Applying heat and humidity using stove boiled water for decontamination of N95 respirators in low resource settings

Global shortages of N95 respirators have led to an urgent need of N95 decontamination and reuse methods that are scientifically validated and available world-wide. Although several large scale decontamination methods have been proposed (hydrogen peroxide vapor, UV-C); many of them are not applicable in remote and low-resource settings. Heat with humidity has been demonstrated as a promising decontamination approach, but care must be taken when implementing this method at a grassroots level. Here we present a simple, scalable method to provide controlled humidity and temperature for individual N95 respirators which is easily applicable in low-resource settings. N95 respirators were subjected to moist heat (>50% relative humidity, 65–80°C temperature) for over 30 minutes by placing them in a sealed container immersed in water that had been brought to a rolling boil and removed from heat, and then allowing the containers to sit for over 45 minutes. Filtration efficiency of 0.3–4.99 μm incense particles remained above 97% after 5 treatment cycles across all particle size sub-ranges. This method was then repeated at a higher ambient temperature and humidity in Mumbai, using standard utensils commonly found in South Asia. Similar temperature and humidity profiles were achieved with no degradation in filtration efficiencies after 6 cycles. Higher temperatures (>70°C) and longer treatment times (>40 minutes) were obtained by insulating the outer vessel. We also showed that the same method can be applied for the decontamination of surgical masks. This simple yet reliable method can be performed even without electricity access using any heat source to boil water, from open-flame stoves to solar heating, and provides a low-cost route for N95 decontamination globally applicable in resource-constrained settings.


S1.1 Details of Heating Protocol for Experiments Conducted In Mumbai, India
Temperature and humidity readings were taken with an Elitech GSP-6 meter, with the probes taped onto the mask surface. Experiments on N95 masks were carried out in Mumbai between 14-21 July 2020, with the ambient temperature and humidity varying between 28-32°C and 75-92%, respectively. Tissue paper was dosed with 1 ml water for the initial trials, since this resulted in humidity levels above 90%, the water content was reduced to 0.5 ml. Given the varying wind and humidity over the period of the experiment, after the first trial, a cloth was wrapped around the outer vessel to maintain the heat for a longer time and obtain better reproducibility. Since the temperature and humidity probe wires had to come out of the vessel, sealing the inner vessel was important. Without this, the near saturated humid air in the outer vessel would get inside (as found in the first trial). This was done using aluminium foil. However, the high humidity levels had no detrimental influence on the filtration efficiency of the mask during this first trial, also illustrating the robustness of this process to changes in conditions.

S1.2 Experimental Details of Filtration Efficiency Measurements Conducted In India
A simple home-built, low-cost, compact, particle filtration efficiency setup was used to evaluate the particulate filtration efficiency at 0.3μm at a flow rate of 10 lpm. The data are shown in Fig. 4 of the main manuscript. The setup uses a Plantower PMS 7003 particle concentration sensor air quality monitor chip (which can separately measure particle counts in different size channels between 0.3-10 μm)and a ESP8266-based WiFi microcontroller. The data transmitted from the ESP8266 module to a web server is visualized and analyzed through a simple intuitive HTMLbased web interface, which also has an in-built efficiency calculator. All the construction details, diagrams, source codes for the micro-controller and interface are available open-source at the GitHub repository https://github.com/shescitech/TIFR_Mask_Efficiency. A schematic diagram of the setup is shown in Fig. S1.
A background of fine particles was generated using normal saline solution in a standard medical nebulizer to create a fine aerosol. Air was sucked through the particle counter using an oil-free diaphragm pump and the throughput of particles in the 0.3 μm channel measured with and without a mask at the input. The entire mask was placed on a plastic ball, and taped all around at the sides to exclude ingress of particles from the sides. We estimate a ±1% accuracy for measurements made with this setup.

S1.3 Potential Route to Scale-Up Involving Dumpling Steamers
The scalability of this setup to a multi-mask arrangement using a home "idli-steamer" was shown in Fig. 9 of the main manuscript. We suggest that this technique can easily be scaled up to much larger numbers using steamers commonly used in typical institutional canteens. Fig. S2 shows a typical large sized "idli-steamer" used in a typical Indian canteen. With small modifications, such a system could easily be configured to hold 40-50 masks (in 2 layers) for a moist-heat based decontamination process. We also note that multilayer similar steamers for making dumplings like "momos" are also common in parts of India and Asia.

S1.4 Experimental Details of Decontamination Experiments on Surgical Masks Conducted In Mumbai, India
In a separate set of experiments, a similar humid-heat based decontamination treatment was explored for high-quality 3-ply surgical masks containing a melt-blown polymer layer. We used a Venus 1010 surgical mask, a widely used brand in India, which is compliant to the IS:16289 Indian Standards for surgical face masks, and had a PFE of ~98-99% in the pristine state. These experiments were carried out in Oct/Nov. 2020. The same 5L aluminum outer vessel with a 2L stainless steel inner container was used for this study.
The temperature and humidity profiles for 5 different cycles are shown in Fig. S3. The measured temperature and humidity exceeded the target values in all the cycles. In most cases the temperature remained > 65 o C for ~60 minutes. Even in the one case with a potential leak in the insulation, the minimum duration of 30 minutes was achieved, showing, as in the case of the N95 masks, that the process is quite robust to small variations in the initial conditions. The particulate filtration efficiency at 0.3um was measured at a flow rate of 10 litres per minute for the pristine and heat-treated masks after every decontamination cycle. Within limits of experimental error, there is no degradation in efficiency due to the humid-heat treatment.