Mannequin Mask Study — “Considerable relative aerosol concentration levels can be reached at a 2 m distance with masks”

This study was cited by Anchorage’s chief medical officer Dr. Savitt in the Anchorage Assembly meeting on 9/13/21 at 3:21:45. It’s clear now that Dr. Savitt’s explanation of this study was misunderstood. In assemblyman John Weddleton’s 4-minute justification for voting for the mask mandate (while touching his mask 21 times in 4 minutes), he claimed this “10% effectiveness” of mask would be helpful. BUT THE STUDY DIDN’T SAY “10% EFFECTIVENESS.” They were measuring “EFFICIENCIES.” The 3-ply cloth masks stopped 10% of the viral particles, allowing 90% to go right through, of which many particles could be measured at 6.5 feet.

Dr. Savitt said the cloth mask is “about 11% effective.” The article stated that the cloth masks were 3-ply, and were 9.8% effective in stopping particles from releasing through the mask. That means 90.2% of the particles went right through, not that the masks were 9.8% effective in stopping people from getting Covid, which some misunderstood, because Dr. Savitt compared it to Assembly Person Crystal Kennedy’s Bangladesh study, which was trying to quantify overall effectiveness also using 3-ply masks. The Bangladesh study is not peer reviewed, and has many problems that they actually admit in the study. They were claiming about a 5% benefit from 3-ply cloth masks.

– –

https://www.researchgate.net/publication/353484161

Experimental investigation of indoor aerosol dispersion and accumulation in the context of COVID-19: Effects of masks and ventilation

July 2021 The Physics of Fluids 33(7)
DOI:10.1063/5.0057100

IV. CONCLUSIONS The present study experimentally investigates the dispersion and accumulation of aerosol particles in indoor environments in the con- text of the guidelines proposed by national health agencies to control the transmission of COVID-19. Experiments were conducted in a controlled laboratory environment with a test manikin in a seated position mimicking relaxed exhalation through the nose, typical of an average adult. The manikin was equipped with five different commercially available masks that have seen widespread use throughout the course of the COVID-19 pandemic. Both near-field flow visualizations and far-field particle concentration measurements allow for a holistic investigation of the effect of masks and ventilation in the test room, and provide a quantitative measure relative to aerosol concentrations and mask efficiencies of interest for transmission risk assessment, model development, and implementation of adaptive health and safety practices at workplace. The results highlight that (i) considerable rela- tive aerosol concentration levels can be reached at a 2 m distance from the subject in an unventilated space, and even when the subject is equipped with a mask, the relative concentrations are notably higher than those expected based on the ideal/rated efficiency of the masks; (ii) fit of the mask to the face, in terms of limiting leakage around the mask perimeter, is critical for limiting aerosol dispersion in an unven- tilated space, especially for high efficiency masks (e.g., N95/KN95); and (iii) increased ventilation/air-cleaning capacity significantly reduces the transmission risk in an indoor environment, surpassing the apparent mask filtration efficacy even at relatively low air-change rates (2 room volumes per hour).

The baseline filtration characteristics for the various masks tested in this study indicate that more than 50% of aerosols (polydisperse, 1lm mean diameter) can pass through the material of commercially available cloth and surgical masks in ideal conditions (zero leakage due to fit), whereas ideal filtration efficiency is 95% (or higher) in the case of KN95 and R95 masks. Flow visualizations and velocity mea- surements in the near-field (immediate vicinity of the face) indicate that none of the tested masks is performing at their ideal filtration effi- ciencies due to leakages through gaps in the fit of the mask. This occurs around the cheeks, below the jaw, and at the bridge of the nose, with the latter being the most significant for all masks. Aerosols are seen to escape through these leakage sites in the form of concentrated particle clouds that do not mix quickly with the ambient air on account of relatively low flow velocities and hence low levels of turbulent mixing. The degree of leakage varies between masks, with high-efficiency masks, such as the KN95, performing better. Factors affecting leakage at the mask perimeter include mask geometry, strap style and elasticity, and whether or not the mask is equipped with a deformable nose piece that can be tightly shaped to the nose. Furthermore, although the present study does not characterize the effectiveness of masks during inhalation, the aforementioned loss of filtration efficiency due to perimeter leakage is also expected to be pre- sent during inhalation, although it is to a lesser extent due to the improved sealing effect produced by the negative pressure difference relation to the ambient.

