Moisture lingers in the air and on surfaces long after the mirror clears. Keeping the fan running for an extra 15 to 20 minutes after showers helps dry areas like grout, ceilings, and corners where moisture tends to settle.
Why it works: Bathroom exhaust fans work by pulling the hot, humid air up and out allowing less humid air to flow in. Mold spores thrive in humid environments. The post-shower extra ventilation lowers the moisture level making it harder for mold and mildew to grow and spread. The EPA suggests reducing moisture levels to below 60% relative humidity to reduce mold growth.
As an additional form of humidity control, installation of a humidistat can take the guess work out of the equation. A humidistat would automatically turn the fan on when humidity reaches a certain level. These devices can be installed next to, or in-combination with, the light switch and fan switch.
Additional tip: Make sure that your bathroom fan actually vents to the outdoors, and not to your attic space where this moisture can cause even bigger problems.