The UV index is a measure (number) of the amount of ultraviolet (UV) radiation in the sunlight that reaches the ground on earth. This page contains more information about this type of radiation.

The radiation from the sun is made up of particles of light called photons. Each of these photons has a wavelength, which is typically expressed in nanometres. A nanometre is very small, measuring one billionth of a metre. The wavelength determines whether we can see a photon and what it looks like to us.

  • UV radiation consists of photons with a wavelength between 100–400 nanometres. These are invisible.
  • We see photons with a wavelength of 450 nanometres as blue.
  • We see photons with a wavelength of 580 nanometres as yellow.
  • We see photons with a wavelength of 620 nanometres as red.
  • Photons with a wavelength greater than 700 nanometres are known as infrared light. Although we cannot see this, we can feel it as heat.

UV radiation

UV radiation is divided into three categories: UV-C, UV-B and UV-A. These are explained below:


UV-C radiation is filtered from sunlight by the ozone layer and therefore does not reach the earth’s surface. UV-C radiation has a wavelength of 100–280 nanometres. Radiation of this type generated by lamps is used for disinfection. UV-C rays with shorter wavelengths do not penetrate the skin, while the longer ones remain in the upper layer of the epidermis and do not reach the stem cells in the basal layer.


Of the different categories, UV-B radiation is most strongly influenced by the thickness of the ozone layer (and the further composition of the atmosphere). Because this is different everywhere, UV-B rays are also different all over the world. UV-B rays have a wavelength of 280–320 nanometres and do not pass through glass. Structures of important molecules in the skin, such as DNA, have dimensions similar to the wavelengths of some UV-B radiation. Because of this, UV-B radiation is biologically significant, despite the low intensity of the UV-B rays we receive on the earth’s surface. UV-B radiation penetrates through the stratum corneum and epidermis to the basal layer, where the stem cells of the skin are located. If these cells are affected and mutation occurs, it can result in skin cancer.


UV-A radiation has a wavelength of 320–400 nanometres (where the spectrum of visible light begins). This radiation is not filtered by the ozone later and can pass through glass. Exposure to UV-A radiation ages the skin. UV-A radiation cannot cause direct DNA damage. However, it does penetrate deep into the skin down to the dermis, where it can trigger the release of free radicals. These free radicals can continue to cause DNA damage hours after sun exposure and are thought to contribute to melanoma formation.
UV radiation during the spring and summer
In the spring and summer months, the daily UV index in the Netherlands is usually between 3 and 7. This is based on measurements over the past 26 years. On 15% of days in the summer months, the UV index is higher than 7. However, the UV index is also lower than 3 on 15% of days in the summer months. In April, the UV index can sometimes be higher than 5. In the past 26 years, the UV index in the Netherlands has only been higher than 8 a few times.
The UV index can be much higher in other countries. In Suriname, for instance, it is regularly higher than 13.

Grafiek met het verloop van zonkracht per maand met de percentielen. Uitleg staat onder het kopje ‘UV-straling tijdens de lente en zomer’