A humidifier takes standing water and puts it into the air you breathe all night. Anything growing in the tank goes with it. That is why humidifier maintenance is a health task rather than a housekeeping nicety, and why the manufacturers' guidance is consistently more frequent than people expect: rinse daily, disinfect weekly, descale as needed.
DAILY: EMPTY, RINSE, DRY. Every morning, pour out any water left in the tank and the base — do not top up on top of yesterday's water, which is how a biofilm establishes itself. Rinse both with clean water. Wipe the base dry with a clean cloth and leave the tank open to air while the unit is off. Refill with fresh water shortly before you use it. If you use tap water, expect more mineral scale; distilled or demineralized water dramatically reduces both scale and the fine white dust that ultrasonic units disperse.
WEEKLY: DISINFECT AND DESCALE. Unplug the unit and take it apart as far as the manual allows. For scale, fill the tank with undiluted white vinegar or a 50/50 vinegar and water mix, leave it 20 to 30 minutes, and use a soft brush on the mineral crust in corners and around the nozzle. For disinfection, follow with a diluted bleach solution if the manufacturer permits it — a common instruction is about a teaspoon of household bleach per gallon of water, left to stand 20 minutes. Never mix vinegar and bleach; rinse thoroughly between the two steps and again at the end until no smell remains. Air dry every part completely before reassembly.
FILTERS, WICKS AND CARTRIDGES. Evaporative humidifiers use a wick filter that traps minerals and should be replaced on the manufacturer's schedule, typically every one to three months, and sooner if it is stiff, discolored or smells. A crusted wick both stops working and becomes a growth surface. Demineralization cartridges in ultrasonic units also have a service life. Cleaning a spent filter does not restore it.
WHAT NOT TO DO. Do not put essential oils in a tank that is not designed for them — oils degrade plastics and void warranties, and the residue is difficult to clean. Do not run the unit continuously without checking humidity; over-humidifying a bedroom above roughly 50 percent encourages dust mites and mold in the room itself. Do not use abrasive scourers on the tank, since scratches give bacteria somewhere to hold. Do not skip the final rinse after bleach.
END-OF-SEASON STORAGE. Do a full deep clean, replace or discard the filter, dry every component completely, and store the unit disassembled with the tank cap off. Storing a humidifier damp is the reliable way to find it unusable next winter.
SIGNS IT NEEDS ATTENTION NOW. A musty or sour smell when it runs, visible pink or black film in the tank or base, white dust settling on bedroom surfaces, reduced mist output, or a new cough and congestion that appear only on nights you run it. Any of those means stop, deep clean, and replace the filter.
PICKING A HUMIDITY TARGET. Most guidance puts indoor relative humidity between about 30 and 50 percent. Below that, dry air irritates airways and sinuses; above it, the room itself becomes a favorable environment for dust mites and mold. An inexpensive hygrometer takes the guesswork out and is more useful than any feature on the humidifier.