Caltech Science Exchange Presents Conversations on COVID 19: Why Masks Work
Good morning and thank you for joining us. My name is Emily Velasco, and I'm a science writer with Caltech's Strategic Communications. It's my pleasure today to welcome you to the first at a new series of webinars: Conversations COVID-19, hosted by the Caltech Science Exchange. This series is designed to give all of us a chance to hear directly from researchers in the community who are bringing insights and solutions to the COVID-19 pandemic. The first of those researchers is Rick Flagan, the Irma and Ross McCollum-William H. Corcoran
Professor of Chemical Engineering and Environmental Science and Engineering. Professor Flagan's research centers on the control of air pollutants and aerosol processes which gives him a unique perspective on the spread of disease and how masks work as an important disease control measure, professor, Thank you for joining us. Thank you for having me.
Yeah, absolutely. How are you today? Just fine. Alright. So, we have some questions for you. And I think we can just start, you know, as I mentioned in the intro, you have typically worked on air pollution aerosols in your research but you've decided to shift that research to looking at COVID-19 and masks recently. Can you describe for us the moment when you made that shift? As we went into this crisis, it became clear that we had a had a need to protect Not just ourselves, but to protect the entire community.
The question came up very early on as to how was Caltech eventually going to reopen the result were a lot of groups on campus, who were looking at getting masks, in order to be able to protect people, but there was really no basis for judging, which masks were good, which were not. I have a laboratory that's fully equipped with instruments for measuring particles. And so it was natural to start applying those to characterizing the masks initially focusing on on the Caltech reopening but looking down much more broadly. Okay, so, break it down for us if you would, what do we know about the relationship between COVID-19 and aerosols. Well, we know that people who are infected with COVID-19 do emit, at least the viral material.
That was seen very early in the pandemic when the Diamond Princess. passengers who had the disease were taken to the University of Nebraska hospital, they found the COVID-19, SARS-CoV-2 viral RNA in the air. They've later confirmed that it was live active virus in the air. I've been involved in some studies done on patients in Wuhan, helping to analyze data, and and in Beijing. They're done direct measurements, looking at the emission of SARS-CoV-2 from patients. And we see the viral RNA in their exhaled breath.
All right, so you've worked with some other faculty members on campus to look at the different kinds of masks that are available today, and which ones are most effective. Can you talk a little bit about that? Well Joe Kirschvink started some work characterizing masks with a very simple system, quite early on, and keep up with some ways to try to clean masks. I got involved when I learned of that work with him.
Trying to validate whether or not the methods work we're doing the cleaning as requested, as desired, but also looking much more broadly at masks. And so we've been collaborating, although the detailed work has been done in my laboratory. Okay. So, so what did you find? What has your research found so far? Well, we found that there is a great variability in masks. The cloth masks are moderately effective. Good cloth masks will reap move about two
thirds of the particles that you might inhale. There are others that will pass through 90%. Yeah. Yes, you can tell, for some by inspection, that they will pass particles through. If you can see through them, they're not doing a very effective job. So, not only it's not just what you inhale but masks also prevent what you exhale as well, right? Yes, the masks are really about community protection, as well as about personal protection.
Okay. So besides the cloth masks, what are some other kinds of masks that are out there. And can you talk a little bit about the effectiveness of those different kinds of masks? So, we have the cloth masks, which are made in many different ways. Out of many different fabrics, it's very difficult to know what a particular mask that you might buy is made from and how it will work. There are the pleated masks, masks that are designed so that they can be expanded remaining somewhat narrow around the sides, but giving a larger area to cover the nose, the chin so that they try to restrict the airflow. There are many different varieties of these, this is a relatively inexpensive one, it removes about 60% of the particles. It's about like a cloth mask in its effectiveness. There are some of the pleated mass that will remove particles with much better than 90% efficiency.
But you can't tell by looking at them which is which. Okay. We, we did not know what this was until we tested it. Okay. Is there any way that people can tell. Right now, it's very difficult. Okay, if you look at quality masks that are
certified by some standard. There are many different varieties there there are N95 masks, I don't have a proper N95 mask. I do have one here, that is, I bought for woodworking and it's very good for that. It's lousy for dealing with this pandemic because it's got a valve, something you definitely don't want. But the one thing it does have on it is an indication of who made it.
And that is been certified, according to the standards. If you buy a mask that claims to be an n95 mask and it doesn't have that, it's not. If you buy one that has ear loops the claims to be an N95 mask, it's not, the other N95 masks use bands to go around the head, in order to give more secure fit.
