One year of surgical mask testing at the University of Bologna labs: Lessons learned from data analysis

Abstract

The outbreak of SARS-CoV-2 pandemic highlighted the worldwide lack of surgical masks and personal protective equipment, which represent the main defense available against respiratory diseases as COVID-19. At the time, masks shortage was dramatic in Italy, the first European country seriously hit by the pandemic: aiming to address the emergency and to support the Italian industrial reconversion to the production of surgical masks, a multidisciplinary team of the University of Bologna organized a laboratory to test surgical masks according to European regulations. The group, driven by the expertise of chemical engineers, microbiologists, and occupational physicians, set-up the test lines to perform all the functional tests required. The laboratory started its activity on late March 2020, and as of the end of December of the same year 435 surgical mask prototypes were tested, with only 42 masks compliant to the European standard. From the analysis of the materials used, as well as of the production methods, it was found that a compliant surgical mask is most likely composed of three layers, a central meltblown filtration layer and two external spunbond comfort layers. An increase in the material thickness (grammage), or in the number of layers, does not improve the filtration efficiency, but leads to poor breathability, indicating that filtration depends not only on pure size exclusion, but other mechanisms are taking place (driven by electrostatic charge). The study critically reviewed the European standard procedures, identifying the weak aspects; among the others, the control of aerosol droplet size during the bacterial filtration test results to be crucial, since it can change the classification of a mask when its performance lies near to the limiting values of 95 or 98%.

Keywords: Bacterial filtration; Breathability; COVID-19; Pandemic spread prevention; Surgical masks.

Fig. 1

a) apparatus layout according to the EN standard; b) picture of the setup at the University of Bologna.

Fig. 2

a) apparatus layout according to the EN standard; b) picture of a detail of the setup developed at the University of Bologna; c) 6-stages Andersen impactor; d) petri dishes collected from stage 4 of the impactor and after 24 h incubation at 37 °C: control sample (no mask) and test sample (applying a Type II mask to the setup).

Fig. 3

Droplet size distribution coming from the nebulizer.

Fig. 4

Layout and picture of the splash test apparatus.

Fig. 5

Success rate of the surgical mask prototypes tested, with details of the mask type for compliant masks and the reason for failure for non-compliant masks.

Fig. 6

Number of tests performed and percentage of compliant masks with production period.

Fig. 7

SEM analysis of a two layers cloth cotton mask and of a three layers surgical mask at different magnifications. External layer of a cloth mask at 400× (a) and at 3000× (c); internal layer of a cloth mask at 400× (b) and at 3000× (d); external layer of a surgical mask at 400× (e) and at 3000× (h); middle layer of a surgical mask at 400× (f) and at 3000× (i); internal layer of a surgical mask at 400× (g) and at 3000× (j).

Fig. 8

Performance of the 42 compliant surgical masks and of other prototypes that passed the differential pressure test and for which the composition was known: a) Differential pressure variation with grammage, b) BFE variation with grammage. Lines are guide to the eyes.

Fig. 9

Penetration of droplets as a function of their size in a Meltblown (MB) and in a Spunbond (SB) mask.

Fig. 10

Results of the prototypes testing classified according to the number of layers.

Fig. 11

Bacterial filtration efficiency (BFE) as a function of the differential pressure for the surgical masks tested.