Does the heat in sauna or steam rooms trigger melasma?

I do believe heat can worsen one's melasma. In one study we did, we found that the brown was intimately associated with underlying blood vessels that dilate and get worse with heat. If the pigment is being fed by those blood vessels it would make sense the pigment would worsen as well, if not at least make it appear so.

Melasma can be caused by heat, steam, UV exposure, and hormones. Our office specializes in melasma treatment and we ask patients to begin application of Melarase AM and Melarase PM to combat the discoloration. Raffy Karamanoukian MD FACS

Yes, sauna and steam room use certainly can trigger melasma. As can standing over a stove or working in a restaurant kitchen or working outside in the heat. It's not just sunlight that causes the melanin increase so anyone with a tendency to hyperpigmentation needs to be fairly diligent in avoiding "hot" conditions as much as is possible.Sunscreens and blocks will help protect from the UV light exposure but won't have any protective factor for increased body heat caused by other situations.If you have to be in a heated situation, try to use a fan or water mist to keep your facial skin cooler.