Legionnaires’ disease is a severe form of pneumonia caused by the Legionella bacteria, which thrive in warm, stagnant water systems. While humidifiers are generally safe and widely used to improve indoor air quality, they can become a source of risk if the water reservoir is neglected. Contracting the disease from a humidifier is rare and depends entirely on a lack of proper maintenance, allowing the bacteria to multiply and be dispersed into the air.
Understanding Legionnaires’ Disease and Transmission
Legionnaires’ disease is a serious lung infection caused by the bacterium Legionella pneumophila, which is found naturally in freshwater environments like lakes and streams. When this organism enters human-made water systems, especially those that allow water to stand, it can multiply to dangerous levels. The disease is one of a group of illnesses collectively known as legionellosis, which also includes the milder, flu-like Pontiac fever.
Infection occurs when a person inhales small, aerosolized water droplets containing the bacteria deep into their lungs. Common symptoms of Legionnaires’ disease resemble other forms of pneumonia, including fever, cough, muscle aches, and shortness of breath. The disease is not spread through drinking water or transmitted from person to person. Any device that creates a mist or spray from a contaminated water source is a potential route of exposure.
When Humidifiers Become a Risk
A humidifier becomes a health risk when it provides the environment for Legionella to multiply before aerosolizing the contaminated water. The bacteria proliferate best within a temperature range of 68°F to 113°F (20°C to 45°C), which is often reached in a room-temperature water tank. Stagnant water left in the reservoir for more than 24 hours provides the necessary time for colonization.
Legionella flourishes inside biofilm, a slimy layer that builds up on the internal surfaces of the water tank and tubing. Mineral deposits and scale from tap water act as nutrients, encouraging the formation of this protective biofilm where the bacteria can hide and grow rapidly.
The risk level depends on the type of humidifier used. Ultrasonic and impeller (cool mist) humidifiers pose a greater risk because they use vibration or rotation to create a fine mist. This process effectively aerosolizes water, dissolved minerals, and pathogens present in the reservoir.
In contrast, steam vaporizers boil the water, which effectively kills the bacteria before the moisture is released. Evaporative humidifiers use a fan to blow air across a saturated wick, producing a larger water droplet less likely to carry bacteria deep into the lungs. However, these types can still harbor Legionella if the wick remains perpetually wet and contaminated.
Reducing the Risk Through Proper Maintenance
Preventing Legionella growth relies on removing the three conditions the bacteria needs: stagnation, warmth, and biofilm-supporting nutrients.
The most effective action is to empty the water tank completely and wipe all internal surfaces dry every day. This simple step eliminates stagnant water and prevents initial colonization.
For weekly cleaning, the device should be disinfected using a solution that destroys developing biofilm. Manufacturers often recommend a diluted bleach or vinegar solution, which must be run through the system and allowed to stand for a specific period of time. After disinfection, thoroughly rinse the tank with clean water to remove chemical residue before reuse.
The type of water used significantly impacts scale and biofilm formation. Using distilled or demineralized water instead of tap water is highly recommended, especially for ultrasonic and impeller models. This minimizes the mineral buildup that acts as a nutrient source for Legionella.
When the humidification season ends, the device must be thoroughly cleaned, disinfected, and completely dried before storage. Store the unit in a dry, cool place to prevent residual moisture from supporting microbial growth.