The strategy of reusing respirators might be a very reasonable practice to conserve available supplies for further use during this unprecedented threat of COVID-19. Nevertheless, ineffective sterilization of respirators can result in contamination, transmission and self-inoculation of mucous membranes resulting in COVID-19 infection among the reusers. To our knowledge, there has only been limited data that describe experimental models of coronavirus inactivation on the N95 respirator. SARS-CoV-2 is primarily transmitted through respiratory droplets and contact routes [7–9] but airborne transmision is possible even in the absence of aerosols-generating procedures [10]. Therefore, the safety of healthcare workers caring for the COVID-19 cases during the current SARS-CoV-2 pandemic has been a major concern. Previous studies have shown that heat effectively inactivated SARS-CoV and MERS-CoV at the lowest temperature range 56–65 °C, after an average time of 30 min [2, 11, 12].
Prior to the present study, published literature showed that viruses could be inactivated by UV-C but the reported effective dose of UV-C varied between studies. The UV-C exposure dose required for 90% inactivation (D90) for coronaviruses including SARS-CoV varied between studies from 3-3046 J/m2 in air or liquid media [1, 13–17]. However, the relatively high UV-C dose of 1,000–18,000 J/m2 was used for inactivating the majority of influenza viruses and coliphage on contaminated N95 facepieces [18–20].
Our study found that the emitted radiation in different parts of the UV-C sterilizer box ranged between 0.05–0.93 mW/cm2. The effective time of viral inactivation by UV-C in our study was observed at 20 min. The calculation of UV dose in the sterilizer box, excluding the corners of the box, for 20 min and 10 min were 3,600 − 11,160 J/m2, and 1,800-5,580 J/m2, respectively.
Although heat alone had no effect on virus, the combination of heat 54–55ºC and UV-C 10 min showed no detectable viruses. The fluctuation in temperature may have been the reason for the poor viral inactivation in the heat mode. Alternatively, this could be interpreted as the effectiveness of 10-mins UV-C exposure or the synergistic effect of heat combined with UV-C. However, the MERS-CoV was still detectable after heat at 56ºC after 30 min in a previous study [11]. Therefore, it is likely that 10-min UV-C is sufficient for the inactivation of the virus.
UV radiation degrades polymers. This may result in the loss of protective function of N95 respirators. However, a study using relatively high UV irradiation at 120 J/cm2 (1,200,000 J/m2) found minimal increase in particle penetration (1.25%) and had little effect on the flow resistance. This high UV dose decreased the strength of the layers of respirator material by 5–42% depending on the models and layer of respirators, but it had very small effect on the strength of the straps [21]. Although we did not test particle penetration or flow resistance in our study, we used the physical appearance of the fibers, as determined by SEM, and fit testing as secondary measures to imply any changes in respirator function that might occur after UV-C decontamination. The SEM showed no change in the fiber arrangement and spaces between fibers after 240 min of UV-C exposure, compared to the structure of unexposed UV-C samples.
Inactivation of organisms by UV-C depends on the type of organisms, energy of the light source, distance between light source and object surface, and the time of exposure. All these factors must be integrated in order to determine the optimal disinfection strategy. Our approach using a UV-C sterilizer box, with a 4 watt bulb, at the distance of 5-8.5 cm, for at least 10 min should be enough to inactivate the coronaviruses, according to our test result, and previous studies.
There were several limitations in our study. We did not perform the viral and SEM test on the strap of N95 respirator. Viral inactivation requires direct UV-C exposure of the contaminated surface. It is important to ensure that all contaminated surfaces are exposed to the UV-C radiation. Previous study suggested that UV-C had minimal effect on the respirator straps. A UV-C dose of 590 J/cm2 (5,900,000 J/m2) decreased the strap strength by 10–21% [21]. The filtration efficiency after reuse was not conducted in our study. However, no structural integrity change was observed using SEM in this study, and the fit testing has been shown to correlate well with the particle-size-selective protection factors [22]. Following the recommended guidelines, a seal check must always be performed prior to each reuse.
Our disinfection strategy of using a home device has several advantages. UV-C sterilization devices are already available in the markets. The process takes only 10–20 min. Users take responsibility for the disinfection process and keep their own N95 respirators. The disinfection process is easy, as no new invention or additional workers are required for the process.
We have implemented N95 respirator extended use and decontamination program using UV-C sterilizer boxes at our institution since March 19, 2020. The consumption rate of N95 respirators reduced by a factor of four within two weeks. We have distributed these devices to many hospitals in Thailand, especially rural hospitals, through a public donation network. The ministry of public health has adopted the idea of UV-C decontamination in the national strategy.