University of Cincinnati Medical Center Report On the Performance of Surgical Masks Operating with the Air Purifiers
Environmental Health Foundation
DEPARTMENT OF ENVIRONMENTAL HEALTH UNIVERSITY OF CINCINNATI
This memo is to summarize the results of the Phase 1 tests that have recently been performed with surgical masks sealed to a manikin (with an absolute fit, i.e., no leakage). In this memo, I would also like to inform you about some preliminary findings of Phase 2, which was initiated to test the masks worn on a subject (the Phase 2 study design enables us to address the leakage issue).
This phase included the following:
- the indoor air cleaning efficiency evaluation of your five products, such as Vortex VI-3500*, Minimate AS180i (positive and negative)*, Automate AS1250B*, and Sanimate AS250B*, conducted with the ELPI in a virus-size range;
- the filter performance tests conducted with N95/R95 respirators sealed on a manikin when operating in the presence of high ion flows emitted by your products; and
- the latter tests conducted with surgical masks.
The indoor air cleaning efficiency data for all the Wein ionic purifiers tested with the ELPI have been submitted to you earlier. The indoor aerosol concentration decreased significantly due to the ionization, especially when using the Vortex VI-3500* air purifier: a 30-minute operation of this air purifier in a typical room (volume = 25 m3) removed about 97% of 0.1 µm particles and about 95% of 1 µm particles from the air. You have also received the data on the performance of the N95/R95 respirators sealed to a manikin, which demonstrated an ionizer-driven improvement of the filter collection efficiency by a factor ranging from 1.6±0.1 (Automate AS1250B*) to 4.5±0.7 (Vortex VI-3500*).
My report below is focused on the performance of surgical masks operating with the Wein air purifiers.
First, we tested a 3M surgical mask (Model 1838, widely used, very popular) that was perfectly sealed on the manikin face and operated at the inhalation flowrate of 30 L/min. The collection efficiency was about 80% for submicron particles, including the virus-size range of 0.04 to 0.21 µm that was specifically targeted. This 80% efficiency translates into a protection factor of 5. The protection factor (also referred to as the fit factor, American National Standard – Fit Testing Method, ANSI Z88.10-2001, p.1) is the ratio of the aerosol concentration in the breathing zone outside the mask to that inside the mask. The factor is generally particle size dependent. The most penetrating particle size range is about 0.1 to 0.3 µm. Once the Vortex VI-3500* was switched on, the protection factor started increasing and exceeded the level of 70 in about 3 minutes of the ionizer’s operation (the average value during this time interval). It jumped to about 400 in 6 min and continued further increasing with the time (although at a lower rate). The above effect exclusively represents the enhancement of the performance of the respirator filter material. The rapid decrease of the ambient concentration due to the Vortex VI-3500* was taken into account when determining the protection factor. The Minimate™ AS180i* units (positive and negative) have also demonstrated a considerable enhancement effect: the protection factor increased from about 5 at t=0 to over 70 at t=3 min and was relatively stable at the level of 70 to130 at t = 6-12 min. The Automate AS1250B* showed some enhancement as well; however, the effect was weaker (the reason was previously discussed with you).
Overall, we are excited to see the filter performance effect of this magnitude, although it is understood that Phase 1 was set to test the masks in a perfect fit condition while the surgical masks have generally a very poor fit potential.
This phase was initiated to address the leakage issue. Indeed, in a real life the protection factor depends on the particle penetration through the respirator filter material as well as on the particle penetration through the leakage. The leakage of some size always exists between the face surface and the filter. The face/body movement increases the potential of the particle penetration through the leakage. The standard fit test is performed to determine an individual’s ability to obtain an adequate seal with a specific respirator. For instance, the fit test performed with the N95/R95 facepiece respirators using the Portacount (TSI, Inc.) is supposed to check whether these respirators fit well enough so their overall collection efficiency exceeds the 95% threshold (protection factor >20). If the filter material is very efficient and creates a good barrier, the aerosol tends to flow through a leak, especially if the pressure drop through the filter is high. Once the collection efficiency of a filter material significantly increases (e.g., >>95%), the potential of the particle penetration through the leakage may increase tremendously (as the pressure drop change may result in a rerouting of the aerosol flow). Under certain conditions, this pathway may become a primary one. Therefore, it is important to run the tests not only with a manikin with a sealed mask but also with a human subject, using the Portacount as the standard method. The exploratory part of Phase 2 was performed in collaboration with Dr. Roy McKay who actually conducted the fit testing of the 3M-1838 mask on me since I volunteered to be a subject. The standard fit testing protocol, which utilized the Portacount, included numerous procedures (normal and deep breathing, moving the face and the body left and right and up and down, talking, etc.).
The initial protection factor of the 3M-1838 surgical masks was found to range from 3.5 to 4. These values are slightly lower than those obtained in our Phase 1 experiments carried out with the mask sealed on the manikin. The difference points to the leakage effect. The protection factor determined increased to about 30 (t ≈ 9 min) when the Vortex VI-3500* was operating, thus turning a surgical mask with a poor fit and relatively low filter efficiency into an N95-level respirator in terms of its collection characteristics. Indeed, the collection efficiency of the surgical mask exceeded 95% due to Vortex VI-3500*. The Minimate™ AS180i* unit demonstrated the ionizer-driven improvement of the efficiency from 3.5 to about 9. The enhancement of the mask overall performance was lower than that observed with a more powerful Vortex VI-3500* but still significant: almost 3-fold.
