9Spores of the bacterium Bacillus subtilis were deposited onto the surface of 10 membranes by a process of filtration and exposed to concentrations of 11 hydrogen peroxide vapour between 10 and 90 mg/m 3 (ppm) for times ranging 12 from 1.5 to 48 h. The inactivation data obtained in this way was modelled 13 using the Weibull, Series-Event and Baranyi inactivation models. The Weibull 14 model provided the best fit, and its use was extended to previously published 15 literature obtained at higher hydrogen peroxide concentrations to produce a 16 correlation yielding D (decimal reduction value) values over a range from 10

C. difficile spores are resistant to routine cleaning agents and are able to survive on inanimate surfaces for long periods of time. There is increasing evidence of the importance of the clinical environment as a reservoir for pathogenic agents and as a potential source of healthcare-associated infections (HCAIs). In this context, to reduce the risk of cross-transmission, terminal disinfection of hospital wards and isolation rooms using hydrogen peroxide vapor (HPV) is attracting attention. Spores of C. difficile (ribotype 027) were exposed to constant concentrations of HPV ranging between 11 and 92 mg m À3 (ppm) for a range of exposure times in a specially designed chamber. The inactivation data thus obtained was fitted using the modified Chick-Watson inactivation model to obtain decimal reduction values (D values). D values ranged from 23 to 1.3 min at HPV concentrations of 11 and 92 ppm, respectively. We present a simple mathematical model based on the inactivation kinetic data obtained here to estimate the efficacy of commercial HPV processes used in healthcare environmental decontamination. C. difficile spores showed linear inactivation kinetics at steady HPV concentrations ranging between 10 and 90 ppm. The data obtained here was used to provide estimates of the inactivation efficacy of commercial HPV process cycles, which employ unsteady HPV concentrations during the decontamination process.