If I had to do it again:
When the dentist took no precautions for himself or his assistant, I should have ran from the office screaming...
I would still take charcoal capsules before and after the sitting. Before I even left the office, I would also open a capsule in my mouth (I know it's gross) and swish it around. I'd repeat this several times. Alternatively, ask to use their washroom and swish water the best you can with freezing in. Other than this don't dilly-dally in the dentist office. He'll have some form of ventilation but just keep in mind that the vapour has 80% absorption rate.
Load up on vitamin C for a week before. None the morning of appt because it can interfere with freezing. If I had to do it again, I'd have made an appt with a nautropath for 2 hours after (when the freezing is coming out) and have 10 gms Vit C IV. Otherwise, take 5 or 6 grams a day (or to bowel tolerance) for a few days then down to 4. (Great apes take in about 6 gms a day and it may be what we are supposed to have anyway).
Magnesium - say 700 mg or to bowel tolerance (people with CFS do well with topical oils in which case we just go by symptoms. Malaise and drop in BP are 2 of the symptoms of high levels to be wary of) Vit E 400 - 1000 IU to compliment Vit C. Selenium 200 mcg/day (it binds free floating Hg to keep it out of mischief). Your B vitamins. Zinc 50 mg or so. Consider omega 3 and choline to help with cell repair.
It is going to come down to your genetics. Are you a Hg hoarder? I was, and I got laid-out. I'll cross my fingers for you.