Fungal Load and Composition in Residential Air Conditioners Recontaminated 1 Year After Cleaning

Background and Objective Allergic fungal airway diseases caused by environmental fungal exposure are often difficult to manage with medical therapy alone. We performed a pilot study to investigate fungal load and diversity recontaminating ductless minisplit air conditioners (ACs) within 1 year of high‐pressure washing. Methods Follow‐up surveys of fungi on the internal parts of AC, AC cleaning effluent, indoor air, and house dust were conducted 1 year after high‐pressure AC cleaning in 17 Japanese residences using quantitative PCR, mycobiome analysis, and culturing. The AC operating rates during summer were estimated based on the temperature and humidity in the AC air vents and the room. Results One year after high‐pressure AC cleaning, the estimated fungal genome copy numbers in the internal AC parts returned to prewashing levels. AC operating rates during the summer correlated significantly with fungal burden in AC effluents ( ρ = 0.60, p = 0.01), modestly with those in indoor air ( ρ = 0.48, p = 0.05), but not with house dust. β ‐diversities of mycobiome in AC cleaning effluent, indoor air, and house dust were distinct ( p = 0.001). The frequency of AC use was associated with the proportions of Ascomycota in AC effluent ( p = 0.009) and indoor air ( p = 0.02), whereas house dust communities remained consistent regardless of AC use. Fungal cultures at 37°C identified viable fungi, including Aspergillus spp., in AC. Conclusion ACs can be substantially recontaminated 1 year after cleaning, underscoring the importance of regular AC maintenance to prevent indoor fungal exposure.