Researchers from the University of Cincinnati conducted a study that advises against using two commonly used sterilization methods for N95 masks during the coronavirus pandemic.

Sergey Grinshpun, PhD, shown in the UC College of Medicine. Source: Colleen Kelley/University of Cincinnati Creative + Brand

CDC has allowed the reuse of N95 masks as a crisis capacity strategy in response to the PPE shortage caused by COVID-19. But in a perfect world, healthcare workers would put on a new mask between patients. Due to the reuse, hospitals have had to resort to sanitizing masks between use.

The most popular sanitization methods are using an autoclave or soaking the masks in an ethanol treatment. With the autoclave method, an apparatus works similar to a pressure cooker to sterilize masks with steam, heat and pressure. With the ethanol method, masks are soaked in a 70% ethanol treatment for two hours, then allowed to dry before being reused.

The team tested two brands of N95 face masks and two types of surgical masks with the two sterilization methods. N95 masks and some surgical masks rely on fibers that have an electrostatic charge. The charge captures small particles and helps protect the wearer. Autoclaving and alcohol treatments weaken the electrostatic charge.

The results of the team's study proved that the most mask damage happens in the first use and sanitization. It didn’t matter how many times masks were disinfected, the most performance loss happened in the first use and lasted through the subsequent uses.

During testing, the masks were mounted on a frame. The frame is designed to utilize the entire filtration area of the mask. The airflow rate in the system mimicked the airflow rate of an average healthcare worker on a shift. The team simulated the contamination of protective devices by soiling the masks with a protein. This mimics the average usage of a mask in a healthcare environment. The team performed five soil and sterilization cycles on the masks to simulate daily reuse over five days.

One of the masks tested was the 3M 8210 N95 mask. This mask showed physical damage after going through a single autoclave disinfection. This damage included partial disintegration of the soft sealing material around the nose clip and loss of strap elasticity. This automatically made the mask unusable. They also tested the 3M 1870 N95 mask. With this mask, the test made notable and moderate damage including some detachment and minor deformation of the nose foam. These deteriorations appeared after one cycle.

Other sterilization techniques, like ultraviolet light, are currently being explored by the researchers. The team chose to focus on autoclave and alcohol sterilization because they are most readily available in hospitals.

This research was published in the Journal of Hospital Infection.