Cool mist vs. warm mist humidifiers: which one should you buy?

Key Takeaways

• Both types add the same moisture to a room. Mayo Clinic says warm and cool mist humidifiers are equally effective at humidifying the air.

• The American Academy of Pediatrics considers cool mist safer for young children because there's no risk of burns.

• Heating water takes power: Levoit rates a humidifier that can heat its water at 280W, against 26W for its ultrasonic cool mist model.

• The EPA says ultrasonic and impeller units disperse the most minerals from tap water, while steam and evaporative units release little or none.

• Room size depends on a unit's output, not on whether its mist is warm or cool.

What separates cool mist and warm mist humidifiers

Every humidifier does the same job: it puts water into dry indoor air. The difference between cool mist and warm mist is whether the machine heats that water first. Warm mist units boil it into steam, while cool mist units break it up with vibration, a spinning disk, or a fan blowing across a wet wick.

That one design choice decides most of what you'll notice day to day: how much electricity the unit uses, whether it's safe on a nightstand in a toddler's room, and whether tap water leaves a film of white dust on the furniture. Comfort is where the two are closest. By the time water vapor reaches the lower airways, Mayo Clinic notes, it's the same temperature whichever way it started.

Specs below come from manufacturers' published figures, and we haven't measured these units ourselves.

Cool mist vs. warm mist at a glance

How each type works

The EPA's guide to home humidifiers sorts them into four designs. Three produce cool mist and one produces warm mist.

Ultrasonic models use a small plate that vibrates at ultrasonic frequency and shakes water into a fine mist, with no fan involved. Impeller models fling water into a mist with a rapidly spinning disk, and the EPA groups them with ultrasonic units for how they handle minerals. Evaporative models use a fan to blow air through a wet wick or filter, so the water evaporates into the room without a visible mist and the wick holds back the minerals.

Warm mist humidifiers are steam vaporizers. A heating element or electrodes boil the water, and in warm mist models the steam cools somewhat before it leaves the machine.

Safety around kids and pets

This is the clearest difference between the two. The American Academy of Pediatrics' HealthyChildren.org says cool mist humidifiers are generally considered safer for young children than warm mist humidifiers or steam vaporizers because there's no risk of burns. Mayo Clinic goes further and advises against buying a steam vaporizer at all in a home with children, since hot water inside can burn if it spills.

The EPA's advice is to keep steam vaporizers out of children's reach, and the AAP suggests placing any humidifier near a child but out of their reach. A tank of boiling water on a low table carries the same risk with a dog or cat that might knock it over, so pet owners face the same tradeoff.

Energy use

Boiling water draws far more electricity than vibrating or evaporating it. Levoit's published ratings show the gap within a single brand: its Classic 300S ultrasonic humidifier is rated at 26W, its Superior 6000S evaporative model at 30W, and its LV600HH, a hybrid that can heat its water for warm mist, at 280W. That makes the heating unit roughly 10 times the power draw of the ultrasonic one.

For a humidifier that runs overnight through a long heating season, that difference adds up on the electric bill. If you're watching running costs, a cool mist unit is the cheaper one to leave on.

White dust and water quality

Tap water carries dissolved minerals, and what happens to them depends on the design. The EPA says ultrasonic and impeller humidifiers are very efficient at dispersing those minerals into the air, where some people notice them as a fine white dust on surfaces. Steam and evaporative units, by contrast, aren't expected to release substantial amounts of minerals: steam leaves them behind in the tank, and an evaporative wick catches them.

The EPA's fix for white dust is distilled water, which it calls the most effective way to remove minerals. Demineralization cartridges vary widely in how well they work, by the EPA's account. Whatever the type, the EPA recommends emptying and refilling portable units daily and cleaning them every third day.

Noise

Noise varies more by design than by mist temperature, and published figures are scarce. Levoit lists its ultrasonic Classic 300S at 30 dB or less, and its evaporative Superior 6000S at 28 to 45 dB depending on fan speed. Evaporative units have a fan, so their noise rises at higher settings.

Warm mist makers rarely publish a decibel figure. Vicks markets its V745 warm mist humidifier as "whisper quiet" without giving a number. For a bedroom, check the published rating where one exists and look at the lowest fan setting you'll actually use overnight.

Room size and output

Whether the mist is warm or cool tells you nothing about the size of room a humidifier can handle. That comes from output, which manufacturers state in milliliters per hour or gallons per day and test under the industry standard ANSI/AHAM HU-1.

Levoit's own range shows the spread. The ultrasonic Classic 300S is rated for 215 to 505 sq ft, the LV600HH hybrid for 430 to 753 sq ft, and the large evaporative Superior 6000S for up to 3,000 sq ft. Match the rated coverage to your room first, then choose the mist type.

Colds and congestion

Many people buy a warm mist humidifier for a cold, but the evidence doesn't favor it. Mayo Clinic says cool mist humidifiers may help ease coughing and congestion, that more research is needed, and that some research has found heated humidifiers don't help cold symptoms. Either type adds moisture, and the EPA advises keeping indoor humidity between 30% and 50% and not humidifying above 50%. If the air in your home is already damp, a dehumidifier is the tool you need instead.

Which one should you buy?

For most homes, cool mist is the default. Warm mist makes sense in a narrower set of cases.

• A baby's or young child's room: Cool mist. The AAP and Mayo Clinic both point families to cool mist to avoid burns.

• A home with pets that knock things over: Cool mist, for the same reason.

• Running all night, all winter: Cool mist. Ultrasonic and evaporative units draw a fraction of the power of a heated one.

• Hard tap water and no plan to buy distilled: An evaporative cool mist unit or a warm mist unit, since neither is expected to spread substantial minerals.

• A large open living area: Whichever type has the rated output for the space. One large evaporative model is rated for up to 3,000 sq ft.

• An adult bedroom where you prefer warmer mist: Warm mist works, as long as it sits where no one can tip it over and you accept the higher power use.

Frequently asked questions

Is warm mist or cool mist better for a cold?

Neither is clearly better. Mayo Clinic says both humidify equally well and that some research found heated humidifiers don't help cold symptoms.

Is cool mist safe for babies?

Yes. The American Academy of Pediatrics considers cool mist the safer choice for young children because it carries no burn risk.

Why does my humidifier leave white dust?

Minerals in tap water. The EPA says ultrasonic and impeller units disperse them most, and distilled water is the most effective fix.

Does warm mist use more electricity?

Yes. Heating water takes far more power, and one manufacturer's heated model is rated at roughly 10 times the wattage of its ultrasonic model.

What humidity should I aim for?

Between 30% and 50%. The EPA advises against humidifying above 50% to keep moisture from building up indoors.