The EPA has published the following to warn people about breathing in germs from humidifiers.

Humidifier fever is a disease of uncertain etiology . It shares symptoms with hypersensitivity pneumonitis, but the high attack rate and short term effects may indicate that toxins <eg., bacterial endotoxins> are involved. Onset occurs a few hours after exposure. It is a flu like illness marked by fever, headache, chills myalgia and malaise but without prominent pulmonary symptoms. It normally subsides within 24 hours without residual effects,and a physician is rarely consulted. Humidifier fever has been relate to exposure to amoebae, bacteria, and fungi found in humidifier reservoirs, air conditioners and aquaria. The attack rate within a workplace may be quite high, sometimes exceeding 25 percent.

Bacterial and fungal organisms can be emitted from impeller and ultrasonic humidifiers.

Mesophylic fungi, thermophylic bacteria and thermophylic actinomycetes --all of which are associated with the development of allergic responses -- have been isolated from humidifiers built into forced air systems as well as separate console units. Airborne concentrations of microorganisms are noted during operatiojn and might be quite high for individuals using ultrasonic or cool mist units. Drying and chemical disinfection with bleach or 3% hydrogen peroxide solution are effective remedial measures over a short period , but cannot be considered as reliable maintenance. Only rigorous, daily and end-of-season cleaning regimens , coupled with disinfection, have been shown to be effective. Manual cleaning of contaminated reservoirs can cause exposure to allergens and pathogens.

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Therefore although the use of humidifiers and vaporizers can be beneficial, it is vitally important that these units be cleaned and maintained properly when used.

What is the Source of the problem?

Actual mold grown in a dirty humidifier
Bacteria and Mold growth within portable consol humidifiers has long been a concern due to the ability of the humidifier to emit this bacteria into the surrounding room where children, elderly and other family members are at risk of serious illness from inhalation of these microorganisms. Studies have shown that regular cleaning of the humidifier as well as the use of treatment products that control bacteria and scale are important for good humidifier health.