[Organizers] When to cancel dance?

With the current temps in the US at an alarming high, our dance hall has been running hot, with their one A/C unit cooling to 75-78 degrees while dancing. We keep it pretty windy, with lots of fans (quietest ones in the hall are small Lasko units and louder ones in the back). That being said, at what point do you cancel the dance because of heat? Dancers are already complaining and all we can do is keep it windy. Of course, we could cut our losses if minimum capacity of dancers are not coming because of the heat. In the past & at another hall, we used to cancel all of August because of heat, but I don't know how you can make that call because you really don't know how it will feel (or how many dancers will attend) until you get there. Deanna Palumbo Chattahoochee Country Dancers, Atlanta, GA