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

Abstract

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.

Fig 1. (a) Kimberly-Clark N95 respirator inside 1.65L (7 cup, 1.75 quart) container, (b) Folded 5x5cm wet paper towel, two halves of the respirator and BME280 sensor inside the container (c) Large 5.7L cooking vessel, removed from stove after heating 2L water to vigorous boil, with a closed 1.65L Pyrex container respirator floating inside, (d)Vessel covered with a plate and allowed to sit for at least 45 minutes while data is logged.

Fig 2. Temperature and humidity inside the container holding N95 respirators during the 5 treatment cycles after which they underwent filtration testing.

Rapid decrease in humidity in the end is due to opening of the container at the end of the decontamination treatment.

Fig 3. Filtering efficiency test setup to test filter efficiency of a N95 mask using LightHouse handheld particle counter to detect particles of size 0.3 μm—10 μm produced by burning an incense stick.

Fig 4. Filtering efficiency of particles sized 0.3–4.99 μm, at an intake rate of 2.83 l/min, of Kimberly Clark N95 respirators undergoing cycles of heat treatments using the method detailed in this protocol with 2L of boiled water.

Fig 5

a. (left): Temperature and humidity inside the container over a single long cycle when placed in a vessel with 2L boiled water (solid lines) and 3L boiled water (dotted lines with circular markers). b (right): Time spent simultaneously above both humidity (50%) and temperature (65 & 70 °C) thresholds for 2L and 3L boiled water volumes.

Fig 6. (a) Kimberly Clark N95 respirator inside a Ziploc bag with a wet folded 5x5cm paper towel, (b) Ziploc bag with respirator resting on steel saucepan inside pressure cooker holding boiled water after being taken off stove.

Fig 7. Filtration efficiency of particles sized 0.3–4.99 μm at an intake rate of 2.83 L/min as outlined previously, undergoing cycles of heat treatments while placed in Ziploc bags.

Fig 8

(a) Large aluminium 5L vessel with 2L of boiling water (b) smaller 2L steel vessel with Venus 4400 N95 FFR with temperature and humidity probes taped to the FFR. (c) Outer vessel covered with a plate, with a cloth wrapped around it for insulation to retain heat longer.

Fig 9. Multiple respirator decontamination possibilities using the boiled hot water technique.

(a) 4-stand “idli-cooker” insert used for cycle 6, shown with 4 Venus 4400 respirators. (b) shows the nested vessels (without lids) used for this experiment.

Fig 10. Temperature and relative humidity inside the container holding the single N95 FFR during the 5 treatment cycles and the 6^th cycle in an “idli-cooker” (container with 4 FFR capacity).

A sudden decrease in humidity followed by a recovery is noticed on opening the container at the end of the run. (For two cycles in which there was either no sealing or a leak in the seal of the inner vessel the RH values are close to saturation).

Fig 11. Particulate filtration efficiency at 0.3 μm for the pristine FFR and after each decontamination and heating cycle.