Bathroom Mold: Why It Comes Back

EPA guidance puts moisture control first for exactly this reason.

EPA guidance on mould and moisture is explicit that controlling moisture is how mould is controlled, and that cleaning without fixing the water problem means the growth returns. Before scrubbing, decide which source you have: shower steam that never leaves, a slow plumbing leak, condensation on a cold surface, or water entering from outside.

The relevant number is drying time, not humidity at one moment.

Run the shower as usual, then check the mirror and the ceiling twenty and forty minutes later. A bathroom still damp an hour after use is supplying the conditions mould needs, daily. Run the exhaust fan during the shower and for a stretch afterwards rather than switching it off with the light, and leave the door ajar so replacement air can reach the room.

A running fan is not proof of ventilation.

Hold a single sheet of tissue to the grille with the fan on; it should hold there without help. Then confirm where the duct goes. A fan discharging into an attic or a joist space simply relocates the moisture, and that ends as a larger problem above the ceiling.

Growth on the coldest surface points at the surface, not the room.

If the pattern follows an exterior wall, a ceiling under an uninsulated space, or a cold-water pipe, moist air is condensing where the surface sits below the dew point. Raising that surface temperature through insulation, air sealing, or gentle background heat stops the deposition. The indoor humidity guide covers reading a hygrometer against dew point rather than guessing.

Area, source, and health conditions all set the boundary.

EPA guidance treats mould covering roughly ten square feet or less as a job a householder can generally handle, with larger areas and contaminated-water events belonging to professionals. Stop and get advice sooner if the growth follows a hidden leak, sits inside HVAC equipment, or if anyone in the home has asthma, allergies, or a compromised immune system. Never mix cleaning products; some combinations release toxic gases.