Well, actually, that's untrue. Carbonylhemoglobin formation due to metabolic and environmental CO, and subsequent breakdown releases CO. There are, in fact, CO breath analyzers. Smokers and smoke inhalation patients have much higher levels, whereas ordinary healthy people have detectable, nonzero levels. For the point of detecting crowds, it's still meaningless.

>> I believe they used a combination of sound dB meters and carbon monoxide detectors to determine approx number of people and how loud they were.

> Carbon dioxide* detectors, people don’t produce carbon monoxide

Wouldn't smokers produce carbon monoxide when they smoked? That might be something they'd be legitimately interested in.

Thanks for the note! I tend to get all those things mixed up, I will try to remember this.