Yesterday I played the Northboro contra dance with Kingfisher, and I made the same CO2 measurements I did at BIDA a few weeks ago. This is a very different environment:

• 30 vs 80 attendees
• Eight big windows with no fans vs two doors with big barrel fans
• Maybe 1/3 the size

Here's what I measured, compared to last time:

The overall shape is very similar: rising as dancers arrive and start dancing, falling during the break, rising again after the break, and then falling at the end. BIDA's break was probably slightly later, and it took a little longer for the dancers to leave the home at the end of the night.

Because atmospheric CO2 is ~400ppm, this chart makes the two cases look a bit more similar than they really are. Let's subtract 400 ppm from each and look at CO2 levels above baseline:

Looking at the area under the curve, a dancer at Northboro likely experienced about 40% of the risk of one at BIDA (448 vs 1,116 ppm hours of excess CO2).

Putting fans in the windows would likely cut this even further, especially if you have them blow across the hall and match them to the prevailing wind.