Humidifier care: keep it at 30 to 50% and clean it every third day

Aim for 30 to 50% on a hygrometer

CPSC and Mayo Clinic both put the target for home humidity between 30% and 50%. EPA’s humidifier page sets the ceiling plainly: don’t humidify above 50% relative humidity, because higher levels may encourage the growth of mold and other organisms. Its mold guide says to keep indoor humidity below 60% if you can, ideally between 30 and 50%. [S1] [S2] [S3] [S4]

You can’t judge that by feel. A hygrometer, sold at hardware stores, reads the humidity in the room, and some humidifiers have a built-in humidistat you can set to a level. [S1] [S3] [S4]

Water condensing on windows, walls or pictures is a sign of too much humidity. EPA says to move the humidifier, lower its setting or run it less, and not to let carpet, drapes or tablecloths near it get damp. [S1] [S2]

Empty, dry and refill every day

CPSC says bacteria and fungi often grow in the tanks of portable and console humidifiers and can come out in the mist. Breathing that mist may cause lung problems ranging from flu-like symptoms to serious infection. [S3]

EPA’s routine for portable units: unplug it, empty the tank, wipe every surface dry and refill with fresh water, every day. CPSC adds to empty the tank before you fill it rather than topping it off. For console models, follow the maker’s instructions for changing the water. [S1] [S3]

Mayo Clinic says the daily change is especially important for cool-mist and ultrasonic models. [S4]

Scrub it every third day

EPA says to clean portable humidifiers every third day. Unplug it, empty the tank, and use a brush or other scrubber to remove any scale, deposits or film on the tank and interior surfaces, then wipe everything dry. [S1] Mayo Clinic gives the same three-day schedule. [S4]

Film or scum on the water, on the tank or on exposed motor parts may mean bacteria or fungi are growing, according to CPSC. Crusty scale is minerals settling out of the water, and it gives germs a surface to grow on. Scrub carefully: CPSC says not to scratch the tank or damage the motor. [S3]

1. Unplug it first. EPA, CPSC and Mayo Clinic all say this. [S1] [S3] [S4]
2. Use what the manual says. EPA says to follow the maker’s advice on cleaners and disinfectants. Without one, it suggests a 3% hydrogen peroxide solution on every surface that touches water. [S1]
3. Rinse well. After any cleaner, EPA says to rinse the tank with several changes of tap water so the chemical isn’t sprayed into the room. [S1]
4. Deal with the filter. Change a filter at least as often as the maker suggests, and sooner if it looks dirty. CPSC says to clean or replace sponge filters or belts when needed. [S3] [S4]
5. Add nothing to the water. Mayo Clinic says not to add anything to the water or use disinfectants that aren’t approved. [S4]

Distilled water and white dust

Ultrasonic and impeller humidifiers are very efficient at spraying tap water minerals into the air, EPA says. Those minerals can settle as a fine white dust on furniture, and CPSC notes the dust may contain particles small enough to reach the lungs. Minerals also build scale inside the tank, which EPA calls the most compelling reason to find an alternative to tap water. [S1] [S3]

Not every bottled water helps. EPA says water labeled distilled will likely have less mineral content than most tap water. Deionized and reverse osmosis water remove most minerals but generally less than distillation, and “purified” water can come from any of those processes. Spring, artesian and mineral water haven’t been treated to remove minerals. [S1]

Demineralization cartridges or filters are worth using if your humidifier came with them or the maker recommends them. EPA says how well they work varies widely; white dust is the sign they aren’t keeping up. [S1] [S3]

Your humidifier type changes the risk

EPA groups home humidifiers by how they make moisture. The ones that spray water as a cool mist generally disperse the most germs and minerals from the tank. The ones that evaporate or boil it disperse less, if any, though they can still grow germs if they hold standing water. [S1]

Putting it away for the season

When you’re done for the year, or won’t use it for a while, EPA says to clean it as directed and make sure every part is dry. Throw out used demineralization cartridges, cassettes or filters, and store the unit somewhere dry. When it comes back out, clean it again and dust off the outside. [S1] [S3]

Mayo Clinic suggests thinking about replacing an old humidifier, since buildup can become hard or impossible to remove and makes bacteria more likely to grow. [S4]

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