How is this possible? Probably a damaged metal halide bulb in the community center gym where the dance was held. Apparently this is a problem that occurs sporadically in gyms all over the country, as I found out after a google search: 

I'm very fair-skinned and burn easily, but I'm not the only one who was affected — the musicians got sunburns as well, and two of them have (mild) eye irritation symptoms. It's only when we compared notes yesterday that we realized what had happened. 

We will all be ok, and thankfully there haven't been any reports of dancers being affected — it was a large and lively crowd, and we kept them moving! Other than the burns, it was an absolutely wonderful dance.

The problem had not occurred before at this monthly event because the bank of lights that we now think the culprit bulb is a part of is not usually on. It was needed that night because some of the other lights had burned out. 

I would like to commend the dance organizers for taking the problem very seriously. Once informed, they immediately contacted the community center management, who in turn promised to address the issue right away.