Ultraviolet radiation-induced erythema in human skin
Introduction
Erythema, or skin redness caused by inflammation, is the most widely used clinical endpoint in human skin photobiology. Erythema is also used in determining the sun protection factor (SPF) of sunscreens. Despite its widespread use, we know little of the basic biology of ultraviolet radiation (UVR)-induced erythema and the reader is referred to a review by Clydesdale et al. [1].
Individual sensitivity to UVR is assessed by the minimal erythema dose (MED), most often 24 h after irradiation. The MED is defined as the lowest UVR dose that will cause either a just perceptible redness or redness with a definite border. One study has shown that the former definition is more reliable [2].
The MED is judged by eye after a predetermined UVR dose series has been given to the individual in question. However, this subjective approach, in which the MED is a threshold in a dose–response curve, discards a large amount of potentially valuable data. The eye, although excellent at assessing threshold responses, is poor at the quantification of color. The best that can be done is semiquantitative grading such as +, ++, +++. The development of reflectance devices such as the Dia-Stron erythema meter enables the quantification of erythema and the construction of dose–response curves [3], [4], [5]. Erythema meters work by measuring the decreased green reflectance, relative to red, from hemoglobin in dermal blood vessels [3], i.e., the change in green to red ratio.
Several workers have determined action spectra (relative effectiveness at different wavelengths) for the MED in human skin [6], [7], [8], [9] using “monochromatic” radiation, in reality narrow- to broad-spectrum radiation, from a monochromator. Some of the earlier studies have been pooled to generate a reference erythema action spectrum that has been adopted by the Commission Internationale de l'Eclairage (CIE) [10]. These studies show that UVB (280–315 nm) is orders of magnitude more effective per unit dose (J/m2) than UVA (315–400 nm). For example, the median MED at 300 nm is whereas that at 360 nm is [9]. A comparison of the action spectra for erythema and epidermal DNA photodamage provides strong circumstantial evidence that DNA is a major chromophore for erythema [9]. The Anders et al. [7] action spectrum, determined from truly monochromatic laser radiation, shows a secondary peak at 362 nm, which is strongly indicative of an additional chromophore for UVAI (340–400 nm) erythema and is almost certainly mediated via reactive oxygen species.
The MED is a measure of individual sensitivity to UVR. Recently, a new term, the standard erythema dose (SED), has been proposed [11]. The SED is a fixed dose of that is biologically weighted by the CIE erythema action spectrum [10] and the emission spectrum of the UVR source. Thus, the SED is independent of the emission spectrum of the UVR source.
Our laboratory has had considerable experience in the assessment of human erythema with different UVR sources and in different skin phototypes. This article describes our protocols, data analysis, and interpretation. We have also transformed our MED data to SED to assess the value of the SED concept. The data come from a variety of studies, all of which had approval from the local ethics committee.
Section snippets
Volunteer selection
Volunteer selection was based on the Fitzpatrick phototype [12]. Complexion, ability to tan, and capacity to sunburn in summer sunlight were all taken into consideration by interview and questionnaire when assigning skin type.
Dosimetry and UVR sources
Emission spectra and irradiances were determined with a DM150BC double-monochromator spectroradiometer (Bentham Instruments, Reading, UK) using an integration sphere and gratings blazed at 250 nm. The spectroradiometer was calibrated using a deuterium lamp that had been
Results
Most of the data are plotted as a function of MED and SED using either the CIE [10] or the Anders et al. [7] erythema action spectrum. The former covers the entire range of the SSR source whereas the latter does not provide data beyond 374 nm.
Concluding remarks
We believe that our data confirm the importance of quantifying the erythema response [3], [4], [5] and that this can be readily done with an instrument such as the Dia-Stron erythema meter. The erythema response to UVR exposure may be given or expressed as a function of either biological dose (MED) or physical dose (J/m2) as shown by Kelly et al. [18]. When data are expressed as J/m2 there is a clear difference between the skin types. However, when erythema is plotted as a function of MED, the
Acknowledgements
We thank Deirdre Kelly and John Sheehan for the use of erythema data generated from their Ph.D. studies, and Professor Brian Diffey for providing us with the interpolated Anders et al. [7] action spectra data. The work described was supported by Contracts from the EC (ENV-4-CT97-0556) and the UK Department of Health (121/6379).
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