What is UV light?

UV light (ultraviolet light), also referred to as ultraviolet radiation and UV radiation is invisible to the human eye. It occurs naturally in sunlight and is produced artificially, normally by either heating a body to an incandescent temperature or by excitation of a gas discharge.

UV light is electromagnetic radiation and is transmitted in the form of waves, which can be described by their wavelength. Wavelength is the length of one complete wave cycle. UV light wavelengths are measured in nanometers (nm), where 1 nm = 1 millionth of a millimetre. It occupies a portion of the electromagnetic radiation spectrum which lies between x-rays and the blue end of visible light. This is the region from 100 nm to 400 nm.

A UV light source is best defined by its UV light emission spectrum which is the energy distribution as a function of wavelength. An example is provided, where the emission spectrum is shown relative to peak wavelength.

The biological, physical and chemical effects of UV light vary enormously with wavelength.

The International Commission on Illumination (CIE 1970) has defined the following spectral range classification bands, according to broad biological effects. The region 315 nm to 380 nm or 400 nm is designated as UV-A, 280 nm to 315 nm as UV-B and 100 nm to 280 nm as UV-C.

However, it must be emphasised that the dividing lines between the UV light classification bands are chosen as convenient distinctions. They do not represent boundaries where sudden, large changes occur in the biological effects of UV light. UV light energy is inversely proportional to wavelength, therefore biological and indeed physical and chemical effects change with UV light wavelength across the bands. The classification band(s) of a UV light source will however broadly indicate its potential to cause biological damage. The level of risk for adverse health effects to the unprotected skin and eyes from UV light exposure is determined by the UV light wavelengths present, UV light irradiance values and an individual’s exposure time. 

It is worth noting that some photobiologists consider the dividing line between UV-A and UV-B to be 320 nm, because most acute and chronic effects of sunlight exposure on biological systems are believed to occur at wavelengths below 320 nm. In addition, the division between UV-B and UV-C is sometimes re-defined at 290 nm, since this is the approximate lower limit of terrestrial radiation. 

The ICNIRP guidelines take the lower limit of the UV-C region to be 180 nm. This is because UV light below 180 nm is readily attenuated in air, and is therefore of little practical biological significance. UV light below 180 nm is known as vacuum UV (VUV) as for practical purposes it only exists in a vacuum.

The table below summaries the UV light classification bands and broadly speaking their potential to cause adverse health effects.

UV light classification band Spectral range Common descriptions Potential to cause adverse health effects
UV-A 315 nm – 400 nm Longwave or blacklight Least potential to cause adverse health effects due to the lowest energy. UV-A represents the largest UV component of sunlight, (approximately 90 %).
UV-B 280 nm – 315 nm Middlewave or erythemal Greater potential to cause adverse health effects than UV-A due to significantly higher energy. UV-B is partially absorbed by the ozone layer. It is the most aggressive component of sunlight and largely responsible for sunburn (erythema).
UV-C 100 nm – 280 nm Shortwave or germicidal Generally, the most potential to cause adverse health effects due to the highest energy. Only normally encountered from artificial UV light sources, since it is totally absorbed by the earth’s atmosphere. UV-C sources, particularly wavelengths less than 220 nm, in the presence of oxygen will produce ozone.

The depth of penetration increases with the increasing wavelength of UV light. Therefore UV-A light penetrates deeper than UV-B light which penetrates deeper than UV-C light.