The data suggest that the leakage represent a clear limitation of the respirator performance enhancement effect, which could have been over an order of magnitude greater if the mask’s fit was perfect. We did not observe any fit improvement due to the ionization. Thus, we could not expect a perfect fit from a surgical mask because of its design. We anticipate that the average leak size remained about the same while the filter material exhibits a much better protection due to the ionization. It is believed that since the particles and the filter fibers charged unipolarly by the ions, the repelling forces decreased the particle flow toward the filter. This consequently reduced the number of particles that could potentially penetrate through the mask and be inhaled. In spite of the fit factor limitations, the overall performance of a surgical mask against virus-size particles seems to drastically improve due to the constant ion flow produced by the Vortex VI-3500* and Minimate™ AS180i*.
In addition to our tests with the surgical masks on a human subject, we conducted one run with the R95-type respirator that fits to the face much tighter than a surgical mask. Due to its rigid periphery, it can be easily adjusted to a specific shape and thus has a better fit potential. When operating the Vortex VI-3500* located at a distance of 40 cm from the human face, the overall protection factor demonstrated a 4-fold increase, exceeding 1000 for certain procedures (normal breathing and deep breathing). The above improvement of the respirator performance agrees well with our Phase 1 results obtained with this respirator sealed on a manikin (4.5±0.7). The slight difference can be attributed to the leakage. The leakage effect was not as significant for the R95 respirator as the one observed for a surgical mask.
It is understood that the above-described Phase 2 findings are preliminary and we are interested in continuing this phase beyond the exploratory level.
The above-summarized data are being further analyzed from the statistical viewpoint and presented in a non-dimensional graphical form. The detailed data report on Phase 1 and the above- summarized data on Phase 2 will be submitted to you within a week.
All the objectives and specific aims proposed for Phase 1 and the exploratory stage of Phase 2 have been met. Two issues were addressed beyond the work scope, originally outlined for Phases 1 and 2 (see below).
Air cleaning, exposure to infectious agents and overall risk reduction:
We have concluded that an ionic air purifier exhibits two mechanisms, which decrease the number of infectious particles inhaled by a person wearing a respirator mask: the reduction of the indoor concentration upstream of the respirator and the enhancement of the respirator performance. The sample estimate presented below exemplifies the cumulative effect resulting from these two mechanisms. The calculations were performed based on the data obtained with the Vortex VI-3500* unit. Please keep in mind that this is only estimation but not a full-fledge risk assessment!
After the Vortex VI-3500* air purifier operates continuously for 15 min in a 25 m3 room, the concentration of submicron particles in this room decreases by a factor of 6. The overall protection factor of a surgical mask, enhanced by the ionic purifier, is about 30 (this takes into account the finding that the considerable improvement of the filter characteristics was partially suppressed by the leakage effect, see Phase 2 results). Thus, the number of submicron particles inhaled by a person is reduced by a factor of 6x30=180, instead of about 3.5 - 4 provided by a surgical mask alone. Let us assume that the concentration of influenza virus in an indoor environment is 1000 m-3. Its infectious dose, ID, is 79 viruses (inhaled). The air volume inhaled during one hour is 1.8 m3 assuming that the breathing rate is 30 L/min. Thus, an unprotected person will inhale 1,800 viruses (> ID50); the person wearing the surgical mask will receive 1,800/4=450 viruses (> ID50); and a person walking into a room where the Vortex VI-3500* was operating for about 10 min will inhale about 1,800/180=10 viruses (< ID50). This example shows the potential of the ionic air purifiers for the exposure reduction when they are used together with respirator masks. More comprehensive assessments that include infectious characteristics of other viruses/bacteria can be performed upon your request.
The ion emission rate and the aerosol particle mobility (or how much should the ion production rate be increased?). The particle charges were measured in our experiments with the ELPI in its charge distribution mode (the software was obtained from Dekati, Inc., Finland). The particle charge distribution was also assessed using the diffusion charging theory (described by Hinds in his “Aerosol Technology” book, 1999, Chapter 15). The experimental and theoretical data are in a good agreement. It was found that the particle charging level is close to the highest possible. In my opinion, this aspect deserves to be further investigated. There is a saturation charge level for every particle size. The performance of ionic air purifiers depends on the particle mobility, which is a complex function of their size and charge. Ideally, any newly-developed ionic air purifier should be evaluated as to its ion emission rate. If this rate is too low, it may be insufficient to drastically affect the particle mobility. On the other hand, starting from a certain level, any further increase in the ion production will probably not affect the air cleaning performance since the aerosol particle charging has essentially reached a plato. In the latter case, a constant supply of ions is needed to maintain the air cleaning efficiency level, but the performance would not improve if the ion concentration increases. Thus, it is important to know how much effort should be devoted to the increase of the ion flow emitted by a specific model.
Please let me know if you have any questions. We are certainly excited about our findings and look forward to working with you further on Phase 2. Dr. McKay has agreed to continue collaborating on the project if so requested.
Sergey A. Grinshpun, Ph.D.
Surgical Mask (3M 1838) enhancement factor
*This document orginally pertained to the Vortex VI-2500, Minimate™ AS150MM, Automate™ AS1250, and Sanimate™ AS250B Air Supply products,.