Aerosol transmission through new vs. reused N95 masks

The world is grappling with the worst pandemic in over 100 years. Caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus which emerged in China in December 2019 but has now spread across the globe. With a rapid rate of transmission, SARS-CoV-2 has now affected over 4.88 million people and resulted in over 322,000 deaths.

One of the most effective tools in preventing viral transmission through respiratory aerosols is the disposable N95 mask. Officially named the N95 filtering facepiece respirator (N95 FFR), it is crucial in preventing the spread of the virus through healthcare facilities, both to and from healthcare workers, staff, and patients.

The massive demand for these masks has led to the recycling of used masks for repeat wear a limited number of times.

The N95 Mask

The N95 FFR is a mask that stops up to 95% of airborne particles. This rating is arrived at based on a NIOSH test, which is meant to certify the occupational setting usage limits of the device tested. However, the specific tests which are used to derive this rating do not simulate the aerosol spread correctly. This is important because, without such simulations, it is difficult to understand what and how the healthcare worker is exposed while they are carrying out any procedure that increases the risk of generating aerosols. It is also hard to comprehend the implications of reprocessing such a protective device.

Study: Transmission of aerosols through pristine and reprocessed N95 respirators. Image Credit: Janthiwa Sutthiboriban / Shutterstock
Study: Transmission of aerosols through pristine and reprocessed N95 respirators. Image Credit: Janthiwa Sutthiboriban / Shutterstock

Transmission and Reprocessing

Now a new study by researchers at the University of Toronto and several Toronto hospitals looks at how the aerosols in an experimental setting could penetrate through the filter media and leak to cause potential viral transmission around any defect of the face-mask seal. The study is published on the preprint server medRxiv* in May 2020.

The researchers tested both pristine or new and reprocessed N95 masks of three commonly used healthcare models, namely, 3M 1860S, 3M 8210, and 3M 9210.

These were first tested in the pristine state and then subjected to reprocessing for 1, 3, 5, or 10 cycles using seven disinfection methods that can be used in hospitals. These include 70% ethanol vapor, forced-air dry heat at 100 oC, hot and humid conditions (75 oC at 75 % relative humidity, hydrogen peroxide gas plasma (HPGP), hydrogen peroxide vapor, and ultraviolet irradiation.

Testing Mask Transmission of Aerosols

Fit testing was used to assess the fit of the mask to the face and evaluate leakage. Penetration was assessed with a polydisperse challenge aerosol under simulated conditions that replicated the actual properties of an infectious-particle carrying aerosol, such as those encountered in healthcare facilities. In other words, the particle and droplet size, the densities of the solid and liquid components, and airflow characteristics are modeled on actual conditions.

The researchers found that penetration was inversely proportional to aerosol size, with a rapidly falling gradient as the particle size increased. With a new or unused mask, penetration was expected to be from 0.09% to 0.19% with a size of 0.1 μm, 0.02% and 0.03% at 0.3 μm and 0.01%, above 0.5 μm. It is noteworthy that aerosol penetration cannot be detected below 0.01% - the detection limit.

After ensuring proper mask-face fit, aerosol leakage was measured at 0.49% for all three models. This is the detection limit. In short, the overall transmission (due to leakage and penetration) was ≤0.68% for aerosols of size 0.1 μm, which have the highest penetration. They also found that pinching the nose clip increases the amount of leakage.

Performance of Reprocessed Masks

Looking at reprocessed masks, they found that with the aerosol size of 0.3 μm, the overall transmission was less than 1.5% for masks treated with hydrogen peroxide vapor for up to 10 cycles. With dry heat and humid hot air, the transmission was low for up to 3 cycles only, and for one cycle only with ultraviolet and HPGP.

Ultraviolet radiation broke down the material by photochemical changes, while HPGP caused leakage using reactive oxygen species that destroyed the insulating foam around the nose foam.

After a single reprocessing cycle, HPGP of these five techniques allowed a low leakage rate of 0.6% during the studied cycles.

Autoclaving was unsuitable for pleated masks because of the physical deformation caused by the process, leading to leakage. However, the molded mask model escaped unhurt. The increase in penetration was suggested to be due to changes in the filter charge.

Once-reprocessed N95 masks that had undergone dry heat, HPGP or hydrogen peroxide performed just as well in limiting aerosol transmission, contrary to the traditional perception. However, none showed equivalent results up to 3 cycles.

Application for COVID-19 and Influenza

In the current situation, the aerosols containing SARS-CoV-2 and influenza virus particles are larger than 0.1 μm in size. With a pristine N95 mask, the transmission of the aerosol is as low as <0.68%. The use of hydrogen peroxide vapor, HPGP, ultraviolet germicidal irradiation, heat and humidity, and dry heat all keep the reprocessed mask in the same transmission condition as a new mask, or contain transmission to below 1.5%.

The researchers, therefore, highlight the danger of improper wearing of the mask, which causes leakage. The paper concludes: “Pristine and properly reprocessed N95 FFRs effectively protect against infectious aerosols, but that care must be taken during use and reprocessing to mitigate degradation of filter charge, avoid deterioration of straps and nose foams, preserve mask shape especially for molded models and limit noncompliant wear.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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