Microbiological Safety and Environmental Efficacy of Disposable Bedside Cool-Mist Humidifiers

Background:

Bedside room humidifiers can harbor pathogens, and have been linked to nosocomial outbreaks. Our hospital policy requires humidifiers to be filled only with sterile water and discarded after 3 days. This study explores potential risks and benefits of humidifiers when adhering to the policy.

Methods:

Cool Mist humidifiers (Kaz 4100) filled with sterile water were placed in an empty patient room for 5 days. Daily humidity and temperature readings were obtained from a sensor on the bed (1 meter from the humidifier, within the vapor vector). The experiment was duplicated 22 times: 15 using humidifiers and 7 control runs with no humidifier. The reservoir was sampled daily; 0.5 mL of water was inoculated onto blood agar plates (TSA with sheep blood) and incubated for 7 days at room temperature. On days 3-5, blood agar settle plates were placed at 1, 3, and 5 meters from the humidifier within the vapor vector. Plates were exposed for 1, 5, and 10 minutes at each distance. Statistical analysis was performed with SAS.

Results:

We found a significant difference between humidifier and control experiments in both room humidity (38% and 25%, respectively, p<0.001) and temperature (75.2°F and 73.9°F, respectively, p=0.003). There were also significant differences between summer and fall/winter humidity and temperature readings (p<0.001). When controlling for season, the humidifier effect on temperature disappeared, but the effect on humidity remained significant (p=0.006). In multivariable analysis, a higher number of colonies grew from reservoir cultures and settle plates in summer than in fall/winter (p=0.002). Contamination appeared as early as day 1, increased with experiment day (p=0.003), and accelerated after day 3. Settle plate colony count diminished with increasing distance from the humidifier (p<0.001). Further controlling for temperature and humidity did not affect these results. Organisms that grew included skin flora and molds.

Conclusion:

Humidifiers had a modest effect on room humidity, but became contaminated over time despite the use of sterile water. Contamination with potential pathogens spread in vapor may pose a risk to immunosuppressed patients, particularly during the summer. This risk is only partially mitigated by replacing humidifiers after three days.