Both can grow mold so frequent filter changing is necessary with either, but cool mist is typically recommended bc the warm one has a heating element which can be a fire hazard around small children.
At least this is what my research has yielded. There was also a big thread on this a few months ago in case you'd like to search for it.
One additional thing I have found about humidifiers is make sure to get a filter less one. Our first one had a filter and within a couple uses became gross. Now we have a Safety 1st Cool Mist and love it.
I have had both and while I felt like the warm air humidifier helped my sinuses feel less dry faster I prefer a cool midt humidifier. I have a tear drop Crane cool most humidifier and have been using it for a while now and it is so much easier to clean plus the water in the tank lasts much longer than the sunbeam brand warm midt humidifier I used. (The sunbeam couldn't run all night on low with a full tank. Where the Crane have run all night and well into the next day) I also don't worry as much about mold or bacteria growth with the cool mist and there isn't a fear that one of my dogs will knock it over and burn themselves as the warm mist got hotter than I thought it would.
I decided to spend a little more and got an ultrasonic one that will do both warm and cold mist. Depending on what you need it for can change which will work better.
We have 2 cool mist (Crane) and a warm mist. I have to use the warm mist in the coldest room in the house, otherwise the cool mist makes it seem colder and damp. The warm mist has an auto-shut off when it runs out of water.
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Around children they recommend a cool mist. DH and I only used cool mist ones for years and I noticed my sinuses never really improved. Gave in and bought a cheap warm mist one a couple months ago, LOVE it!!! As long as you follow the cleaning instructions, you shouldn't have a mold issue but either one can have a mold issue. I would buy cheap ones though and always make sure to follow instructions to clean properly!
ETA: all the expensive or "nice" ones we have bought in the past were disappointments. The cheap little ones have always worked the best long term.
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Re: Humidifier which one is best warm mist vs cool mist humidifier?
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At least this is what my research has yielded. There was also a big thread on this a few months ago in case you'd like to search for it.
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ETA: all the expensive or "nice" ones we have bought in the past were disappointments. The cheap little ones have always worked the best long term.