@cwal Here's the advice from the NTM expert researcher, Joseph O. Falkinham, III Ph.D. I cut and paste the pertinent parts from his latest research (2021) . Once you read it you can make an informed decision. Net net: a HOT water steam vaporizer properly maintained is acceptable.

(11) Humidifiers: A Big Problem. Humidifiers, whether small room-size or whole house HVAC systems, transfer water to air. They can also transfer microorganisms, especially NTM, Legionella pneumophila, and Pseudomonas aeruginosa. Transferring NTM from water to air is the problem. Humidity helps breathing, but it exposes one to NTM-laden aerosols. As humidifiers come in two sizes, room and house-size with different considerations, they will be dealt with separately.

Room-Size Humidifiers. Small, room-size humidifiers can generate aerosols with high numbers of NTM; even from reservoir water containing relatively low numbers of NTM (500 CFU/mL). In particular, the new ultrasonic humidifiers generate a high density aerosol mist that are rich in NTM (Hamilton and Falkinham, 2018). If you need to humidify your air (especially during winter), only use a room evaporative humidifier. Evaporative humidifiers boil water and the mist is blown by a fan or simply rises. Boiling will kill any NTM or bacterial cells and thereby be relatively free of NTM. The ultrasonic humidifiers do not heat the water, but use ultrasonic vibration to produce aerosol droplets. If a room-size humidifier blows water through a wetted filter (called an evaporative humidifier), don’t use it as the water wetting the filter will have NTM that can be aerosolized.

Remember that NTM will grow in biofilms in the humidifier’s water reservoir. Therefore, scrub the surface of the reservoir before refilling to reduce the number of NTM and other waterborne pathogens in the biofilm. I suggest you use a bathroom scrubbing agent. You can even disinfect the reservoir by adding Clorox® (1 cup to 10 cups water) for 30 min. Then be sure to rinse repeatedly until you can’t smell the chlorine before use.

Household-Size Humidifiers (HVAC). In an on-going study of NTM-patients in Philadelphia (the same hospital and area where the elderly, slender women were first identified at risk for NTM pulmonary disease), our colleagues at the Lankenau Medical Research Institute (led by Dr. Leah Lande) discovered that all the NTM-infected women have whole house humidifiers that are simple fabric or plastic woven filters with a channel above with holes for tap water to drip down through the filter. Those systems are like the “swamp coolers” used in the desert southwest to cool homes. The wet filter cools the air during passage. However, NTM adhere to the filter material where they grow and are transferred the household air that is drawn through the filter. Such house-size humidifiers are difficult, if not impossible to disinfect. Theoretically, if the incoming water to the filter was sterilized by 0.2 micrometer filtration or ultraviolet (UV) irradiation, the filter was disinfected regularly like a showerhead, the duct work leading in and out of the humidifier throughout the whole house was free of NTN, and the incoming air was free from outside dust, the humidified air would be relatively free from NTM.

Room- Versus House-Size Humidifiers. I prefer the room-size as they are portable and easier to clean (biofilm-removal) and disinfect (Clorox®). Successful employment of a whole house humidification system requires disinfection of the existing ductwork, a daunting task.