Yeah, there's also the KN95. Now this is a different standard your stamp this KN 95 on the other side is a clear indication of the specific standard, that is. Now, this is a Chinese standard. They are efficient in the material sense but what the, what they lack is this very rigid structure that is designed to keep it off your face so you get a large surface area for breathing. This one has a flange inside that seals very effectively against the face. This one does not have that structure. They come folded flat. It has a metal band,
you can shape to your nose and you have to shape it to get it to seal around the eyes, dealing from the bridge of the nose to the cheekbone is a hard place to seal. It doesn't have any special sealing and it has ear loops which are not very effective at fastening it to the mask at fastening it to the face, that won't hold it tight enough. I tend to put a very simple fix a slipknot that I can pull tight, to make it a little bit tighter on my face.
Yeah. So, a couple of follow up questions. You mentioned that your woodworking mask for example has a valve and you said that's not ideal. So can you tell us a little bit about why a valve is a bad idea on a mask? The valve is there to make it really easy to exhale, so you don't have warm air accumulating in the mask you exhale, it all goes out very easily. That includes everything that you may emit. So the emission of the virus from someone who is infected who may not be showing any symptoms is not stopped at all by that mask. Slowed slightly but not significantly.
That is not a suitable mask for dealing with a pandemic. Okay. And then, you mentioned the nose band, and I've seen people talk online and elsewhere. They talked about, if, like you and I both wear glasses. And they said,
if your glasses fogged up when you wear your mask, then your mask is not sealing right so like can you talk a little bit why is it so important to get that seal around the bridge of your nose. Well, it's the seal all the way around your face the bridge of the nose, depending upon the shape of your nose, you have a large gap, and unless the mask is pushed down inside that region. So when I put this on. It's pushing down here, if it's pulled tight on my face. It doesn't leak significantly it doesn't fog my glasses, but it can also leak around the edges, if you look at the surgical type masks, they tend to leak around the sides as well. Yeah. and it's just stopping the emission.
Yeah. So maybe we can give people a little bit of news they can use, I know that you know, like N95 masks are not something you wash but a lot of people were cloth masks, and you want to wash them because they do get dirty but is there, Have you found anything in your research about, is there a limit on the, on the number of times you can wash a mask, like how often do you need to wash them ask? Did you find anything out like that? I have not found anything and we have not tested long enough to see the degradation in the fabric. The woven fabric has holes that it as you wash it, you lose some of the fine fibers that are actually doing the filtration and twist of threads are less, much less effective. You may find that it's degrading with time. We have not gone through many wash cycles we would have to do to evaluate that. Yeah.
For disposable masks that is there any kind of a limit for the number of times you can wear disposable mask and is there any way to clean them that you found? Well there's one way to clean them, that works, that is actually been approved for the N95 masks, and that is to use vapor phase hydrogen peroxide, which would be very dangerous to breathe. But it can do a very good job at deactivating, any microbes or virus that's on the mask. Beyond that, there are a lot of attempts to deactivate virus on masks. None of them have proven effective without degrading the masks. washing the nonwoven masks tends to degrade the material and in particular the N95 masks are built with charge stored in the mask. When you wash them you wash away that charge.
And that dramatically decreases the efficiency. As you wear the mask you deposit particles you deposit oils from your face to put a moisture into it. If gradually degrades.
We have done tests on masks that have been worn several days. By with with the KN 95 masks the Chinese standard masks, we have found that after about four days, the efficiency of the mass decreases, and the pressure drop increases so they become less comfortable. Yeah. So, I don't think we need to get too in depth on this but you mentioned that the masks have like a charge built in. I assume that that has it's somewhat along the lines of how ionizing air filters work, in some sense, but can you real briefly explain what you mean when you say that mask has a built in charge? So, there are many different ways they do it, they. One way is just to expose
one side to positive ions the other side to negative ions and deposit those on the mask. You deposit enough charge on the mask you produced low, very small scale local high electric fields that induce dipoles in particles as they're passing through the mask and enhance their deposition. They collect neutral particles, as well as charged particles. In that way. So that the charge helps the particles including particles that would contain virus stick to the mask better so they don't travel out into the air around you is that right? they help them reach the surface of the mask, of the fibers better once they reach the surface of the fibers. The forces Van Der Bal's forces between the fiber, and the particle are very powerful. If
the particle is wet capillary forces come in the particles will tend to stick quite effectively. Okay. So we know that cloth masks, don't work as well as N95 masks and we know that you can't use N95 masks indefinitely. So I think for a lot of people, the reality is going to be. It's going to be hard for them to find N95 masks. Still, and that they, you're going to have to go through a lot of them to keep them effective so for people who only have a cloth mask, if we know that scientifically it doesn't work as well as an N95. Is it still worth wearing of class mask and why? Well, if you if you consider if you're wearing a mask, you're reducing your exposure by, for a good mask by about two thirds, you're reducing your dose to one third of what it would be without the mask.
