Think Twice About Reusing KN95 or Surgical Masks

Masks sterilized once with chlorine dioxide showed a marked reduction in overall filtration efficiency especially among KN95s and surgical masks, reported Changjie Cai, PhD, and Evan Floyd, PhD, both of the University of Oklahoma in Oklahoma City, in a JAMA Network Open research letter.

Mean filtration efficiencies prior to sterilization were 97.3% for N95 masks, 96.7% for KN95 masks (a Chinese version of N95s), and 95.1% for surgical face masks.

When post-sterilization filtration efficiency was stratified by aerosol particle size, the mean for KN95s and surgical masks dropped to below 50% for particles of approximately 300 nm when sterilized with chlorine dioxide. N95 mask efficiency decreased to approximately 86% with chlorine dioxide for particles of 300 nm, though overall filtration efficiency was approximately 95%.

Little degradation was seen with plasma vapor hydrogen peroxide at all aerosol sizes, the researchers found.

"To better protect health care personnel in hospitals, we recommend measuring the respirator's filtration efficiency by aerosol size instead of only measuring the overall filtration efficiency," Cai and Floyd wrote.

They noted that, due to potential personal protective equipment (PPE) shortages during COVID-19, various sterilization methods have been evaluated in attempts to reuse N95 masks. However, no studies tested the feasibility of reusing KN95 masks or surgical masks.

After sterilization with plasma vapor hydrogen peroxide, both N95s and KN95 masks had at least 95% efficiency, though efficiency of the surgical mask was described as "reduced." However, after chlorine dioxide sterilization, overall filtration efficiency dropped to 76.2% for KN95 masks and 77.9% for surgical masks, though it was still 95.1% for N95 masks.

Examining sterilization by filtration efficiency by aerosol size for particles of approximately 300 nm, the authors advised "caution should be exercised" when using KN95s and surgical masks, whose mean filtration of 300 nm particles dropped to 40.8% and 47.1%, respectively, following chlorine dioxide treatment.

"In addition to considering the overall filtration efficiency, the filtration efficiency for particles similar to infectious agents should be considered," the authors cautioned.

An important limitation of the study was that effects of repeat sterilization cycles were not examined; Cai and Floyd stressed that "filter material may degrade further after multiple cycles, which should also be investigated."