Buying a humidifier
Humidity is important for comfort or health. If you are buying a humidifier for your home, there are a few things to consider.
There are so many different categories and types. I'll go them briefly.
Ultrasonic - it uses a metal diaphragm vibrating at high frequency to spray mist into the air
Impeller - uses a rotating disk to spray mist into the air
Evaporative - there is a wick which is wetted. A fan pulls the air through the wick and the water evaporates. Most home furnaces have this - air from the supply is recirculated to the return where it is passed over a wetted wick.
Atomizer - high pressure water or compressed gas is added to the water to create a fine spray. This is not found in residential settings.
Cool Mist - this is a category that refers to ultrasonic or impeller humidifiers, but sometimes also includes evaporative humidifiers.
Adiabatic Humidifiers - this refers to all the previously mentioned ones (ultrasonic, impeller, evaporative, atomizer) which do not heat the water to create steam.
Hot mist/vaporization/isothermal/steam - different names for humidifiers that create steam by boiling water.
I'll deal with the most common ones for homes: ultrasonic/impeller, evaporative and steam.
The EPA has a good resource here.
Ultrasonic/impeller/cool mist are the biggest risk. If you use tap water, the dissolved particles will be sprayed into the air. You might see particles land on surfaces and you'll be inhaling it.
Aside from minerals in the water becoming aerosolized, biological growth is a concern. If you are using these humidifiers, you need to:
1. use demineralized or reverse osmosis water
2. disinfect frequently or have UV light
Even when I disinfected mine often, I still got pink mold. I've stopped using ultrasonic humidifiers, but you can if you are very careful about the risks. Other ones are preferable.
I don't have personal experience with portable ones.
I had an evaporative humidifier attached to my furnace that was useless. I needed to use ultrasonic humidifiers as well and could barely keep RH above 30%.
These are effective but use the most energy. They also heat the air, so the energy isn't wasted. There is burning risks if it's used around small children. I've had on installed on my furnace and am very satisfied so far.
Risks of aerosolized particulate matter and biological growth are lower with evaporative or steam humidifiers than ultrasonic/impeller humidifiers. You can still get mineral buildup in the humidifier if you don't use demineralized water.
There are components - wicks, filters or absorption pads, that might need replacement from mineral deposits.
Any time you have sitting water, there can easily be biological growth, so make sure you clean and replace the water as stated in the manual.
If you are humidifying in the winter, you don't want to go over 40% RH. Keep an eye on your windows. If you see condensation on the bottom of the window wipe it off and back off on the humidification. Mold growth is a bigger concern.
This table from ASHRAE shows the maximum humidity before condensation starts based on window type and outdoor air temperature.
There are other considerations like cost, noise, appearance and maintenance. I can't go into all of them. Here are some other references.