When turned all the way up can old fashion gas heaters still be dangerous to one's health...the fumes?

Exhausting how?: It depends--some were put in the room with no external exhaust--these can cause c02 and carbon monoxide buildup. Properly installed and vented floor / wall gas heaters should not pose that kind of risk and should only "smell" if dust/paint on surfaces too hot when run high. Need to have a carbon monoxide detector in home and if smelly, have service check of heater.