Vitamin D and Health

3. Photobiology of vitamin D
Ultraviolet radiation
3.1 The sun is the main source of ultraviolet radiation (UVR) for most of the population. Artificial sources
of UVR may provide a significant proportion of the exposure for specific groups including those who
use artificial tanning facilities and those receiving UVR medical treatments.
3.2 Solar UVR forms the part of the electromagnetic spectrum from wavelengths of about 100-400 nm.
The International Commission on Illumination (CIE, 2011) 23 has defined sub-regions of the UVR
spectrum which take account of the transmission of the UVR in human tissue and potential health
effects, into the following categories: UVA (315–400 nm); UVB (280–315 nm); and UVC (100–280 nm).
3.3 The broad spectrum and intensity of the UVR emitted from the sun are due to its high surface
temperature. The quantity and spectral distribution of solar radiation at the Earth's surface depend
on the power output of the sun, the path of the radiation through the Earth's atmosphere and the
transmission properties of the atmosphere. Solar UVR undergoes absorption and scattering as it
passes first through the outer layers of the atmosphere and then the stratosphere and the
troposphere before reaching the Earth’s surface. The most important of these processes are
absorption by molecular oxygen and absorption by ozone.
3.4 The stratospheric ozone layer, formed between 10 and 40 km above the Earth's surface, prevents
almost all UVR of wavelengths less than 290 nm (UVC) and a substantial proportion (70–90%) of UVB
radiation from reaching the Earth. Therefore, the ground-level component of the solar UVR spectrum
consists of wavelengths in the range of about 290 to 400 nm. This means that only UVA and UVB are
relevant to human health. UVB accounts for about 5% of terrestrial UVR, the remainder being UVA.
3.5 Ground-level UVR consists of two major components: radiation received directly from the sun and
radiation that has been scattered by the atmosphere. The ratio of the scattered to direct radiation
varies with wavelength and with solar zenith angle (at 0° the sun is directly overhead and at 90° is on
the horizon from a horizontal viewpoint). The ratio increases as the wavelength decreases and the
solar zenith angle increases: UVB is scattered more than UVA and the amount of scattering increases
as the sun moves from above towards the horizon.
3.6 Human exposure to solar UVR depends on the amount of sunlight available (climate), then the time
spent outdoors and the level of exposure. The amount of sunlight available is determined primarily by
solar elevation (which depends on time of year and time of day) and weather (which will influence
outdoor activity and skin exposure). At middle-high latitudes where there are distinct seasons, the
winter months are characterised by low solar elevation, short day length and cloudy skies, which all
reduce the available solar UVR.
3.7 The solar zenith angle depends on season, time of day and latitude (Webb, 2006). When the solar
zenith angle is small (in summer, noon, at low latitudes) the sun is high in the sky and UVR has a
relatively short path through the atmosphere. When the solar zenith angle is increased (in the early
morning, late afternoon, during winter and at high latitudes), UVR has to pass through more ozone
which means that less UVB reaches the Earth’s surface. In the UK, the spectral UVR irradiance
(wavelength 300 nm) is theoretically at a maximum at solar noon (GMT), when the solar zenith angle
23 From its French title: Commission Internationale de l’Eclairage.
18