If the person near you, is also wearing a mask. They're reducing the amount they're emitting by the same factor. And so if you have one third protection for you and one third protection from emission, multiply those together, and you've got down to about a 10% probability. So you've reduced, your exposure by a factor of 10. And we don't know that there's someone in the room that has the disease.
Right. If you look at the statistics in the, in the statistics as of this last week, 11.1% of the tests in the Los in the Southern California area are coming back positive. At that rate of positive tests, if that were representative, the whole population. If you were in an area with 20 people, you'd have a 10% probability of not having an infected person in the room. So 90% probability that someone in the room is infected.
You'd like to reduce your exposure and potential exposure of others as much as possible so if everyone's wearing a mask that gives you another order of magnitude reduction. Great. Yeah, I mean, those are those are good odds one-tenth is a lot better than 100%. I know that some, some cloth masks come with insertable filters that you can replace. Has your research looked at those at all and and do those present any more of an effective solution than just a plain cloth mask? You can use removable filters, effectively, there are masks that are an elastomer, a soft rubbery polymer that have a filter on on those filters can be extremely effective. When you put a filter into a pocket on a cloth mask, and we have tested some, unless you've actually sealed the cloth where the filter is not, you still have air passing through the cloth itself. With that lower, lower efficiency.
The other thing we've seen with the ones we have tested, is that the filters that were provided, which were labeled as PM 2.5 filters. What I'm sorry What is pm 2.5 million real briefly? PM2.5 is, is the standard used for looking at particles in the air with regard to potential health effects.
Okay, particles smaller than two and a half microns. All right, I'm sorry, go ahead The pressure drop was high. Okay. Um, so there are a lot of different kinds of masks out there , and you're armed with more knowledge than a lot of people from your research, if you were going to design a mask, what would the essential features of that mask be what what would your ideal mask look like? It would use a high efficiency filtration medium. But it would not be made would probably not be made entirely from that. Just yesterday there
was a discussion of establishing new standards for masks that are made of these rubbery polymers that would then have separate filters. Okay. And this I think that is is likely to be what what we will end up with. If we look at worldwide and we need to address this problem, not just in the US,
we need to address it in the entire world if we want to get this pandemic fully under control. We've got to figure out ways to provide protection to people in areas where they don't have the wealth to purchase elaborate masks. And for that, it may be something as simple as a piece of a high quality filter medium held in place by a frame that pulls it tight against the face and is shaped to fit over the areas where you get leakage. I think there's likely different solutions for different regions, but we really need to address the problem, not just in the US but also globally. Sure. So what comes next for your research, you've been at this for for a few months already so where does this research go next what what do you hope to look at, going forward, and what directions might might you take? Well, near term.
The thing we've focused on so far has been on the filtration ability of the material. We've got to go beyond that to look at the test. And so over the, over the next week or two, we will be modifying our system to use a face form, so that we can simulate fit and evaluate how masks work. Now to do that we have to look at the many different facial types that we have in our population, and that there are differences, depending upon your genetic background, and we need to address that. As we look at the mask. One mask will not work for everyone. That's the near term.
Long term, we really need to understand how to, how to, how to quantify the exposure that is occurring. We don't really know quantitatively the emission rates that are occurring we do know that people are emitting. We know that when you just breathe particles are coming out of the deep lungs, they tend to be relatively small. When you're speaking. They're coming. You have
additional particles generated from the vocal cords and from actions in your mouth, your tongue that increase those emissions and lead to much larger particles. People who are singing. There was the big episode up in Skatchet county Washington, where they had a large number of people exposed, from one, one person in a choir practice 60 people for two and a half hours at the end of which 45 develop the disease. We need to understand those sources, much more quantitatively. And we need to be able to do it much more quickly, because when the next pandemic comes, and there will be more, there have been several over the last couple decades, and there will be more.
We have to be prepared to understand what are the mechanisms, we need to consider all the possibilities, not assume that only big droplets are responsible. We need to consider the aerosol, we also need to consider the fomites the particles deposit on surfaces, all the different mechanisms to understand how to protect people. Sorry What is a fomite? I've not heard that term before. Fomite is the medical term used to describe transmission due to particles that have been deposited so if you cough and emit spray out large droplets. Those will fall and settle deposit on surfaces if you touch those if you touch your eye. You can transmit the disease to yourself.
