Is dry heating or UV disinfection safe and effective for reusing N95 respirators?

Short-term shortages and reuse of disposable respirators may be due to aerosol-borne diseases such as COVID-19. Because of these situations, the Centers for Disease Control in many countries and the World Health Organization recommend extending the frequency and duration of use of single-use respirators, if appropriate. However, these organizations have not specified which method of disinfection should be required and how long reuse should be allowed.

Various disinfection options include soap or hot water immersion, alcohol spray, high pressure steam, dry heating, UV radiation, vaporized hydrogen peroxide, oxide of ethylene and moist heat. While previous studies have confirmed that dry heating and UV radiation can be used to disinfect respirators without impairing their protective performance, these disinfection cycles have been performed continuously one after the other. Additionally, the filtration efficiencies reported in previous studies were only for unused and repeatedly disinfected respirators.

To address this shortcoming, investigators of a study titled “Filtration Efficiency of N95 Filtering Facepiece Respirators Over Multiple Cycles of “8-Hour Simulated Donning + Disinfection” recently published in The Journal of Hospital Infections noted that in addition to not using soiled respirators to verify filtration efficiency, “there is a lack of investigation of the change in filtration efficiency before and after each disinfection cycle for reused respirators”. Investigators also explained that “existing research primarily adopts constant flow for testing respirators, which neither reflects the respiratory flow pattern nor simulates warm, humid exhaled air…Given [these] research gaps, this study quantitatively explored changes in the filtration efficiency of N95 respirators over multiple “8-hour simulated donning + disinfection” cycles.

Investigators expected search results from 3 N95 filtering respirators to provide a benchmark for duration and frequency of reuse of multi-cycle disinfected respirators.

Multiple cycles of dry heat and UV radiation treatments on reused N95 respirators (i.e., multiple donning for 8 hours) have minimal effect, greater than 0.5%, on filtration efficiency of the respirator. All N95 respirators tested are capable of maintaining filtration efficiencies equal to or greater than 95% for at least 30 hours or 4 “8 hour donning + disinfection” reuse cycles, even at 85 L/min. In addition, lower breathing flow (15 L/min) plus exhalation valve can further extend the operating time of the N95 respirator up to 140 hours or 18 reuse cycles of “8 hours donning + disinfection” . As the respirator is worn longer, aerosol penetration slowly increases according to a quadratic function with a negative second-order coefficient, and the increment of penetration during each 8-hour donning cycle is less at 0.9%. Therefore, reusing the N95 respirator with multiple cycles and dry heating or disinfection by UV irradiation is realistic.

“However, since most current respirators use electret filters as filtration materials, disinfection treatments in the form of aqueous solutions or vapor, such as soap or hot water immersion, spraying alcohol and high pressure steam, will neutralize the static charge on the filters, resulting in a significant decrease in the filtration efficiency of the respirators,” the investigators wrote. “Dry heating, UV radiation, and hydrogen peroxide vaporized hydrogen are disinfection methods that have been shown to be effective in inactivating microorganisms without compromising the filtration efficiency of the respirator.”

Of these options, vaporized hydrogen peroxide, which requires specialized equipment, is too difficult compared to microwave ovens for dry heating and ultraviolet lamps for UV radiation.

Other experts agree. “The VHP is the longest, requiring a 40-45 minute decontamination period, not including the time required for several other critical process steps, including the vaporization of liquid hydrogen peroxide,” Christina Yen, MD, Ahmed Abdul Azim, MD, Preeti Mehrotra, MD, MPH, wrote in Infection control today®. Therefore, dry heating and UV disinfection methods are applicable to the disinfection of reused respirators.

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