Elemental hg is highly toxic . It vaporizes at room temperature and is absorbed in the alvioli ,then the blood where it becomes highly toxic hg2+. It can also be directly absorbed in the brain and oxidised to hg2+.With hg ,the dose determines the poisoning. If a child plays with a drop of hg for an hour ,the exposure is not that high. If you break a thermometer and clean it up that day,your exposure is not that great.If you break a thermometer and don't dispose of the hg for one year ,your exposure is high. I read that the hg vapor level in a room in which a thermometer had recently been broken was 10 microgms/m3.According to govt agencies, .03 microgms/m3 is the threshold level for
toxicity in non industrial settings.
If find it funny that a dentist would encourage one to vacuum the hg. That's the worst thing you can do . If a vacuum cleaner has been used in a hg contaminated room ,it should be thrown out . If the room has carpet, the carpet should be disposed of .
Sulfur turns brown (it is yellow ) upon exposure to hg so one can use it to test for hg.The best thing to do is get a machine to measure the vapor . If one gets the govt involved they might declare your house a superfund site (a slight exageration).
If you wash hg down the sink ,it becomes trapped in the plumbing and exposes you for years.