Microbial contamination of air conditioning and humidifying systems in hospitals and in the built environment: a mapping review to assess potential microbial risks of adiabatic humidification.

Adiabatic humidification is considered an energy-efficient alternative to steam humidification. However, it remains unclear whether these systems can act as reservoirs for pathogens that may subsequently be transmitted to patients. This mapping review assesses evidence on pathogen presence, biofilm formation and dissemination in air and aquatic environments of air conditioners and humidifiers analogous to adiabatic humidifiers. A search was conducted in PubMed, Web of Science and Embase until 6th July 2023. Data related to the 14 clinically most relevant bacteria based on expert assessment, and all fungi and protozoa were extracted. The search identified 468 studies, 22 were included, covering 184 air handling units (AHUs), none involving adiabatic humidification. In total, 216 bacterial and 149 fungal pathogens and 26 protozoa were classified. Pseudomonas aeruginosa and Burkholderia cepacia were occasionally found in system air. Mycobacterium spp. and Acinetobacter spp. were found on cooler surfaces and in drain pans. High-risk pathogenic fungi were reported in ambient air surrounding coolers. Mucor spp. and Aspergillus fumigatus were found in water and ambient air surrounding various components, whereas Fusarium spp. were more airborne. None of the studies examined the role of biofilms in AHUs. A variety of pathogenic bacteria and fungi were present in systems similar to adiabatic humidification, highlighting the need for risk-managed water safety, maintainable and hygienic design and routine microbiological surveillance, as deviations from recommended operation and maintenance may promote growth. The absence of adiabatic humidification studies necessitates further research to evaluate their microbiological safety in hospitals.