Is the Current N95 Respirator Filtration Efficiency Test Sufficient for Evaluating Protection Against Submicrometer Particles Containing SARS-CoV-2?

The National Institute of Occupational Safety and Health procedure No. TEB-APR-STP-0059 recommend of measuring the respirator filtration efficiency using sodium chloride aerosol with count median diameter of 75 nm ± 20 nm and geometric standard deviation ≤1.86. This study showed that this method would overestimate the respirators’ ability to protect against submicrometer particles. In this study, we converted both mobility diameter and equivalent volume diameter to aerodynamic diameter for comparison. The results showed that one unqualified KN95 respirator (with the filtration efficiency of 72%±3% for ≥300 nm sodium chloride aerosol) still passed the test with a measured overall filtration efficiency of 98%±3%, due to its larger most penetrating particle size compared to the typical N95 respirator. In addition, after three cycle H₂O₂ plasma vaporous sterilizations, the most penetrating particle size for the N95 grade respirators also shifted to 250 nm – 500 nm, in which size the particles carried the peak concentration of the SARS-CoV-2 in hospitals. This size shift caused the significant difference between the size specific (250 nm – 500 nm) filtration efficiency and overall filtration efficiency using the same NaCl test aerosol. For example, after three cycle H₂O₂ plasma vaporous sterilizations, the size specific filtration efficiency of the N95 was 55%±2%, however, the measured overall filtration efficiency was still 86%±5%. The size Specific filtration efficiency of the KN95 was 69%±2%, but, the measured overall filtration efficiency was still 90%±3%. In order to protect health care personnel adequately, we recommend increasing the test aerosol size, and measuring the size specific filtration efficiency to evaluate the N95 alternatives (e.g. KN95), and the reuse of N95 level respirators. In addition, multi-cycle sterilization with ultraviolet germicidal irradiation appears to have fewer negative effects than H₂O₂.