How to Keep a Humidifier From Growing Mold (and Your Home Too)

Why humidifiers grow mold

Standing water at room temperature is an ideal place for microbes. EPA notes that scale from minerals can be a breeding ground for microorganisms, and MedlinePlus says bacteria can grow in standing water. Wicks in evaporative units stay wet for weeks. Ultrasonic and impeller units then turn whatever is in the tank into droplets in the room, which is why EPA describes them as very efficient at dispersing what is in the water.

The routine

From EPA's "Use and Care of Home Humidifiers", CHOP and MedlinePlus.

When	What to do	Source
Every day	Empty the tank, wipe it dry and refill with fresh water	EPA; MedlinePlus says drain and clean daily
Every third day	Clean following the maker's instructions; if none, 3% hydrogen peroxide on surfaces that touch water	EPA
Every 2 to 3 days of use	Clean; CHOP suggests soaking parts in 1 part bleach to 9 parts water	CHOP
After any cleaner	Rinse with several changes of tap water	EPA
Before storing	Clean, discard the wick and dry fully	CHOP; AIRCARE manual

Where your manual gives a different method, follow the manual. Levoit's Classic 200 manual, for example, says to clean every 3 days, change the water daily or every other day, descale with distilled white vinegar and never mix vinegar and hydrogen peroxide. AIRCARE's MA1201 manual uses a vinegar soak followed by a teaspoon of bleach per reservoir, and recommends cleaning at every wick change.

Cleaning products: what to use and what never to mix

• White vinegar removes mineral scale. Levoit and AIRCARE both use it.
• 3% hydrogen peroxide is EPA's default disinfectant when the maker gives no instructions.
• Dilute bleach is CHOP's suggestion (1 part to 9) and part of AIRCARE's routine. EPA's mold guide warns never to mix chlorine bleach with cleaners that contain ammonia.
• Never combine products. Levoit warns against mixing hydrogen peroxide and vinegar. Rinse thoroughly between steps.
• Use only cleaners recommended for humidifiers. AIRCARE's manual says never to use flammable, combustible or poisonous materials.

Wicks and filters

Evaporative wicks trap minerals and stay damp, so they age. Honeywell says to replace its Filter A wick every 30 to 60 days, and AIRCARE says to replace the MA1201's MAF1 wick at the start of every season and every 30 to 60 days of operation, more often with hard water. Some makers, AIRCARE among them, sell an EPA-registered bacteriostat to add to the water; use only the product your manual names.

Water choice

EPA recommends water labeled distilled, which removes the minerals that form scale. MedlinePlus and CHOP give the same advice. Fewer minerals means less scale for microbes to colonize and no white dust from ultrasonic units. See distilled vs tap water.

Preventing mold in the room

A clean humidifier can still cause mold if it overdoes it. EPA says excess moisture can encourage dust mites and molds, and its mold guide recommends keeping indoor humidity below 60%, ideally 30 to 50%. MedlinePlus warns that running a humidifier too long can leave surfaces constantly damp. CHOP adds that an oversized unit can cause condensation that fosters bacteria and mold.

• Set it to 30 to 50% and check with a separate hygrometer. See the ideal indoor humidity level.
• Do not point the mist at walls, curtains or furniture; keep it a few feet from them.
• Wet windows are EPA's sign of high humidity: turn it down.
• Do not humidify a basement or bathroom; those rooms usually need a dehumidifier or exhaust fan instead. See the humidity level that prevents mold.

Choosing a humidifier that is easy to keep clean

• Wide tank openings you can reach into with a cloth.
• Few parts and no hidden channels.
• Dishwasher-safe tanks. Honeywell says the HCM-350's tank is dishwasher safe.
• A humidistat or sensor to stop over-humidifying.
• Evaporative if you use tap water, since EPA says those units are not expected to disperse substantial amounts of minerals.

When to stop using it

EPA advises stopping use and contacting a physician if you have respiratory symptoms you believe are linked to a humidifier. Replace a unit with visible growth you cannot clean out, cracked tanks or a wick housing that stays slimy. For ranked picks, see the best humidifiers; for children's rooms, baby humidifier safety.