Since the early nineteen-seventies, there has been a significant increase in the incidence of skin cancer, which is observed among peoples with predominantly pale complexion around the world. This is closely related to personal habits regarding sun exposure and its UV components, as well as public opinion that tanning is desirable and healthy. The value of the solar UV index is an important parameter that determines the need to use protective equipment when exposed to direct UV radiation. Everyone is exposed to ultraviolet radiation from the Sun and from many artificial sources used in industry, commerce and health recovery procedures.

The spectrum of the Sun’s radiation includes light, heat and ultraviolet (UV) rays. The ultraviolet range covers the wavelength ranging from 100 to 400 nanometers (nm) and is divided into three parts: (1) UVA (315-400nm); (2) UVB (280-315nm) and (3) UVC (100-280nm). When sunlight passes through the atmosphere, all UVC and approximately 90% of UVB radiation are absorbed by ozone, water vapor, oxygen and carbon dioxide. The atmosphere has less effect on UVA radiation. Therefore, the radiation that reaches the Earth’s surface consists mainly of UVA radiation and small amounts of UVB components.

UV index data for the territory of Bulgaria are accessible from NIGGG-BAS’s equipment .

What are UV Radiation levels dependent on?

  • Latitude – As one approaches the equator from the pole, the level of UV radiation increases.
  • Position of the Sun – The higher the Sun, the stronger the UV radiation. Its levels vary according to the time of day and year, as the position of the Sun changes in different hours and seasons. Outside the tropical regions, the maximum levels are reached when the sun assumes its highest position – at noon in the summer.
  • Cloud cover – The level of UV radiation is highest when there are no clouds. It can remain relatively high even in their presence due to reflection from different surfaces. Cloudy weather does not exclude the possibility of getting sunburned. The sun’s rays contain UV radiation, which causes sunburn and skin cancer and can also penetrate clouds.
  • Altitude – As altitude increases, the atmosphere gets thinner and absorbs less UV radiation. Every 1000 meters of increase in altitude correspond to 10% to 12% increase in UV radiation.
  • The ozone layer absorbs a significant portion of UV radiation that would otherwise reach the Earth’s surface. Ozone levels vary throughout the year and even throughout the day.
  • Reflection from the Earth’s surface – UV radiation is reflected or scattered to a certain extent by different types of earth surfaces. For example, fresh snow can reflect up to 80% of UV radiation, dry beach sand up to 15%, and sea foam up to 25%.

What is the UV index?

The UV index is a measure of the intensity of UV radiation on the Earth’s surface, which has an effect on human skin. Sun protection is required for UV index values above 3 and mandatory for values 8 and above. The UV index is different at different times of the day. One day the UV index may reach a value bigger than 3 for less than 30 minutes, while on a different day it may stay above 3 for a few hours.