Research Note Know Your Standard: Clarifying the CIE Erythema Action Spectrum Ann R. Webb* 1 , Harry Slaper 2 , Peter Koepke 3 and Alois W. Schmalwieser 4 1 School of Earth Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK 2 Laboratory for Radiation Research, The National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands 3 Meteorological Institute, Ludwig-Maximilians-University, Munich, Germany 4 Unit of Molecular Physiology and Biophysics, University of Veterinary Medicine, Vienna, Austria Received 24 August 2010, accepted 23 November 2010, DOI: 10.1111/j.1751-1097.2010.00871.x ABSTRACT The standard erythema action spectrum provides an internation- ally accepted representation of the erythema-inducing effective- ness of wavelengths in the UV part of the spectrum. The action spectrum forms the basis of the UV index used for public health information, defines the standard erythema dose unit and the minimum erythema dose and is the default response spectrum aspired to by a range of UV radiometer manufacturers. However, there are several versions of this erythema action spectrum in use, and only one of them has been endorsed as a standard. While the differences in erythemally weighted radiation incurred by choice of action spectrum will be no more than a few percent, this uncertainty is unnecessary. Here we detail the differences in the different versions of erythema action spectra, illustrate the resulting effects in quantifying UV doses and encourage readers to use only the standard version of the action spectrum in the future. INTRODUCTION The erythema action spectrum is widely used in assessing the negative effects of UV radiation sources (most frequently the sun) on human skin. Strictly, the action spectrum is for erythema (sunburn), but it is also used as a proxy in quantifying UV for other UV effects, e.g. skin cancer, vitamin D synthesis and even effects on plants and sometimes for weathering. The action spectra for these effects are similar but often not precisely known. Thus, many UV radiometers are in use that measure the erythemal UV directly with a single channel that has a response mimicking the erythema action spectrum (1). This is in part because it takes more detailed spectral measurements, which need more complex instruments, to precisely define alternative biologically weighted radiation. The erythema action spectrum is also the basis of the UV index (2,3), which is used globally in weather forecasting and for public health information. The UV index is simply erythemally effective UV scaled to give a dimensionless number between 1 and 12 (or more in extreme cases) that indicates the instan- taneous sunburning potential of the solar radiation. The total erythemal radiation received can be expressed in terms of standard erythema dose unit (SED) where 1 SED is defined as 100 J m )2 erythemally effective radiation (4). The minimum erythema dose (MED) is also based upon the erythema action spectrum, being the minimum dose of erythemal radiation that produces a faint but perceptible reddening of the skin 24 h after irradiation (5). However, the MED is not a standardized term but a personal measure of susceptibility to sunburn. Thus, both common instruments and derived units are based on the erythema action spectrum. The erythema action spectrum has come to refer, through common use, to the action spectrum proposed by McKinlay and Diffey (6,7). The action spectrum is a mathematical fit to experimental data and is represented by three straight lines on a semilog plot. It was the work of a Technical Committee of the International Commission on Illumination (CIE), intended to replace existing ‘‘Standard erythemal curves’’ from CIE, 1935 (8) and DIN (9), which did not account for the UVA portion of the spectrum. In addition to information in the UVA, the ‘‘new’’ erythema action spectrum was based on statistical considerations of (what was then) relatively recent experimental data, and it was deemed to represent a good ‘‘average erythema curve’’—recognizing that there is inevitable variation in human skin and its response to radiation. This CIE action spectrum gained broad acceptance and widespread use, and in 1998 was published as a CIE standard (10) and thereafter adopted by ISO (11)—we will refer to this version as CIE 1998 throughout. However, it is not widely recognized that there are subtle differences between the 1987 and 1998 versions of the CIE erythema action spectrum. Here we identify those differences, discuss their implications and reiterate the standard form of the erythema action spectrum. MATERIALS AND METHODS The 1987 versions. In the original publications (6,7) the authors showed a table for the erythemal weighting function with values with two significant digits (see Table 1, column 3) and the following set of equations (Eq. [1]). *Corresponding author email: ann.webb@manchester.ac.uk (Ann R. Webb) Ó 2011 The Authors Photochemistry and Photobiology Ó 2011 The American Society of Photobiology 0031-8655/11 Photochemistry and Photobiology, 2011, 87: 483–486 483