Humidifiers are great for adding moisture to the air in dry climates, but if they aren't clean they can spew nasty bacteria into the air. Some humidifiers market themselves as "bacteria-free", but only a few types of humidifier actually stay relatively clean. The Consumer Reports YouTube channel investigated three major types of humidifier — vaporisers (which emit steam), ultrasonic humidifiers (which emit a fine mist into the air), and evaporative humidifiers (which blows air over a wet wick filter) — to see which types were the most likely to encourage microbial growth. After testing 34 different units for three days, they found that all humidifiers with anti-microbial claims weren't any better at preventing bacterial growth or preventing bacteria from being emitted. None of the evaporative wick models, however, emitted any bacteria. So, if you're looking to keep your air bacteria-free, go with an evaporative wick model. Also, regardless of what type of humidifier you use, be sure to empty, rinse, and dry out the water reservoir at least once a day to avoid any bacterial build up.
Bacteria-breeding Humidifiers [YouTube]