Overview

This study evaluated the effectiveness of surgical, N95, and N100 respirators in filtering airborne viral particles using a highly controlled experimental setup. We employed a 1,000-liter Lexan chamber specifically designed for bioaerosol experiments, fitted with multiple sampling ports to measure airborne particulates. To generate a viral aerosol, we used a six-jet Collison nebulizer dispersing MS2 bacteriophage particles into the chamber.

Inside this chamber, a manikin connected to a respiratory simulation system was positioned. This system comprised a polyvinyl chloride (PVC) throat with a midget impinger—an air sampling device—to collect the inhaled particulates. The system also included a filter along the inspiratory flow path to prevent bioaerosol entry, and a Lifecare® PLV-100 mechanical piston ventilator simulated the breathing patterns of an adult at rest. The ventilator was calibrated to a tidal volume of 0.70 liters and a breathing rate of 16 breaths per minute, with an inspiration-to-expiration (I:E) ratio of 1:2.5, achieving a peak inspiratory flow rate of 60 liters/minute.

In this study, we used Schlieren imaging to visualize airflow patterns and identify leaks in the fit of surgical masks. Schlieren imaging, a method that exploits variations in air density to visualize flow patterns and turbulence, clearly illustrated the general inadequacies in the fit of surgical masks. Despite being designed to filter out particles, these masks often had visible leaks, particularly under the chin and around the nose, as well as at several other points along the mask's perimeter. These gaps allowed air to escape without filtration, reducing the overall effectiveness of the masks. However, the imaging also confirmed that a significant portion of breath was indeed passing through the mask material, indicating some level of filtration was occurring. This demonstrates that while surgical masks do not seal perfectly, they are capable of providing partial protection by filtering out airborne particles as air passes through their fabric.

Figure 1: Schlieren imaging of surgical mask on manikin. Image shows air currents during exhalation.

Viral Protection Results

Our findings indicate that none of the masks provided complete protection against inhalation of infectious aerosols. Surgical masks were particularly ineffective, blocking only about 11.9% ± 10.2% of viral particles and predominantly allowing air (and potential viral particles) to escape laterally, rather than through the mask material itself. The Food and Drug Administration (FDA) has already indicated that surgical masks do not offer adequate filtration of bioaerosols and are designed to be loose-fitting.

The N95 respirators showed a somewhat better performance, with a 39.2% ± 19.5% reduction in particle inhalation. They primarily directed exhaled air through the mask material, although some gaps near the nostrils were noted. However, even this level of protection is below the FDA’s recommended 4-log (99.99%) reduction for air purifying devices.

N100 masks demonstrated the highest filtration efficacy among the tested masks, with an 82% ± 30.5% reduction, equivalent to a 0.74 ± 0.16 log reduction in inhaled bioaerosols. Despite this, the performance still falls short of the FDA’s standards.

Figure 2: Reduction of inhaled virus by mask type compared to controls (no mask).

Summary

In conclusion, while the material of N95 and N100 masks can theoretically filter out most viral particles, real-world factors such as fit and wearer movement significantly reduce their effectiveness. This study underscores the need for better-fitting respiratory protection, particularly in medical and public health settings, to prevent the inhalation of infectious aerosols. The results and graphical data on the net reduction of inhaled viral load for each mask type clearly illustrate the varying levels of protection offered by these respirators.

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

Full Report – Web Version

Mask Study – One Page