries. Deaths and disfigurement from skin cancer likewise pose a large burden on health care systems. Up to 20% of cataract operations could be prevented or delayed, and the incidence of skin cancer greatly reduced, by informing the public about UV-induced health effects and the simple techniques required to protect against excessive exposure.
Methods for personal protection from solar UV exposure include adequate clothing, hats and the proper use of sunscreens to protect UV-exposed skin. For eye protection, UV absorbing sunglasses are needed.
Changes in behaviour could minimize solar UV exposure. These include staying out of the sun, either indoors or in shaded areas, during the four-hour period around solar noon when UV levels are at their highest. During summer, when daylight saving time is in effect, solar noon in most of Europe is at 14.00 hours (2 p.m.); in the UK and countries with a similar longitude, it is at 13.00 hours (1 p.m.).
Broad-spectrum (UVB and UVA) sunscreens should be used when other means of protection are not feasible, and then to reduce exposure rather than lengthen the period in the sun. While topical applications of sunscreen are preferred for absorbing UVB, some preparations do not absorb the longer wavelength UVA effectively; moreover, some have been found to contain ingredients that are mutagenic in sunlight. People using sunscreens should use those with a high sun protection factor and be aware that they are to protect from the sun and not for tanning purposes.
The reflective properties of the ground influence UV exposure. Grass, soil and water reflect less than 10% of incident UV. However, fresh snow reflects nearly 80% while sand reflects 10-25%, significantly increasing UV exposure for skiers and bathers.
INTERSUN - the Global UV Project
INTERSUN is a collaborative project between WHO, UNEP, the International Agency for Cancer Research (IARC), ICNIRP and specialist agencies at the international and national level. Centres collaborating with WHO on INTERSUN include:
The objectives of INTERSUN are to:
INTERSUN aims to assist national authorities with advice on programmes to prevent excessive UV exposure and thereby reduce health care costs.Further information on the INTERSUN project can be obtained from the Project home page at: http://www.who.ch/uv/.
For more information please contact Health Communications and Public Relations, WHO Geneva, Tel (41 22) 7912599, Fax (41 22) 791 4858, E-Mail: firstname.lastname@example.org; or Dr. Michael Repacholi, Environmental Health Research, Global Hazards Assessment and Radiation Protection, Tel (41 22) 791 3427; E-Mail: email@example.com. All WHO Press Releases, Fact Sheets and Features as well as other information on this subject can be obtained on Internet on the WHO home page http://www.who.ch/