There is nothing wrong with these indoor venues. They are great venues and have provided shelter and warmth throughout the bitter winter months to everyone with the need to "get their groove on", but these venues are rarely adequate for the spring months. Noise restrictions that make it necessary to keep all doors and windows shut create a situation where heat and humidity build up quickly turning any party into a virtual sauna, or worse.
When selecting a venue, those with no central air conditioning are the first to be viewed as completely inadequate by promoters, for obvious reasons. What some do not realize though is that regardless of whether a venue has A/C or not, when the temperature and humidity inside start to climb, it is almost always for the same reason.
While setting up for any particular dance event, a lot of organizers sometimes unknowingly contribute to the problem that arises later in the night. They are usually running in and out of doors, gathering up equipment and setting it up. They may have all of the doors flung open for easy in-and-out access. They might even have some of the windows open. Unfortunately, while all this has been going on, huge amounts of humid air have filled the venue. It doesn't matter if it's warm or cold. If it is humid at all, that humidity is going to fill the air of the venue and it is going to be a while before the A/C can remove that humidity from the air again.
The dance crowd is also part of the problem, though they are more a symptom a multitude of other conditions. The human body hovers around 98° and generates one heck of a lot of heat just standing still. Dancing muscles create even more heat and sweat. The human body can sweat up to four liters per hour; for longer periods (up to 6 hours), 1 liter per hours is common. In addition to the sweat, your body also loses water vapor during respiration.
The event is being held for people to dance and they are there to dance, and so can't really be blamed, though they remain part of the problem. Most of the responsibility for temperature regulation falls on the organizers of the event. The temperature inside the venue may only increase 5° F or so and be only mildly uncomfortable to the hundred or so that are already there. The A/C might even be able to keep up at this point, but the more populous the party becomes the more the temperature and humidity will rise. After a certain point, there might just as well not be any A/C. This is especially true if there are no outside facilities to get fresh air, no outside facilities for those that smoke and "No Re-Entry". The perfect venue has now become the not-so-perfect sauna.
There are things that can be done to keep it from getting too bad though. Obviously, always try to use a building with A/C. Summer or Winter, doesn't matter. If you can't get a building with A/C, get a lot of fans or plan on having the event outdoor. Even if the venue does have A/C, get fans. You don't know that the A/C is going to be able to handle the number of people that will be in attendance.
Open any doors or windows you can, as soon as is necessary. Let some fresh air in. If there are noise restrictions, find out what time they are and make sure you get the venue opened up for a little while before it is no longer an option.
If you are attending the event, be sure to find out as much as you can about the venue and dress accordingly. If you think it's going to be really hot, dress in layers and strip 'em off, if you need to.
Drink Water. Whether you like it or not, drink it. Alcohol will dehydrate you more, so if you are going to drink alcohol, drink water too. Evaporation of your body water can happen quickly and sometimes before you are aware of it and your body functions are severely limited when you have lost 10% of your weight due to dehydration.
Don't block the fans! If you are standing in front of a fan to cool yourself off, you are preventing the fan from circulating the air properly and depriving about 100 other people from being able to cool off.
Piling in to a corner with a bunch of other people is not a really good idea either. The air needs to be able to circulate around your body so that the heat can dissipate faster, thereby keeping you cooler. Bodies piled on top of each other only creates more heat and a more dangerous situation.
There are a bunch of other things that can be done to keep things cool. The best thing to do is just use a little common sense, regardless of whether you are the organizers or the audience. It is more important than you think and people will remember it. Besides all of that, nobody wants to leave a party in an ambulance.