Performance evaluation of monochromatic and combined GUV wavelengths for airborne E. coli inactivation in an HVAC duct

• UV rate constants were quantified for airborne E. coli at 222, 254, 265, and 365 nm. • 265 nm showed highest efficacy; 365 nm alone caused no measurable inactivation. • Larger aerosols (2.1–7.0 μm) were less susceptible due to UV shielding effects. • Sequential UVA-before-UVC exposure resulted in significant synergistic inactivation. • Simultaneous and UVA post exposure to UVC showed only additive disinfection effects. This study quantifies the wavelength-dependent disinfection performance of germicidal ultraviolet (GUV) for airborne Escherichia coli in a pilot-scale HVAC duct system. Monochromatic UV sources at 222, 254, 265, and 365 nm were tested at three airflow rates (29.05, 52.16, and 79.00 m³/h) under controlled indoor conditions (25 °C, 40% RH). UV fluence was determined using localized irradiance mapping and computational fluid dynamics (CFD) based exposure time modeling. Among the tested wavelengths, 265 nm exhibited the highest UV rate constant ( k = 1.151 m²/J), followed by 222 nm ( k = 0.480 m²/J) and 254 nm ( k = 0.420 m²/J). No measurable inactivation was observed under 365 nm, even at fluences as high as 223 J/m². Disinfection efficiency decreased with increasing bioaerosol size, with smaller particles (0.65–2.1 μm) exhibiting higher inactivation than larger ones (2.1–7.0 μm) due to internal UV shielding. Dual-wavelength combinations of UVA (365 nm) with UVC were evaluated under both simultaneous and sequential exposure modes. Simultaneous, and UVC followed by UVA exposures resulted in no additional effects beyond UVC alone. However, sequential UVA pre-treatment significantly enhanced UVC efficacy across all tested combinations, indicating a synergistic effect likely driven by oxidative stress and inhibition of microbial repair mechanisms. These results demonstrate that the wavelength sequence critically influences airborne disinfection outcomes. This work provides new evidence supporting UVA before UVC configurations for enhanced microbial inactivation and informs the design of multi-wavelength GUV systems for safe and energy-efficient air disinfection in HVAC duct applications.