face mask cleaned with oven

Face masks shown to be effectively sterilised using electric rice cooker

Image credit: Chamteut Oh

A study has shown that N95 respirator masks, often used to treat patients with coronavirus, can be effectively sterilised in an electric multicooker that many people already have at home.

Just 50 minutes of dry heat in an electric cooker, such as a rice cooker or instant pot, was found to decontaminate N95 respirators inside and out while maintaining their filtration and fit, according to the researchers from the University of Illinois, Urbana-Champaign.

Since the coronavirus pandemic took hold earlier this year, the amount of waste generated by single-use masks is piling up. A report in May estimated that reusing masks could save 66,000 tonnes of contaminated plastic waste.

The newly discovered sterilisation process could enable wearers to safely reuse limited supplies of the respirators, originally intended to be one-time-use items.

N95 respirator masks are the gold standard of personal protective equipment that protect the wearer against airborne droplets and particles.

“A cloth mask or surgical mask protects others from droplets the wearer might expel, but a respirator mask protects the wearer by filtering out smaller particles that might carry the virus,” said professor Thanh Nguyen.

High demand during the Covid-19 pandemic has created severe shortages for healthcare providers and other essential workers, prompting a search for creative approaches to sanitisation.

“There are many different ways to sterilise something, but most of them will destroy the filtration or the fit of an N95 respirator,” Verma said. “Any sanitation method would need to decontaminate all surfaces of the respirator, but equally important is maintaining the filtration efficacy and the fit of the respirator to the face of the wearer. Otherwise, it will not offer the right protection.”

The researchers hypothesised that dry heat might be a method to meet all three criteria - decontamination, filtration and fit - without requiring special preparation or leaving any chemical residue. They also wanted to find a method that would be widely accessible for people at home. They decided to test an electric cooker, a type of device many people already have in their home.

They verified that one cooking cycle - which maintains the contents of the cooker at around 100°C for 50 minutes - decontaminated the masks, inside and out, from four different classes of virus, including a coronavirus. It achieved this more effectively than using ultraviolet light.

“We built a chamber in my aerosol-testing lab specifically to look at the filtration of the N95 respirators and measured particles going through it,” said professor Vishal Verma. “The respirators maintained their filtration capacity of more than 95 per cent and kept their fit, still properly seated on the wearer’s face, even after 20 cycles of decontamination in the electric cooker.”

They noted that the heat must be dry heat, with no water added to the cooker; the temperature should be maintained at 100°C for 50 minutes, and a small towel should cover the bottom of the cooker to keep any part of the respirator from coming into direct contact with the heating element. However, multiple masks can be stacked to fit inside the cooker at the same time, Nguyen said.

The researchers see potential for the electric-cooker method to be useful for healthcare workers and first responders, especially those in smaller clinics or hospitals that do not have access to large-scale heat-sanitisation equipment.

The demand for N95 masks, and their global shortage, has prompted researchers to more closely investigate issues around them. In July, University of Cincinnati researchers demonstrated that two frequently adopted sterilisation methods for cleaning disposable surgical masks and N95 respirators - autoclaving and soaking them in 70 per cent ethanol for two hours - could actually be making the masks less effective with each use and wash cycle.

Meanwhile, a research team from MIT has demonstrated a reusable face mask primarily made from silicone rubber with small, replaceable N95 filters, which can be sterilised multiple times and should be able to effectively stop viral particles.

Yesterday, it emerged that the UK Government spent £252m on 50 million face masks earlier this year that were deemed “not fit for purpose” due to their potentially loose fit. They will now not be used by the NHS, their intended destination.

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