Don't know what they're called exactly, and haven't worked in a dental office in a few years, but they were similar to the badges that check for x-ray exposure, but they were mercury sensors that I assumed checked mercury vapor in the air.
Did a quick google check and this page mentions 3M lapel badges to monitor different hazardous fumes, mercury vapors being one of them.
http://www.hoslink.com/Ellis/ALARM.htm