My OB told me to wait until 14 weeks to dye my hair, that it's not just the fumes, but what gets into your blood stream. You are putting chemicals straight onto your scalp! I've dyed my hair for 10 years so used the time I was pregnant to give it a rest and grow it out.
As for the dentist, I kept reading to avoid dental procedures while pregnant. In talking with a hygenist I met, she said they actually like pregnant women to see them for cleanings and check ups because pregnancy can wreak havoc on teeth-they get weaker and start having problems. I just had my first cavity. But to get a cavity filled, you get novicaine, so I'd check with your OB first. The first trimester is CRUCIAL for development.