The near-field velocity measurements indicate that the forward momentum of breath exhaled through the nose is reduced significantly and redirected when the subject is equipped with a mask. Furthermore, this attenuation of the forward momentum increases with the filtration efficiency of the mask material when a proper fit is ensured. Thus, the present results endorse the use of high-efficiency, unvalved masks with a proper fit when the recommended social dis- tancing guidelines cannot be maintained between individuals.

Measurements of aerosol concentration at a 2 m distance from the subject show a characteristic increase in average particle concentra- tion with time in the absence of ventilation, following the first order response based on the well-mixed room model. Across all cases, rela- tive particle concentrations saturate at elevated levels, indicating accu- mulation of aerosol particles within the room. When the subject is not fitted with a mask, the saturation concentration is the highest among all the cases tested. A decrease in saturation concentration is seen for all mask types; however, the effective filtration is notably lower than the ideal filtration efficiency of the material due to leakages in accor- dance with a mask’s ability to decrease the number of particles released into the room per breath. Thus, the apparent filtration efficiency of a mask (gAFE) is estimated based on the relative difference in saturation concentration at the measurement location between cases with and without a mask. This metric provides a more representative measure of mask efficiency and is of particular interest for future modeling studies and continuous occupancy risk assessment.

The results show that a standard surgical and three-ply cloth masks, which see current widespread use, filter at apparent efficiencies of only 12.4% and 9.8%, respectively. Apparent efficiencies of 46.3% and 60.2% are found for KN95 and R95 masks, respectively, which are still notably lower than the verified 95% rated ideal efficiencies. Furthermore, the efficiencies of a loose-fitting KN95 and a KN95 mask equipped with a one-way valve were evaluated, showing that a one-way valve reduces the mask’s apparent efficiency by more than half (down to 20.3%), while a loose-fitting KN95 provides a negligible apparent filtration efficiency (3.4%). The present results provide an important practical contrast to many other previous experimental and numerical investigations, which do not consider the effect of mask fit when locally evaluating mask efficiency or incorporating mask usage in a numerical model. Nevertheless, if worn correctly, high-efficiency masks still offer significantly improved filtration efficiencies (apparent and ideal) over the more commonly used surgical and cloth masks, and hence are the recommended choice in mitigating the transmission risks of COVID-19.

The directivity of aerosol dispersion was assessed through con- centration measurements at a 2m distance and at locations in front of (0), to the side of (90), and behind (180) the subject with a surgical and KN95 masks. For all the cases, the effect of orientation was less than about 10% of the local particle concentration and indicated a rel- atively minor directivity effect at a distance of 2 m. It is conjectured based on the flow measurements in the vicinity of the manikin face that significant directivity effects are confined to the relatively close proximity of the host.

The effect of ventilation/air-cleaning was consider using a HEPA air purifier at the recommended pre-pandemic air-change rates (ACH ¼1:7–3:2h 1). The results show that ventilation air-exchange or purification is effective in decreasing both the final saturation con- centration and the time required to reach the saturation state. Based on the apparent filtration efficiency, tests performed with no mask at an air-change rate of 1:7h 1(and higher) outperform cases with high-efficiency masks (KN95 and R95) and no room ventilation. However, at these low ventilation rates, a notable particle concentra- tion is still present at a 2 m distance, which is indicative of higher ven- tilation rates needed to ensure negligible aerosol build-up over prolonged occupancy.

Leave a Reply

Your email address will not be published.