The respective roles of UV photons and oxygen atoms in plasma sterilization at reduced gas pressure: the case of N2-O2 mixtures

In the reduced-pressure (≤10 torr) afterglow stemming from discharges in O₂- containing mixtures such as N₂-O₂, the test-reference spores are ultimately inactivated by UV photons through destruction of their genetic material (DNA). To show this, we assume the inactivation to result from a sufficiently large number of successful hits of the DNA strands by UV photons. This implies that the higher the UV intensity, the shorter the time required to reach the lethal dose. Simultaneously, the increased erosion of the spores by the oxygen atoms as time elapses reduces the incident number of photons required to meet the lethal dose. Erosion, as observed by scanning electron microscopy, also increases with the O₂ percentage in the mixture. Actually, sterilization time is found to be the shortest when the O₂ percentage in the mixture is set to maximize the UV emission intensity, which occurs at O₂ percentages typically below 2%, where erosion is low. This proves the predominant role of UV radiation over erosion as far as spore inactivation is concerned. In any case, plasma sterilization always implies some erosion of the test spores, in contrast to what happens with conventional sterilization methods.