UVC Inactivation of Black Mold is Wavelength-Dependent, and its Growth in HVAC Systems is Preventable Using Periodic Dosing with commercially available UVC LEDs

Mold growth in HVAC systems poses a threat to human health, increases facility management operating costs due to decreased air flow efficiency, and damages buildings, impacting buildings' attractiveness in an ever more competitive property market concerned with both carbon footprint and ventilation quality. Although UVC treatment of HVAC systems has been shown to reduce growth, efficient design requires a specified amount of UVC dose coupled with a specific dosing strategy. UVC disinfection efficacy and wavelength sensitivity of spores from the most common and notorious black mold, Cladosporium halotolerans is not known. This study investigates the sensitivity of C. halotolerans and demonstrates that growth of black mold on HVAC coils can be effectively prevented with a periodic dosing scheme using commercially available UVC LEDs. Multiple UVC LED arrays with varying wavelength peaks in the range of 252-280 nm were used to demonstrate the spectral sensitivity of C. halotolerans by keeping the wavelength specific UVC dose equal in order to avoid bias. The data obtained for doses of 25, 75, 125, 175, and 225 mJ/cm2 was used. The data obtained was used to determine a dose response curve and susceptibility patterns. Disinfection performances for the arrays were determined using log reduction value (LRV) by comparing the controls against the irradiated treatments. To study mold growth prevention on HVAC coils, the 267 nm array which had the best disinfection efficacy was utilized. The coils were placed in a chamber with a temperature set at 24 °C and relative humidity (RH) at 96%. Unlike the controls, the test coil was irradiated with a dose of 28.8 mJ/cm2 every 12 hours. The coils were monitored, and observations were recorded using time-lapse videos. The highest disinfection level of black mold observed in the range of 250-280nm occurred at 267 nm, with 225 mJ/cm2 obtaining 4.03 LRV. Linear regression analysis at 95% for 252 nm (R2=0.9637, p=0.0030), 261 nm (R2=0.9711, p=0.0021), 267 nm (R2=0.9723, p=0.0020), 270 nm (R2=0.9819, p=0.0010), 273 nm (R2=0.9878, p=0.0006), and 280 nm (R2=0.9914, p=0.0003) displayed significant association between arrays' peak wavelengths and disinfection performances against C. halotolerans. The study revealed that a higher dose of 225 mJ/cm2 is required to disinfect C. halotolerans by 99.99% (4.03 LRV). However, using a periodic dosing strategy utilizing 28.8 mJ/cm2 prevented any mold growth, while the fungal levels in the positive control increased. The study allows for the design and implementation of mold growth prevention strategies in HVAC systems to improve health and lower operating costs. ### Competing Interest Statement All authors receive salaries from Crystal IS, an Asahi Kasei company that manufactures UVC-LEDs.