1. Introduction
The demand for face filtering respirators has grown continuously to cope with various environmental hazards including fine dust, liquid mist, bioaerosol, and droplets. In the meantime, the outbreak of COVID-19 has sculpted a new way of life. Since pathogens can be transmitted via bioaerosol or droplets generated by coughing or sneezing, the use of respirators is now an everyday necessity as the front-line safety tool to protect both the wearer and others from the exposure to such infectious matters [1,2]. According to the Institute of Medicine (IOM), a six-week influenza pandemic results in a demand for 90 million respirators [3]. Likewise, the prevalence of the COVID-19 pandemic has led to a worldwide shortage of respirators, and this situation resulted in the undiscerning reuse of disposable respirators, although most users are not informed of proper methods of respirator maintenance. While little information is available with respect to standardized cleaning methods, commonly used methods to disinfect the used respirators include ultraviolet (UV) irradiation, microwaving, sunlight exposure, laundering, ethanol-spray, heating with a hairdryer, and ironing [4,5]. However, there is a lack of scientific evidence on whether such treatments are indeed effective and safe in disinfecting biological matter and preserving filtration performance [6]. Therefore, it is imperative to investigate the validity and effectiveness of disinfection methods for disposable respirators.
As a contingency strategy for the capacity crisis of disposable respirators, the Centers for Disease Control and Prevention and the National Institute of Occupational Safety and Health announced the guidelines of potential disinfection methods for the reuse of disposable respirators, which include the application of ultraviolet (UV) germicidal irradiation, vaporous hydrogen peroxide (VHP), and moist heat [5]. Table 1 shows the summary of previous studies on respirator disinfection methods [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]. As for the bactericidal efficiency, autoclaving, UV irradiation, and chemical solvent treatments using bleach, ethanol, and VHP showed up to a 99% reduction of tested bacteria or virus [21,25,26,27,28,29,30]. Nevertheless, some of the results lacked the coherence, and not every method was tested for the bactericidal effect. Thus, a comprehensive investigation is called for to understand the effect of disinfection methods on filtration performance and bactericidal effectiveness.
The particle filtration of an electret media is attributed to the mechanical capture and the electrostatic attraction of particles [31,32]. Many previous studies associated the deteriorated performance of disinfection-treated respirators with the loss of electrostatic charges [7,9,11,15,17,18,19,22,25], yet the direct evidence of charge deterioration was often missing. Additionally, it has hardly been examined for the effect of treatment on the structural integrity of the filter media, which may affect the resistance of respirators. This study aims at divulging the effects of disinfection treatments on inactivation/removal of bacteria, deterioration of filtration performance and structural integrity. To this end, commonly-applied disinfection methods were employed as reuse treatments of respirators, which included microwaving, oven-dry, UV irradiation, immersions in hypochlorite (ClO−), ethanol (EtOH), and isopropanol (IPA), and laundering with and without detergent. The influence of disinfection treatment on bactericidal effect was investigated using Escherichia coli (E. coli) bacteria, as a common Gram-negative bacteria with viability in diverse environments [33]. The change of filtration performance after treatments and the probable causes for the change were examined by measuring surface potential, wettability, chemical property, and the morphology of filter fibers. The approach of this study is significant in that the validity of various disinfection methods were extensively investigated, associating the deteriorated performance with the physicochemical changes of electret media after treatments. Rarely has it been conducted for this level of inclusive investigation to reveal the impact of disinfection treatments. This study intends to provide practical yet fundamental information on the effect of disinfection treatments in multifarious aspects including bactericidal and filter performance.