Ozone concentration, not concentration-time dose, governs inactivation of dried human coronavirus on surfaces

Gaseous ozone reaches surfaces that liquid disinfectants and ultraviolet light cannot. Decontamination cycles are commonly specified by the concentration-time product CT, on the assumption that equal products give equal outcomes. We tested that assumption for human coronavirus 229E dried on polystyrene, using ozone generated from ambient air by a packed-electrode volumetric dielectric barrier discharge reactor. At the standard viral load, doubling the contact time raised the dose to 24,000 ppm·min and gave 2.8 log 10 , whereas raising the concentration from 200 to 250 ppm reached only 15,000 ppm·min yet gave 3.0 log 10 . With a fivefold larger inoculum the separation widened: 24,000 ppm·min at 200 ppm achieved 1.4 log 10 , while 15,000 ppm·min at 250 ppm achieved 2.3 log 10 . At both loads the larger integrated dose, delivered at the lower concentration, produced the smaller reduction. Exponents derived from the standard-load conditions are 1.2 for concentration and 0.3 for contact time, so concentration is roughly fourfold more influential. We attribute this to ozone diffusing into a dried organic film that consumes it. Across published surface studies the required CT spans more than three orders of magnitude. A cycle specified by CT alone can therefore under-decontaminate a surface while nominally meeting its dose target.