Dermal dysplasia

From Memory Alpha, the free Star Trek reference.

Dermal dysplasia is better known as sunburn. It is caused by prolonged exposure to the ultraviolet radiation from a star.

In 1996, Lieutenant Tom Paris attempted to convince Lieutenant Commander Tuvok to remove his shirt and "enjoy the sunshine." Tuvok refused because he did not want to risk dermal dysplasia. (VOY: "Future's End")