Ultrasonic humidifiers contaminated by microbes have been found to cause humidifier lung (humidifier fever) or infectious diseases. To prevent microbial contamination, I devised a wash-out method. By reducing volume of a transducer chamber of a humidifier below a certain level, a dilution rate surpasses a growth rate of microbes in the water of the chamber, and under such environment microbes cannot develop.
First I analyzed the idea of wash-out method theoretically and calculated the desirable volume of a transducer chamber. Then, a humidifier with the transducer chamber of a little volume was constructed. Moreover, I contrived a mechanism that keep empty in a transducer chamber while the humidifier is not in operation. The improved humidifier was operated daily 8 hour period for 6 weeks. The number of bacteria in the water of the chamber was almost below detectable levels throughout the experimental period. In control experiments, viable counts of bacteria increased with time, and reached a level of 10^5 CFU/ml during the second week. In a load test, initial bacterial concentration in water of the transducer chamber which was 3.3X10^5 CFU/ml decreased with time, and reached 70 CFU/ml in 8 hour of operation. Since the residual water in the chamber was removed while the humidifier is not in operation, viable counts were hardly detected after the following day. Because of the results above, the effectiveness of the wash-out method was fully demonstrated. This method demands no change in the mechanism of a humidifier, no additional expense in manufacture and operation, and causes no safety problem.