according to researchers. The greatest reduction in filtration efficiency after sterilization occurred with surgical face masks.
With plasma vapor(H2O2) sterilization, filtration efficiency of N95 and KN95 masks was maintained at more than 95%, but for surgical face masks, filtration efficiency was reduced to less than 95%. With chlorine dioxide (ClO2) sterilization, on the other hand, filtration efficiency was maintained at above 95% for N95 masks, but for KN95 and surgical face masks, filtration efficiency was reduced to less than 80%.
In a research letterJune 15 in JAMA Network Open, researchers from the University of Oklahoma Health Sciences Center, Oklahoma City, report the results of a study of the two sterilization techniques on the pressure drop and filtration efficiency of N95, KN95, and surgical face masks.
“The H2O2 treatment showed a small effect on the overall filtration efficiency of the tested masks, but the ClO2 treatment showed marked reduction in the overall filtration efficiency of the KN95s and surgical face masks. All pressure drop changes were within the acceptable range,” the researchers write.
The study did not evaluate the effect of repeated sterilizations on face masks.
Five masks of each type were sterilized with either H2O2 or ClO2. Masks were then placed in a test chamber, and a salt aerosol was nebulized to assess both upstream and downstream filtration as well as pressure drop. The researchers used a mobility particle sizer to measure particle number concentration from 16.8 nm to 514 nm. An acceptable pressure drop was defined as a drop of less than 1.38 inches of water (35 mm) for inhalation.
Although pressure drop changes were within the acceptable range for all three mask types following sterilization with either method, H2O2 sterilization yielded the least reduction in filtration efficacy in all cases. After sterilization with H2O2, filtration efficiencies were 96.6%, 97.1%, and 91.6% for the N95s, KN95s, and the surgical face masks, respectively. In contrast, filtration efficiencies after ClO2 sterilization were 95.1%, 76.2%, and 77.9%, respectively.
The researchers note that, although overall filtration efficiency was maintained with ClO2 sterilization, there was a significant drop in efficiency with respect to particles of approximately 300 nm (0.3 microns) in size. For particles of that size, mean filtration efficiency decreased to 86.2% for N95s, 40.8% for KN95s, and 47.1% for surgical face masks.
The testing described in the report is “quite affordable at $350 per mask type, so it is hard to imagine any health care provider cannot set aside a small budget to conduct such an important test,” author Evan Floyd, PhD, told Medscape Medical News.
Given the high demand for effective face masks and the current risk for counterfeit products, Floyd suggested that individual facilities test all masks intended for use by healthcare workers before and after sterilization procedures.
“However, if for some reason testing is not an option, we would recommend sticking to established brands and suppliers, perhaps reach out to your state health department or a local representative of the strategic stockpile of PPE,” he noted.
The authors acknowledge that further studies using a larger sample size and a greater variety of masks, as well as studies to evaluate different sterilization techniques, are required. Further, “measuring the respirator’s filtration efficiency by aerosol size instead of only measuring the overall filtration efficiency” should also be considered. Such an approach would enable researchers to evaluate the degree to which masks protect against specific infectious agents.
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