A Thermophilic Bacteriophage TP-84 Surrogate System for Validating Photocatalytic and Ultraviolet Inactivation of Airborne Viruses, Including SARS-CoV-2

The airborne transmission of SARS-CoV-2 has renewed interest in engineering controls for indoor air; photocatalytic oxidation over titanium dioxide (TiO2) under ultraviolet (UV) light is an attractive, reagent-free basis for such controls. Rational development requires a biosafe, quantifiable challenge organism usable at biosafety level 1 (BSL-1). We validated the thermophilic bacteriophage TP-84 (host Geobacillus stearothermophilus) as a surrogate for airborne viruses, characterising its stability, aerosolisation, recovery from photocatalytic supports and inactivation on TiO2 surfaces under UVA, UVC and UVA + UVC by a reference-normalised mass balance. TP-84 was stable in a simulated saliva matrix, survived ultrasonic nebulisation, and was recovered from porous ceramic supports at 66.0–91.7%. Five minutes of UV on TiO2 surfaces—UVC (254 nm, 540 mJ cm−2) or combined UVA + UVC (2040 mJ cm−2)—inactivated 87.4–89.4% and 61.6–76.5% of phage, respectively; bulk liquid was strongly protective (94.6–97.5% recovery), whereas desiccation reduced recovery to 4.3–16%. Flow-through aerosol assays provided a device test bed. Because TP-84 is a hardy, non-enveloped dsDNA virus, these values are conservative lower bounds that must not be equated with SARS-CoV-2 efficacy; validation with an enveloped surrogate and authentic virus is the next step.