Characteristics of UVA-induced delayed tanning: seasonal variation.

We studied seasonal variation in UVA-induced delayed tanning and compared it to that in UVA + B-induced delayed tanning. The minimal tanning dose (MTD) and the minimal melanogenic dose (MMD) were determined one week after UVA exposure or UVA + B exposure, respectively. The MTD determined in the summer (June to August) was significantly higher than that in winter (December to February). In addition, while MTD in the winter correlated well with skin color, there was no correlation between MTD and skin color in summer. MTD tended to correlate with self-reported tanning history at the initiation of tanning in the winter, but they were not correlated with each other in the summer. The MMD measured in the summer was significantly lower than that in the winter. MMD correlated well with skin color in both seasons; the lighter the skin, the smaller the MMD. Also, a correlation was detected between MMD and self-reported tanning history. These results suggest that the susceptibilities of delayed tanning to UVA and UVA + B are affected by different factors.