Ultraviolet (UV) radiation
What is UV radiation?
The solar spectrum outside the Earth's atmosphere contains radiation in a wide range of wavelengths. Large parts of it are absorbed in the atmosphere. The atmosphere is only transparent for visible light, and parts of ultraviolet (UV) and infrared (IR) radiation. In general, the damaging effect of radiation increases with decreasing wavelength. UV radiation is defined as the radiation between 200 and 400 nm. Nearly all radiation with wavelengths shorter than 280 nm (UV-C) is absorbed by oxygen. The radiation between 280 and 315 nm (UV-B) is absorbed by atmospheric ozone, while the UV-A part (315-400 nm) is much less attenuated. The human skin is particularly sensitive to radiation in the UV-B region.
Which factors influence UV radiation?
The intensity of radiation in the UV-B region is governed by
- the elevation of the sun: the higher the elevation of the sun, the higher the UV-B radiation. This depends on the time of day, the time of the year and the location where you are.
- clouds: clouds mostly absorb (decrease), but can also reflect (increase) UV-B radiation.
- the total ozone content of the atmosphere (mainly stratospheric ozone): more ozone absorbs more UV-B radiation.
- the altitude at which you are: at high altitudes, the UV-B intensity is higher.
- the albedo of the surroundings: higher reflecting surroundings (snow, white sand) increase the intensity of the UV-B radiation.
- other atmospheric constituents (aerosols, trace gases): when present they decrease the UV-B radiation. Aerosols play an important role in climate forcing: they can reduce the amount of UV-B at the ground with about 5 to 35%.
In order to inform the public about the expected level of UV radiation the use of the so-called UV index has been recommended by the WHO (World health Organisation) and WMO (World Meteorological Organisation).
Why is UV-B radiation dangerous?
UV-B radiation is needed for the production of vitamin D (which is responsible for stronger bones and teeth), but it also has a negative impact on human health:
- influence on the skin: human skin is especially sensitive to UV-B radiation. On the short term, overexposure to UV-B radiation will lead to sunburn. On the long term, UV-B radiation influences the aging process of the skin, leading to an increased amount of wrinkles. More importantly, UV-B exposure significantly increases the risk of skin cancer.
- influence on the eyes: our eyes are also sensitive to UV-B radiation. Acute consequences are inflammations to the cornea and the conjunctiva. UV-B radiation is also known to play a role in the development of cataract.
- influence on the immune system: too much UV-B radiation can cause damage to the immune system.
Due to these negative effects of UV radiation, it is very important to pay attention and to use protection (sun screen, sun glasses, protective clothing) in the Sun whenever the UV index is high!
To inform the public about the risks related to the UV radiation, the UV index is forecasted and disseminated together with the weather forecast. An image with both the measured and forecasted UV indices for Uccle is updated several times per day on our website.