[Bacterial filtration efficiency (BFE) of surgical masks used for eight hours and unused masks with Staphylococcus aureus].

Abstract

Objective: Spanish regulations and World Health Organization (WHO) guidelines recommend not using surgical masks for more than four hours. The high demand and low availability of surgical masks during the COVID-19 pandemic highlighted this as a major limitation. The objective of this study was to compare the bacterial filtration efficiency (BFE) between used surgical masks (eight hours) and unused masks (zero hours).

Methods: 208 surgical masks worn for eight hours in a row were studied. They were placed on a culture plate containing blood agar (Agar Columbia with sheep blood plus, Thermo Scientific) and sprayed a suspension of Staphylococcus aureus (ATCC 29313) at a distance of thirty centimeters. Culture plates were incubated at 37 °C for twenty-four hours. A count of colony forming units (CFU) was carried out. As a control for BFE, new non-worn masks were included. A t-test and a z-test were used at a 95% confidence level to compare the means of the two groups.

Results: To establish the BFE, the CFU of the cultures from the used masks were compared with the control cultures without a mask. Likewise, the BFE of used masks was compared to that of new masks. The mean, median and mode values of the BFE of both masks, used and new ones, were greater than 99%. No statistically significant differences were detected between eight hours used masks and not used ones (zero hours).

Conclusions: Surgical masks maintain a BFE equivalent to that new, unused surgical masks at eight hours.