The assessment of sensitivity of the human skin to sun rays is important in connection with photochemotherapy, diagnosis of photodermatoses, photo-aging, photo-carcinogenesis and photo-protection.
Fitzpatrick introduced a Working Classification of Sun-reactive Skin Types, which is based on a person's tendency to sunburn and capacity to tan, along with some racial parameters (1) which is not realistic and not practical for various reasons.
We determined the minimal erythema dose (MED) and minimal melanogenic dose (MMD) after solar stimulated irradiation in 54 healthy volunteers belonging to the six skin types.
We found a wide spread of MED values within any one of Fitzpatrick's skin types and a large overlap of their ranges. In some cases the MED values of one class fully overlaped those of a contigous class. We therefore conclude that the MED' s is neither an accurate nor a reliable parameter to represent the sensitivity of human skin to UV-irradiation.
Rather than relying on the MED, which is only one point on the erythema dose-response curves of our subjects based on quantitative measurements at five points. The UV energy was increased in steps that changed by a constant factor of V₂, so that of five spots irradiated on the forearm at least four showed an increasing redness (without edema or blistering). For the quantitative measurement of erythema and pigmentation responses we applied an objective method: we used a commercially available chromameter (Minolta Chromameter Reflectance II) instead of the human eye (2). In this way we could obtain reliable, and repeatable, measures of the color changes that occurred during the erythema and melanin pigment forming processes. The average steepness of the erythema and pigmentation dose-response curve appeared to be distinctive per constitutional skin color group.
When the 24-hours erythema dose-response curves of individuals with light complexion are measured, it appears that they all have a steep slope of approximately the same angle with the horizontal axis (big erythema dose-response angle), showing that a small increment in the UV dose gives rise to a considerahle increase in erythema (Fig.1).
Similarly, all dose response curves of individuals with a dark complexion make a shallow slope with the horizontal axis, demonstrating that a larger increase in UV energy is required to induce perceptible difference in erythema (Fig;2). As can be expected individuals of middle complexion have an average dose-response angle that lies in between those of the light and dark complexioned individuals.
The 7-days pigmentation dose-response curves are more or less reciprocal to the erythema dose-response curves e.g. individuals showing stronger erythema responses appear to show a larger darkening of the skin relative to the constitutional skin color (Fig. 3 and 4).
The correlation between skin color and dose-response angle is summarized in the scatter diagrams of Fig. 5 for the erythema response and Fig. 6 for the pigmentation response of all 54 individuals we studied.
We propose that, for routine use in clinical practice, the steepness of the dose-response function, in combination with the MED, is a better estimator of the photosensitivity of the human skin than the MED used in isolation.
Our finding that skin sensitivity can be estimated by the angle of the dose-response curves of either the erythema or the pigmentation responses can be of practical value for the phototherapy in light and dark color groups in which the dose increments should be small and large respectively.
It needs, of course, yet to be established whether the relation between constitutional skin color and the dose-response angle is also present in case of PUVA induced erythema and pigmentation. It is also conceivable that in phototesting, abnormalities exist in the steepness of the dose-response angle rather than in the MED range.

(1) Pathak MA et al. In : Dermatology in General Medicine (eds TB Fitzpatrick et al) 3rd ed. 1987, 1507-1516.
(2) Westerhof W et al. Photodermatol 3:310-314, 1986.

W. Westerhof
Academic Medical Center
Meibergdreef 9
NL - 1105 AZ Amsterdam