Evaluation of skin sensitivity to UV radiation is a key component of the assessment of cutaneous photosensitivity, which is essential in the case of photodermatosis, photocarcinogenesis, sun protection, and use of phototherapy.1,2 One of the most commonly used parameters for measuring sensitivity is the determination of the minimal erythema dose (MED), which is defined as the minimum dose of UV radiation capable of inducing perceptible erythema in the skin. It is usually evaluated 24hours after irradiation.3,4

Visual detection of the MED is subjective, and its value can be affected by various factors, such as the light source, the increments used, the area irradiated, previous exposure to UV radiation, the definition of the MED, skin temperature, and intraobserver and interobserver variability.2 Differences in ethnicity also affect variability in the detection of the MED; therefore, measurement must be validated for the specific population.

Potential sensitivity to UV radiation can be evaluated using indirect methods without having to measure the MED. The most widely used is the Fitzpatrick skin type scale,5 although the reliability and validity of this system have been called into question, mainly because it is subjective and prone to recall bias.2,6,7 Nevertheless, Sánchez et al.8 suggest that the scale can prove to be reliable insofar as the evaluation method of this approach can be standardized.

In several countries, the MED has been determined taking into account the skin type, and some studies have shown that there is no favorable correlation between these variables.6,9–12 In Colombia, the only published study was performed in schoolchildren and did not find a correlation between MED for UV-B and phototype.12

Cutaneous erythema has been assessed more objectively using various instruments, including the tristimulus colorimeter (chromameter) and the narrowband simple reflectance spectrophotometer (Mexameter MX 18), which correlate well with the visual evaluation.13,14

The objective of the present study was to determine the MED for UV-A+UV-B, UV-A, and narrowband UV-B in a group of Columbian individuals with different skin types, the degree of correlation between MED and the skin types, and the degree of concordance and correlation between the 2 assessment methods (visual assessment and Mexameter MX 18).

The study population comprised 113 participants with a median age of 27 (18–56) years; of these, 66% were women. No participants had skin types V or VI. Fig. 1 shows the total number of individuals and their distribution by skin type. MED was evaluated for UV-A+UV-B and narrowband UV-B in 111 participants and for UV-A alone in 91 owing to difficulties in remaining for the time necessary to undergo the test. Erythema was not observed at the maximum dose of UV-A administered (100J/cm2) in 21 of the 91 cases (3 were skin type II, 7 skin type III, and 11 skin type IV), and in 14 participants, Mexameter MX 18 did not detect erythema (1 was skin type II, 7 skin type III, and 6 skin type IV). Table 1 and Fig. 2A–C show the median (IQR) MED by skin type and assessment method.

Figure 1.

Study Population. NBUVB indicates narrowband UV-B.

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Figure 2.

A–C, Box and whisker plots for UV-A+UV-B (MEDTOT), UV-A, and narrowband UV-B (NBUVB) for the MED values measured visually and using Mexameter MX 18 for skin types 1 to 4. Note the progressive increase in the median value, especially from skin type 2 to skin type 4. D–F, Diagrams showing the concordance-correlation coefficient between the visual measurement and measurement by Mexameter MX 18. Note the good correlation for the different wavelengths. MED indicates minimum erythema dose; NBUVB, narrowband UV-B.

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In order to compare our findings with the literature on this subject, we calculated the mean MED assessed visually, even though the sample was not normally distributed (Table 2).

In this study, we determined the MED for UV-A, narrowband UV-B, and UV-A+UV-B in a sample of the Colombian population. Investigations based on a solar simulator revealed that the mean MED of UV-A+UV-B for the different skin types varies; however, the values for skin types I to IV in this study population were similar to those reported by Pérez-Ferriols et al.17 in Spain, probably owing to the fact that our population comprises people of Spanish descent as a result of genetic admixture.18 In a study carried out in India, the mean MED value for skin type IV was similar to the results for this skin type in Spain and in the present study.19 For skin type III, the mean MED was similar in Australian Asians,20 Spaniards,17 and Colombians. In China, the mean MED values by skin type are much higher,21 probably because the study was based on buttock skin, which is less sensitive than that of the back,9,22 and to genetic differences (Table 5).

The mean MED value for skin types I–IV in studies carried out with other sources of UV-B (fluorescent UV-B lamps) in Brazil,11 in the USA,23 and for some skin types in the UK9 is similar to that of our population for UV-A+UV-B. In contrast, mean MED values in Korea,6 and more particularly, Singapore,24 are much higher for the same skin types (Table 6). These differences could be explained by genetic variability and because the skin irradiated in Singapore was on the buttocks.

If we compare the results of the present study with those reported from Medellin, Colombia,12 we see that in the latter the mean MED value by skin type was higher, probably because the definition of MED used was the minimum dose that led to erythema with well-defined borders.

Our literature review revealed few publications on MED values for UV-A, perhaps because of the difficulty obtaining a UV radiation source that emitted high doses of UV-A over short periods. The median MED values for UV-A in the present study, with the ranges obtained by Fitzpatrick,5 are similar for types I (22 vs 20–35J/cm2) and II (42 vs 30–35J/cm2), although they are higher for types III (86 vs 40–55J/cm2) and IV (100 vs 50–80J/cm2).

As for narrowband UV-B, the mean values for the Spanish population17 (obtained by applying a conversion factor to the MED results for UV-B) are similar to those of the present study for all skin types. In Detroit, USA, Carretero-Mangolis and Lim25 reported that the median MED was 600mJ/cm2 for skin types I-II and 700mJ/cm2 for skin types III-IV; both of these values are similar to those reported in the present study, that is, 550 and 770mJ/cm2, respectively. In 352 patients with psoriasis in the UK, the median MED values by skin type were lower: 200, 280, 390, and 550mJ/cm2 (skin types I–IV, respectively).26

Consistent with reports from other authors,2,11,12,24 we found that MED tended to increase with the skin type. However, the Fitzpatrick skin-type scale is not sufficient to distinguish well between values. In fact, MED values for UV-A+UV-B, UV-A, and narrowband UV-B overlap in different skin types, so that 2 people could have the same MED value but belong to different skin types.2,6,25,27,28

Some authors report no correlation between MED and skin type,6,9,12,29 whereas others do.11,24,30,31 After exploring this association, the present study found a correlation that can be classed as good for UV-A and UV-A+UV-B and moderate for narrowband UV-B.

The concordance correlation coefficient for MED assessed visually and using Mexameter MX 18 was very good for UV-A+UV-B, UV-A, and narrowband UV-B, as shown elsewhere.13,14 Taking these results into account, both forms of measurement are interchangeable; consequently, visual determination continues to be useful in clinical practice.

The limitations of the present study are its small sample size, especially in the case of skin type I, and the fact that the sample is not representative of the whole Colombian population, since we were unable to include individuals with skin types V and VI, who live in other geographic areas of the country. Furthermore, some individuals did not develop erythema at the maximum dose of UV-A administered (100mJ/cm2), sofor the data analysis, we assumed an MED value of 120J/cm2, which may not correspond to the real MED of these subjects.

In this study, where MED was determined for UV-A+UV-B, UVA, and narrowband UV-B in a group of Colombians with skin types I–IV, we found that MED values overlapped for different skin types. We also recorded a moderate to good correlation with the skin types and a good correlation between visual measurements and those obtained using an objective method (i.e., Mexameter MX 18).

This study was funded by Hospital Universitario Centro Dermatológico Federico Lleras Acosta, Bogotá, Colombia.

The authors declare that they have no conflicts of interest.