Prosthodontics, Special Patient Care, Maxillofacial Prosthetics, and Weintraub Center, UCLA School of Dentistry

N95 respirator testing

In order to prevent viral particles from entering the respiratory system, N95 respirators must meet three important criteria. First, the mask materials must provide excellent filtration efficiency at the virus size (0.1 micron). Second, the mask must minimize aerosol leaks by adapting to facial surfaces during normal facial movements. Third, the filtration membrane must exhibit minimal pressure drop at physiological respiratory flow rates.

The test for mask adaptation to face is called the “fit test” to evaluate leakage around the edges. Not every mask type provides the same fit, and the seal can range from 56.5-99.7%!   Even though N95 masks are less leaky, they can range from 85-99% depending on mask and strap designs. This is why healthcare workers must be properly fitted each time a new N95 mask is introduced into the hospital system.  Fit can be evaluated by a qualitative test involving the use of aerosols from saccharin, and denatonium benzoate. For quantitative fit tests, OSHA and NFPA rely primarily on photometric lasers to count the nanoparticles in the ambient air and compare against the number of particles inside the mask.  The fit factor is determined by dividing the number of particles outside of the mask versus the number of particles inside the mask, per OSHA protocol.

Pressure drop is a measure of the resistance that the air meets as it flows through the respirator filter and into the mask. This is important because the pressure drop affects the comfort and breathability of the mask- specifically a lower pressure drop is desirable as it translates to increased breathability and thus comfort. Since pressure drop is inversely proportional to the surface area of the filter, this information is critical to designing the size of the filter cartridge, and in turn the respirator housing design. To ensure that the filter does not inhibit air flow under normal inspiratory and expiratory pressure, we are designing our own precision instrument to measure differential pressure across the filtration membrane.  The pressure drop standard for N95 face masks is 350 Pa at 85 liter per min (LPM), but not all N95 filters are equal.  On average, those that have high filtration efficiency are also the ones with the largest pressure drop, which make it difficult to breath.  Our team is seeking N95 filter materials that offer the ideal combination of high filtration efficiency and low pressure drop, and these in-house testing capabilities will provide critical data in a timely manner for design, sterilization, and maintenance. These instruments will also allow us to quantitatively evaluate materials options for non-medical community masks.

Reference

https://smartairfilters.com/en/blog/coronavirus-pollution-masks-n95-surgical-mask/