Scientists at Lawrence Livermore National Laboratory (LLNL) have determined that heating N95 respirators up to 75 degrees Celsius for 30 minutes deactivates a surrogate coronavirus without compromising the device’s fit and its ability to filter airborne particles.
This temperature (equivalent to 167 degrees Fahrenheit) is easily achieved in hospitals and field settings allowing for the N95s to be reused once decontaminated. This heat treatment can be applied at least 10 times on an N95 respirator without degrading its fit. The research appears in the Annals of Works Exposures and Health.
Previous studies have reported that the filtration efficiency of N95s is not negatively impacted by these heating conditions.
“These results suggest that thermal inactivation of coronaviruses is a potentially rapid and widely deployable method to reuse N95 respirators in emergency situations where reusing the respirators is necessary and sterilization is unavailable,” said LLNL electrical engineer Travis Massey, lead author of the paper.
N95 respirators are protective devices that filter airborne particles. The “N95” designation means that the respirator blocks at least 95 percent of very small (0.3 micrometers or larger) test particles. They are typically used a single time in health care settings because the respirators can be contaminated when treating infected patients, thereby posing a risk to caregivers who continue wearing a contaminated device, as well as other patients treated by the provider. Limited supply and crisis conservation strategies made reuse a common practice.
“We found that this heating method decontaminates N95s without affecting the overall fit,” said LLNL material scientist Sal Baxamusa, a senior author on the paper. “But overall wear time and the number of times put on and taken off are important factors that likely degrade N95 respirator fit and must be investigated further.”
The team used a mouse coronavirus that does not cause disease in humans as a surrogate for SARS-CoV-2.
To test the heat treatment of N95s, the team enlisted two volunteers. Prior to heat treatment, each volunteer participant briefly fitted a new, unused N95 to their faces and noses to simulate a first-time use. 3M Model 8210 N95s were used for all the heat treatment tests. This particular model is one of the most widely recognizable, is used in the industry and is available in one size as it was designed to seal effectively against most human faces.
Since a goal of the study was to determine how fit from an initially well-fitting N95, as quantified by fit testing, would be affected by single or multiple heat treatment cycles, and not how different N95 models, at different sizes, would perform for a variety of face structures (other studies have investigated this), only one model of N95 was used for the test in the study. After the initial donning/doffing cycle and prior to heating, the N95s were loaded into sterilization pouches.
Once in pouches, the masks were put into a laboratory oven set at 75 degrees Celsius for 30 minutes. They heated the masks from one to 10 times under dry and humid (90 percent relative humidity) conditions and found that the dry heating decontaminated the mask while maintaining fit for further use. Previous studies have shown that humid heating for these times/temperatures will inactivate SARS-CoV-2.
“Our study provides further evidence that virus dried on N95 filter material can be inactivated by heating,” Massey said.
Other Livermore researchers involved in the study include Monica Borucki, Samuel Paik, Kyle Fuhrer, Mihail Bora, Staci Kane and Razi-ul Haque. The research was funded by internal LLNL grants.