Ultraviolet radiation and health

What is UV radiation?

Everyone is exposed to UV radiation from the sun and an increasing number of people are exposed to artificial sources used in industry, commerce and recreation. Emissions from the sun include visible light, heat and UV radiation.

The UV region covers the wavelength range 100-400 nm and is divided into three bands:

• UVA (315-400 nm)
• UVB (280-315 nm)
• UVC (100-280 nm).

As sunlight passes through the atmosphere, all UVC and approximately 90% of UVB radiation is absorbed by ozone, water vapour, oxygen and carbon dioxide. UVA radiation is less affected by the atmosphere. Therefore, the UV radiation reaching the Earth’s surface is largely composed of UVA with a small UVB component.

Environmental factors that influence the UV level

• Sun height—the higher the sun in the sky, the higher the UV radiation level. Thus UV radiation varies with time of day and time of year, with maximum levels occurring when the sun is at its maximum elevation, at around midday (solar noon) during the summer months.
• Latitude—the closer the equator, the higher the UV radiation levels.
• Cloud cover— UV radiation levels are highest under cloudless skies. Even with cloud cover, UV radiation levels can be high due to the scattering of UV radiation by water molecules and fine particles in the atmosphere.
• Altitude—at higher altitudes, a thinner atmosphere filters less UV radiation. With every 1000 metres increase in altitude, UV levels increase by 10% to 12%.
• Ozone—ozone absorbs some of the UV radiation that would otherwise reach the Earth’s surface. Ozone levels vary over the year and even across the day.
• Ground reflection—UV radiation is reflected or scattered to varying extents by different surfaces, e.g. snow can reflect as much as 80% of UV radiation, dry beach sand about 15%, and sea foam about 25%.

Ozone depletion and UV-related health effects

Depletion of the ozone layer is likely to aggravate existing health effects caused by exposure to UV radiation, as stratospheric ozone is a particularly effective UV radiation absorber. As the ozone layer becomes thinner, the protective filter provided by the atmosphere is progressively reduced. Consequently, human beings and the environment are exposed to higher UV radiation levels, and especially higher UVB levels that have the greatest impact on human health, animals, marine organisms and plant life.

Computational models predict that a 10% decrease in stratospheric ozone could cause an additional 300,000 non-melanoma and 4500 melanoma skin cancers and between 1.6 and 1.75 million more cases of cataracts worldwide every year.

Health consequences of excessive solar UV radiation

New WHO report shows breakdown of disease caused by UV radiation

25 JULY 2006 | GENEVA -- Ultraviolet radiation from the sun causes a considerable global disease burden, including specific cancers, a new World Health Organization (WHO) report finds. Much of the UV-related illness and death can be avoided through a series of simple prevention measures.

The report, Global Burden of Disease of Solar Ultraviolet Radiation estimates that up to 60,000 deaths a year worldwide are caused by too much exposure to ultraviolet radiation (UVR). Of those 60,000 deaths, an estimated 48,000 are caused by malignant melanomas, and 12,000 by skin carcinomas.

In total, more than 1.5 million DALYS ("disability-adjusted life years") - a measure of the loss of full functioning due to disease and death are lost every year due to excessive UVR exposure. The most serious consequence of excess UVR is malignant melanoma, which has high cure rates only if detected early. Up to 90% of the global burden of disease from melanoma and other skin cancers are estimated to be due to UVR exposure.

The new WHO report is the first-ever systematic examination of the global health burden due to UVR. It investigates nine adverse health outcomes from excess UVR exposure. The main three, which cause the greatest burden of disease from UVR, are cutaneous malignant melanomas, and non-melanoma skin cancers developing in different cell layers of the skin (squamous cell carcinomas and basal cell carcinomas). In addition, UVR causes sunburn, skin photoageing, cortical cataracts (eye lens opacities), pterygium (a fleshy growth on the surface of the eye), reactivation of herpes of the lip (cold sores) and the rare squamous cell carcinomas of the eye.

"This global assessment of the health risks of UV radiation provides a good basis for public health action. We all need some sun, but too much sun can be dangerous - and even deadly. Fortunately, diseases from UV such as malignant melanomas, other skin cancers and cataracts are almost entirely preventable through simple protective measures," said Dr Maria Neira, Director for Public Health and the Environment at WHO.

The report notes that UVR does have beneficial effects, mainly in the production of vitamin D following skin exposure to the UVB (shorter wavelength) component of UVR. Adequate vitamin D prevents the development of bone diseases such as rickets, osteomalacia and osteoporosis. Moreover, the possible beneficial effects on some cancers and immune disorders are under investigation.

WHO notes, in most cases minimal casual exposure to UVR should be sufficient to maintain vitamin D levels at a range that avoids these health problems. The dangers are much greater from over-exposure to the sun's radiation.

A few easy-to-implement sun safety measures could prevent much of the cancer and other death and disease burden due to UV radiation, WHO says:

• Limit time in the midday sun
• Use shade wisely: seek shade when UV rays are most intense
• Wear protective clothing including hats and sunglasses
• Use a broad-spectrum sunscreen of sun protection factor 15+
• Avoid sunlamps and tanning parlours; for youth under the age of 18, WHO recommends that they do not use them at all
• Know the UV index: when the UV Index predicts radiation levels of 3 (moderate) or above sun safety practices should be taken
• Protect children from the sun

In conjunction with the launch of this new report, WHO and the World Tourism Organization of the United Nations (UNWTO) are joining forces to multiply global outreach efforts to inform the public and reinforce measures to prevent avoidable diseases. Through the UNWTO, WHO will be distributing information, including its new UV flyer on healthy sun habits, to all national ministries responsible for tourism.

http://www.who.int/uv/uv_and_health/en/index.html