This study assessed the effectiveness of surgical, N95, and N100 masks at reducing inhaled viral particulates. A custom 1,000L Lexan bioaerosol chamber with multiple sampling ports was used along with collision six-jet nebulizer to introduce aerosolized MS2 to the chamber.  Placed in said chamber was a manikin attached to a breathing circuit. Such a circuit included a PVC throat bearing a midget impinger for air sampling, a filter to inhibit travel of bioaerosols down the length of the inspiratory flow path, and a Lifecare® PLV-100 mechanical piston ventilator. For all mask trial sets, the Lifecare® PLV-100 mechanical piston ventilator was set to mimic the respiration frequency, tidal volume, and minute-volume of a typical adult during light activity. The ventilator test operation settings were controlled and set as follows: Tidal volume was set at 0.70 L. The breaths-per minute were set to 16 bpm. The Inspiration-to-Expiration (I:E) ratio was set to 1:2.5 with a corespending peak inspiratory flow rate of 60 L/min. Schlieren imaging was performed to observe the airflow preference for each mask during exhalation and to best fit the masks. Schlieren imaging employed a point source light, 114 mm parabolic mirror with a focal length of 1125 mm, and a razor blade to bisect the image at the focal point. This allowed for a difference in air density caused by heat gradients to refract some of the incoming light onto the razor blade and create a color gradient on the image.

Our results show that masks do not fully protect wearers from inhaling infectious airborne viral particles. Particularly, surgical masks provide negligible protection from bioaerosol inhalation, with a percent reduction of 11.9% ± 10.2% and an exhalation airflow preference for the lateral leakage points rather than through the mask material. It is important to note that the FDA states surgical masks are inadequate for the filtration of bioaerosols and are intended to fit loosely, rendering them ineffective at regulating the spread of SARS-CoV-2. N95 masks prove more effective, with 39.2% ± 19.5% reduction and an exhalation airflow preference through the mask material with some gaps near the nostrils. The reduction of inhaled bioaerosols through the N95 mask, while superior to the surgical mask, does not meet the 4 log reduction recommended by the FDA for air purifying devices. Likewise, the N100 mask displayed a 0.74 ± 0.16 log, or 82% ± 30.5%, reduction of inhaled bioaerosols, falling far below the recommended reduction value. Results for the Net reduction of total inhaled virus are shown graphically below for all trials.

Figure 1: Percent reduction of respirable RNA virus when using a respirator. Ventilated human model testing.

This study shows that while the filtration material of an N95 or N100 mask may theoretically provide adequate protection, when used in a real-life scenario accounting for fit to the wearer, these type of masks provide minimal protection against respirable viral bioaerosols.

The full paper and a one page infographic can be downloaded and viewed below: