Due to global connectivity, virus infection diseases and related pandemics are among the most pronounced safety risks of modern open society. Typical epidemics repeat themselves in a cyclic but unpredictable fashion, with initial exponential growth. Successfully tackling such crises depends on the availability of safe, acceptable and high-quality protective equipment as well as governmental readiness to influence citizens' uptake of protective behaviours, besides heavy regulations.
Virus infection diseases and related pandemics are one of the most pronounced safety risks of modern open society (Cirillo and Taleb 2020; WHO 2019). The most common human disease is a flu that is an upper respiratory infection caused by viruses. Respiratory viruses spread among humans via droplets, aerosols and contaminated surfaces (Kutter et al. 2018). Obviously, droplets are essential in distribution of viruses (Milton et al. 2013). The aerosol transmission route is emphasized in the research on SARS-CoV-2 virus (Santarpia et al. 2020) and the use of face masks (Li et al. 2020) and social distancing are recommended actions to prevent the spreading of the infection (Ferioli et al. 2020).
In the context of the COVID-19 pandemic, there is a worldwide shortage of respirators such as filtering face pieces (FFP). Therefore, they should be prioritised for the use by front line health care workers who are at increased risk of getting COVID-19 (European Centre for Disease Prevention and Control 2020b; Nguyen et al. 2020). At the same time, the recommendations for the citizens to wear face masks in public has increased the need for alternative, easily available face masks. Wide variety of mask materials and concepts have been tested (Tcharkhtchi et al. 2021), and there are great differences in the protection level they provide. For example, filtration efficiency for 0.03 − 2.5 µm sized particles is 80 − 90% for nonwovens, while with cloth masks the efficiency is 39–65% (Shakya et al. 2017). Professional N95 and FFP2 protective devices seem to be 1.7 (Rengasamy et al. 2009) and even 33 (van der Sande et al. 2008) times more efficient than non-standard masks. Despite some of their weaknesses, in combination with other measures the personal protection devices seem to be effective (Abaluck et al. 2020).
The shortage has been seen as well in increased price of protective masks causing problems to lower income citizens. On June 2020 CEN workshop agreement CWA 17553 Community face coverings - Guide to minimum requirements, methods of testing and use was published, and it was stated that the community face coverings specified as reusable shall withstand the number of cleaning cycles claimed by the producer (at least 5 cleaning cycles) with a minimum washing temperature of 60°C. For the respirators, several different procedures have been tested for decontamination of respirators to mitigate the effects of shortage (Bailar et al. 2006; European Centre for Disease Prevention and Control 2020a; Rubio-Romero et al. 2020). However, there are not published research studies about cleaning the community face masks available yet.
Requirements for any useful method are that they are simple, inexpensive, effective, remaining mask performance, and safe in means that no harmful contaminants nor residuals after decontamination exists.
Steam sterilization is a routinely used sterilisation process in hospitals, but not suitable for respirators. Respirator deformation or failing fit-test after steam sterilization at 134°C was reported in some types of respirators in a study performed in the Netherlands (Rijksinstituut voor Volksgezondheid en Milieu 2020). Research published in 2012(Lore et al. 2012) demonstrated the effectiveness of microwave generated steam (MGS) in inactivating viral particles of influenza virus on two models of N95 respirators. MGS had shown to reduce > 4 logs viable influenza virus on N95 respirators, with only one of the six models tested showing a slight changes of the foam at the nose cushion (Heimbuch et al. 2011). Additionally, physical deformation for certain N95 models related to inner foam nose cushion was reported, yet maintained adequate aerosol penetration and filter airflow resistance after three disinfection cycles (Bergman et al. 2010). Disinfection using steam bags inactivated 99.99% bacteriophages from N95 respirators (Fisher et al. 2011). The steam had little effect on the filtration efficiency, which still remained above 95%. In a recent pre-print (Liao et al. 2020), it was shown that steam treatment on N95 compatible melt-blown fabric did not considerably impact the efficiency and pressure drop in three steam treatment cycles. In a study (Bergman et al. 2011), the authors reported how three applications of MGS did not cause significant changes (pass rate ≥ 90%) in respirator fit in the three types of N95 respirators tested.
Heat treatment did not cause considerable degradation of filtration properties on melt-blown fabrics (the material out of which respirators are constructed). Heat treatment was performed with static-air oven at 75°C for 30 min per cycle and tests were performed up to 20 cycles (Liao et al. 2020). Minor or no change in filtration efficiency and pressure drop was detected with heat treatment up to 100°C. In this publication the authors highlight that steam may decrease filtration efficiency and that humidity should be kept low when approaching 100°C (Liao et al. 2020). Similar results were obtained when dry heat was used at 70°C for up to 60 minutes on fabric from N95 respirators (Fischer et al. 2020). They found that filtration performance was not reduced after a single decontamination cycle, but after subsequent rounds of decontamination filtration efficacy decreased. Dry heat decontamination also inactivated SARS-CoV-2 more rapidly on N95 fabric than steel. The authors highlighted that dry heat and exposure time long enough ensure the reduction of viruses (Fischer et al. 2020). Viscusi et al. (2009) found that different models of respirators melted and changed filtering properties in different temperatures. They reported melting in some models when temperature above 100°C was applied.
The disinfection methods presented above for FFP decontamination and reuse are only considered as extraordinary last-resort methods due to shortage of FFP supplies. They should be applied after a careful evaluation of the situation and after exploring the possibility of resource-conscious, rational use of FFPs, for example by extending the FFP lifespan, and having in mind the product use instructions provided by FFP manufacturers. After all not much has been published on decontaminating of surgical single use face masks or community face coverings corresponding to the use in public defined by national authorities.
One useful method for microbe deactivation is ultraviolet germicidal irradiation (Fisher and Shaffer 2011). UV-radiation is used widely in different industrial processes and medical applications to decontaminate different substrates. Traditional mercury lamps with wavelength of 254 nm (UVC radiation) have been employed widely. Main inactivation mechanism is pyrimidine-dimer formation between thymine bases which inhibits microbe DNA or virus RNA reproducibility (Kowalski 2009). In recent years more UV sources have been developed and they are becoming less expensive and less risky to use.
CDC (Centers for Disease Control and Prevention) recommends for mask decontamination UV dosage 1 J/cm2 applied preferably in intervals. This is considerably high dosage and e.g. corona virus in water inactivates with markedly lower dosages (Zhao et al. 2020). It is also recommendable to treat both sides of the mask (Derraik et al. 2020). Previous studies have shown 99.9 % decontamination of MS2 and influena virus when applying the recommended dose of 1 J/cm2 (Mills et al. 2018).
Various technologies have been applied to decontaminate respirators from different microbes including viruses. Several of these, like autoclaves and use of hydrogen peroxide vapour, are possible for professional use, but not for common people. In the following we fact test some of the possible low technology last-resort methods without an intention to recommend them in use under normal conditions where respirators are available. Due to the rapidly emerging demand for face masks, more understanding on their re-use is needed. In this study the used methods were chosen from the technologies available at home. A single-use surgical face mask and a home-made cloth face mask were selected for the study, as they are available for citizens and the use of them do not increase the possible shortage of respirators for health care professionals. The following microbe inactivation and removal methods were studied: boiling, machine wash, steam and heat treatments, ironing, long storage at airy room temperature, and intensive UV irradiation. We are discussing the effect of these methods on efficiency of removing microbes and filtration performance.