Protecting healthcare staff from severe acute respiratory syndrome: filtration capacity of multiple surgical masks

Abstract

Guidelines issued by the Centers for Disease Control and Prevention and the World Health Organisation state that healthcare workers should wear N95 masks or higher-level protection during all contact with suspected severe acute respiratory syndrome (SARS). In areas where N95 masks are not available, multiple layers of surgical masks have been tried to prevent transmission of SARS. The in vivo filtration capacity of a single surgical mask is known to be poor. However, the filtration capacity of a combination of masks is unknown. This was a crossover trial of one, two, three and five surgical masks in six volunteers to determine the in vivo filtration efficiency of wearing more than one surgical mask. We used a Portacount to measure the difference in ambient particle counts inside and outside the masks. The best combination of five surgical masks scored a fit factor of 13.7, which is well below the minimum level of 100 required for a half face respirator. Multiple surgical masks filter ambient particles poorly. They should not be used as a substitute for N95 masks unless there is no alternative.

Figure 1

Particle count reductions seen with varying numbers of surgical masks. Median values for one, two, three and five masks were 2.7, 3.8, 4.6 and 5.5, respectively. The boxes outline the 25^th and 75^th centiles, the bars indicate the 10^th and 90^th centiles and the circles indicate the outlying values. A half face respirator such as an N95 mask should achieve a minimum 100-fold reduction in particle count.