Sterilization of disposable face masks by means of standardized dry and steam sterilization processes; an alternative in the fight against mask shortages due to COVID-19.

Sir

The Covid-19 pandemic causes imminent shortages of face masks in hospitals globally. In preparation for that scarcity we performed a study to investigate the possibility of reprocessing disposable FFP2 face masks in order to verify their re-usability with a method that could be applied in practice using already available equipment. Therefore single use FFP2 masks (type 1862+3MTM) were sterilized with a 15-minute procedure at 121 ⁰C, using a dry sterilization process as well as with a regular steam process with the masks in sterilization/laminate bags. The effectiveness of these processes are sufficient to inactivate the coronavirus based on the knowledge of inactivation of such viruses. [1,2] A blind comparison of unused sterilized masks was performed with respect to visual inspection, consistency, face fit and breathing resistance. The results of this comparison were that the investigators were unable to distinguish unused new (slightly curved and folded) masks from reprocessed sterilized masks.

We then tested the functionality of the unused and sterilized masks in several ways. Firstly the permeability properties for bacteria were tested by spraying a bacteria solution of Staphylococcus epidermidis (ATCC 12228) on the masks, while air was drawn through the masks. Unused and multiple sterilized masks showed no differences in the amount of bacteria passed through (data not shown). In these experiments it was also observed that the reprocessing procedures of the masks did not appear to affect the water-repellent mask properties.

We then assessed pressure/flow and performed particle tests. Before sterilization, the batches were individually packed in laminate bags and sterilized with steam sterilization by means of 121 ⁰C in Getinge autoclaves and in combination with permeable laminate bags, Halyard type CLFP150X300WI-S20. The autoclaves were activated on a 121 ⁰C program and validated accordingly. After sterilization, the samples were tested at Delft University of Technology and at Reinier de Graaf Hospital, and benchmarked with new mouth masks. A custom test set-up was built to measure the pressure drop over the masks and outflow with regard to the permeability of the masks. A direct comparison between new and sterilized masks did not show substantial differences. Finally, the filtration capacity of the masks was evaluated using a calibrated Lighthouse Solair 3200 particle counter (Lighthouse, San Francisco). It was shown that the mask permeability of small particles did not change after multiple heat sterilization procedures (Table I ).

Table I

Filter efficiency testing of sterilized masks

	New FFP2	1x Heat 121oC (N=2)	3x Heat 121oC (N=4)	5xHeat 121oC (N=2)	10kGy (N=1)	25kGy (N=2)
Filter Efficiency %
0.3 μm	99.4	96.9	97.4	96.8	55.4	-
0.5 μm	99.8	98.0	98.4	98.7	79.1	57.5
5.0 μm	99.8	95.2	95.5	94.3	98.1	98.7

We openly shared our positive experiences of the steam sterilization process, with other hospitals in the Netherlands that are also preparing for the outbreak. We were informed that their attempts to steam sterilize mouth masks at 134⁰C gave poor results as masks started to deform and became sticky while the elastics lost its resilience.

In addition, we tested Gamma radiated masks, this process did hamper the filter capacity (Table I).

The results of our experiences and experiments indicate that our sterilization process did not influence the functionality of the masks tested. In case of an acute shortage of FFP2 masks, steam sterilization (e.g. in laminate sterilization wrappings) of used masks at 121 ⁰C in laminated bags, is a simple, useful, cost-effective and quick procedure that can be used to make used masks safe for reuse. The sterilization process of available standard autoclaves in hospitals may have to be adjusted in order to use this sterilization method. We also emphasise that we performed these experiments with 3M masks only. However, our method seems to be a potentially useful way to reuse mouth masks; other hospitals facing a shortage of masks may wish to test and validate this approach to reusing masks.