Relative humidity (RH) is how much moisture the air holds versus the most it could hold at that temperature. It is one of the most controllable — and most overlooked — IAQ levers. The EPA's guidance is to keep indoor RH below 60%, and ideally between 30% and 50%. Stay in that band and you discourage the things that thrive at the extremes.
Too humid is the bigger hazard in the Gulf and coastal South. Above roughly 60% RH, mold can germinate on almost any organic surface that carries a little dust, and dust mites — a leading indoor allergen — flourish. The fix in humid markets like Florida and coastal Texas is often a whole-house dehumidifier ducted into the system, which dries the whole home rather than one room. Too dry is the opposite problem: in arid Arizona and during cold, dry northern winters, low RH brings static, cracked wood, and irritated sinuses, and a whole-house humidifier brings it back up.
The practical point is that the same comfort complaint — 'the air feels wrong' — has opposite solutions depending on climate, so the first move is to measure. An inexpensive hygrometer tells you which way you're off before anyone sells you equipment. A good IAQ contractor sizes humidity control to the home and the local climate, not to a one-size humidity gadget.