Evaluation of Filtration Efficiency Test Method of Surgical Mask
Date Issued
2012
Date
2012
Author(s)
Liao, Yi-Ding
Abstract
Surgical masks need to pass bacterial efficiency (BFE) tests equivalent to ASTM F2100 as required by USFDA, to be certified for medical use in many countries. Yet, surgical mask filter efficiency has been found extremely variable in several previous studies. The inconsistency results among certified laboratories during the inter-laboratory comparison tests were particularly troublesome to the regulatory agencies. Therefore, this work aimed to identify the source of the variability of BFE test method and to propose a replacement method that is more consistent and directly associated with the respiratory protection.
Experimental apparatus was build according to the ASTM F2100. Acrylic powder of 0.8μm was used as the surrogate of Staphylococcus aureus during the beginning phase of the project. Aerosol particles were nebulized into chamber and sampled with an Andersen cascade impactor sampler onto 6 stages at 28.3 L/min. The size distribution and number concentration of the aerosol output were measured using an aerodynamic particle sizer. The filtration efficiency of the surgical masks to be tested by ASTM method, was measured using a scanning mobility particle sizer (for particle smaller than 0.7 μm) and an aerodynamic particle sizer (for particles larger than 0.7 μm). The major operating parameters included concentration of the peptone solution, brand of peptone, number concentration of particle in the solution, solution feeding rate, and air flow supplied to the generator. In addition to the in-house laboratory experiments, two certified laboratories (Nelson laboratory in the United States and Taiwan Textile Research Institute, TTRI laboratory) were chosen to carry out the inter-laboratory comparison. Three models of already certified surgical masks with the lowest filtration efficiency were selected to facilitate the statistical analysis of bacterial colony count. The particulate filtration efficiency and the pressure drop across the filter media were measured by using a filter tester (TSI 8130) before sending to certified laboratories. For each model, 15 facepieces were sent to a laboratory for pressure drop, BFE and PFE tests.
The results showed that aerosol number concentration increased with the increasing air flow supplied to the generator. This is because higher air flow (or pressure) tended to break solution into smaller droplets. But the peptone concentration and the solution feeding rate appeared to be the two most influential factors determining the size distribution of the generated droplets. The bacteria (or the acrylic powder) concentration in the solution did not affect the aerosol size distribution, but might change the colony count of BFE test. From the inter-laboratory comparison, the particulate filtration efficiency tests were consistent in terms of pressure drop across the filter media and the aerosol filtration efficiency. However, the bacterial filtration efficiency test results were random and even contradictive. This is likely due to the propagation of the uncertainties embedded in the biological processes of BFE method. Overall, the Bacterial efficiency tests method is inconsistent, tedious, costly and unnecessary. We propose that surgical masks be tested using non-biological particles with the most penetrating size, i.e., 0.3 μm for mechanical filters, and 0.075 μm for electret fitlers.
Subjects
Surgical mask
Particulate filtration efficiency
Bacterial filtration efficiency
Type
thesis
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