Ultraviolet radiation
Ultraviolet radiation (UV radiation) is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X‑rays, i.e., wavelengths from about 10 nm to 400 nm. However, the UV band is usually considered to span the region from 100 nm to 400 nm. In photobiology, UV radiation is subdivided into three main bands: UVC, UVB and UVA, representing the wavebands 100–280 nm, 280–315 nm and 315–400 nm, respectively. The boundaries between these three subdivisions, however, are somewhat arbitrary and vary according to disciplines. For instance, the wavelengths 290 nm and 320 nm are often used as boundaries between UVC/UVB and UVB/UVA, respectively.
UV Quantities and units
The most common UV quantities that are related to human exposure to UV radiation are: the dimensionless quantity UV index (UVI), the physical quantities irradiance and radiant exposure, and the erythemally weighted quantities erythemal irradiance (exposure dose-rate) and erythemally weighted radiant exposure (exposure dose).
Global solar UV radiation
The sun emits radiation throughout the electromagnetic spectrum from gamma‑rays to radio waves. This radiation is strongly affected by absorption and scattering when passing through the earth’s atmosphere. These processes filter out radiation of wavelengths less than about 290 nm (including the UVC band) and reduce the intensity of the UVB band. Therefore, global solar UV radiation is mainly composed of UVA and small amount of UVB. Another way to describe the composition of global solar UV radiation is that global solar UV radiation is made up of two components, direct and diffuse UV radiation.
Factors affecting global solar UV radiation
The main factors that affect solar UV radiation levels on the earth’s surface are:
Extra terrestrial solar UV radiation: This is the radiation that reaches the top of the earth’s atmosphere. This radiation varies depending on the 27‑day apparent solar rotation, the earth‑sun distance, which varies with an annual cycle, and the 11‑year cycle of sunspot activity.
Solar zenith angle (SZA): Global UV irradiance is inversely proportional to the SZA. As the SZA increases, the radiation takes a longer path through the atmosphere and, as a result, more absorption and scattering of the UV radiation will occur before reaching the earth’s surface.
Atmospheric ozone: Atmospheric ozone completely filters out any UVC radiation reaching the earth’s surface. In addition, it modifies the UVB radiation on the earth’s surface by controlling its intensity and cut‑off wavelength. A 10% decrease in ozone with all other factors constant will lead to a doubling of the radiation at 300 nm. Therefore, any variations in atmospheric ozone with season or altitude will influence global UV irradiance levels.
Cloud cover: At a given time, cloud cover affects local solar UV radiation and may lead to short‑term variations. Clouds can decrease the direct radiation mainly by reflecting some radiation back to space, but they can also increase the amount of diffuse radiation reaching the earth’s surface by scattering. The overall instantaneous effect, therefore, could be either an attenuation or increase in the local UV irradiance, depending on a combination of factors such as cloud type, cloud altitude, fraction of cover, cloud composition and geometry. However, attenuation is the dominant process.
Altitude: At higher altitudes, radiation takes a shorter path to travel through the atmosphere and, as a result, less UV radiation will be filtered out. Solar UV radiation could be increased by about 10% with every 1000 metre increase in altitude.
Surface albedo: Surface albedo ranges between 0.025 for grass and 0.8 for snow. The enhancement of global irradiance measured on a horizontal surface by high albedo snow can be as high as 28% for clear sky conditions and increasing significantly under partly cloudy conditions.
Effects of exposure to solar UV radiation on humans
Solar UV radiation has dual effects on human health. Small amounts of UV radiation are beneficial, while excessive exposure is responsible for several diseases. Fortunately, the penetration of UV radiation in human tissues is very weak and therefore the harmful effects are limited primarily to the skin and eyes.
Moderate exposure to solar UV radiation is pivotal for vitamin D production. This vitamin is essential for calcium absorption which in turn is important for healthy bones.
Excessive exposure to solar UV radiation may cause sunburn (erythema), skin photo-aging, skin cancer, photokeratitis and photoconjunctivitis, cataracts and pterygium.