My wife and I moved our business into a new building 18 years ago. The suite we were in also happened to contain the gas furnace that heated the entire building, so the city building inspector required a inspection of the furnace before we could open.
Turns out the furnace was completely buggered and had been belching out carbon monoxide at dangerous levels for who knows how long. We got it fixed of course, and later one of our neighbors in the building told us that his near constant headaches he’d been suffering from for ages went away.
CO detectors are a must I think. Certainly for commercial properties.