Detecting cigar/cigarette smoke levels
I'm not entirely sure if this will be useful in your situation - but the way your typical smoke detector works is by measuring the radioactive decay, apparently Americum is at least used in some of them.

But I just learned something new (thanks, Wikipedia!) - apparently some smoke detectors use optical sensors! 

So you should be able to do something similar in your lounge. It may take several sensors and combine their inputs. Obviously you'll have to deal with the fact that the smoke distribution probably isn't initially uniform. Potentially you could add an extractor to your room (not sure if that's desirable as I don't smoke) and measure the smoke levels there.

Not sure if there are any pre-built solutions, but perhaps you could gut several optical smoke alarms for their sensors.