ISO 11137-1 states that "when irradiation occurs in the presence of liquid water, microbicidal effectiveness can be affected by the operating characteristics of the radiation sources". Based on this normative text, dose transference between different irradiation sources may require additional consideration. This contrasts with what has been previously demonstrated that dose transference between different sources of irradiation is feasible when irradiation is carried out under "dry" conditions (McEvoy et al., 2023) or humidified spores (Tallentire et al., 2010; Tallentire and Miller, 2015). The text in ISO cites a paper by Tallentire and Powers (1963), but to the best of our knowledge, no data is available to compare the microbicidal effectiveness of X-ray, e-beam and gamma when spores are irradiated at different moisture content levels. Therefore, this study aims to investigate the efficacy of all three irradiation sources to inactivate B. pumilus spores at varying hydration levels and determine if dose rate influences the inactivation kinetics. 

When irradiation is carried out on hydrated spores, some researchers have observed that water can have a protective effect (Tallentire and Powers, 1963; Härnulv and Snygg, 1973; Todoriki et al., 2000). However, considering that the dose is initially established during validation, the potentially increased resistance of hydrated spores will be addressed. It is expected that such an impact will be replicated upon dose transference as long as the routine process replicates the validated process. 

These findings may be used to inform the ISO standard (11137-1) on the feasibility of dose transference when irradiation is carried out in the presence of liquid water.