I find it really interesting you raise this topic. It hasn't come up much though I vaguely recall reading someone write about this very thing here or possibly elsewhere. I just wanted to validate your husband's experience.

Sunlight is so powerful as I am always reminded when walking. The difference in skin surface temperature between sun and shade is impressive and I can see where this could trigger some people. The UV may very well be doing something. I have noticed that my affected area will redden in the sun in a blotchy sort of fashion even if it is elevated. So something happens, I just don't know what....