I am in my 50s and lived in many different houses and apartments over the years. Only the last 2 places I have lived have I had this problem (one was a house, one an apartment). The tub drain emits a solvent-like smell that is extremely unpleasant. Its intensity varies and sometimes it is not present at all. But the smell is intolerable at its worst. I will start to get a headache being around it. What am I smelling? Is it dangerous to one's health? How do I get rid of it?

Sounds like rotting hair and grease from the soap. Try this. Tonight pour 12 gallon of bleach down the drain and let it set overnight. Next morning flush the mess out with a large pan of boiling water. If that doesn't get it click on back for plan "B". Good luck, Tom