Okay. That's why washing your hands. If we could back up a little bit. You talked a little bit about changing your testing procedure.
Because you have to test how the masks fit to get a proper like reading of how many particles they filter can you talk like real briefly about like how does that work is it essentially like a mannequin head that you put masks on like how, like what's the process? It's exactly that. Except, we need the mannequin have to have the softness of a real face we need to be able to deal with, with how the mask fits on a real person. What we have is a chamber small vacuum chamber that we've adapted we introduce an aerosol into that chamber we measure the size distribution of those particles going in. And then we sample behind the mask.
And that's what we've been doing now, but we've had the mask, very firmly mounted very firmly sealed so we're measuring the material efficiency. Now what we want to do is to measure with the mask fitted as it would be worn. And look both at inhalation and at exhalation. So that we're getting both, what is the protection of the wear and what is the protection of the community. Yeah. So, like you said, like our human faces are soft but a mannequin face is is rigid, so are you. Is your research looking at like how do you make. When you say let's
make a better model is that, like how do we make a mannequin with a soft face, is that something you're looking at? Fortunately there have been people who have done a tremendous amount with that already. In the field of robotics people have developed robots, whose faces look, human and respond human when they speak they get get the same facial motions that we have when we speak. We can adapt the technologies that have already been developed. Okay. We have just a few minutes left before we start taking audience questions so I wonder if there's anything that are there any common misconceptions that you hear, out, out in the world about the pandemic about the masks, about how to wear masks, about what kind of masks work and what don't? Are there any misconceptions that you think we should talk about? Well, there are many misconceptions that were propagated early on, by the fear of depleting the supply that was needed for protecting healthcare workers. The confusion between what is an aerosol and what, what are the droplets.
Well, the droplets are part of an aerosol and aerosol is particles in a gas. It's not just the particles of particles plus the gas. The particles that can penetrate into the lungs tend to be relatively small smaller than about 10 or 15 microns. The particles that can be emitted depending upon where in the lung they origin are where in the respiratory tract they were, they were only small ones can come from the deep lungs, bigger ones can come from the throat and the mouth or nose.
that's one aspect. The question of whether masks work. Well they only work if you're protecting the entire route by which particles can go in or out. That means it has to cover and seal around the nose and the mouth. For men, if you have a beard, you've provided a leakage path.
Yeah, you know, my brother works at a hospital and he had a beard, and he had to shave it off because the hospital decided they needed better sealing around their masks, so he looks kind of funny now because I've used him having a beard but he's, he's beardless. And we're not for the mask issue I probably would have grown a beard while I'm here at home. But I won't. All right. Well, um, how about we take some audience questions, I think, I think we've got quite a few questions here so let's let's take a look and see what people have asked. Alright so we have a question about. So this question is, with COVID being so highly
contagious, why aren't masks use masks considered to be hazardous material to be disposed off? Used masks should be considered hazardous material that they if they if someone is wearing one who has the virus, or if someone is wearing one who's been around someone who has the virus, one side or the other of the mask has the virus. Right. And they need, need to be treated so if you look at what hospitals have been doing. They have been short on supply. They have adapted systems to do this vapor phase hydrogen peroxide cleaning.
And they process the masks in large numbers, so that they can reuse them. All right. We have another question here this this is interesting, do masks also work for cold, flu, etc. And if so, why hasn't the US, used masks before?
Did you ever wear a mask to prevent flu or anything like that before this pandemic? I have rarely worn a mask. when I've been traveling. If I'm in really crowded places in a packed airplane and people are coughing around me, I will pull one out and put one on. I've done that before. But I have not generally worn one.
Will I in the future? Yes. Yeah. And do we know what does the science say do these masks work as well for things like the flu or the common cold as they do for COVID. They definitely do work.
There's a lot of experience with that, especially in Asia, where people have adapted to wearing masks quite a bit because the original SARS virus, there was definite evidence of that being airborne early on, and masks have become a part of the culture over the last 20 years. Great. We have another question. Do you think that masks will still be necessary after we get the vaccine? I think they will be necessary for some time after we get the vaccine, how long that will be is a wide open question. We don't know what the sources are.
We don't know if someone who has been vaccinated and is then exposed to the virus would have some low grade infection perhaps and be emitting virus, but not showing symptoms, we already know there are people who have the infection and don't show symptoms. Yeah. So until we have direct evidence for that. I would strongly support wearing masks. Okay.
So several people have asked, they want to know what kind of mask, you wear. So what is your preferred mask? My preferred would probably be an N95, but I wear a kn95. So, This is what I'm wearing. When I go outside. And why do you wear a K95 rather than an N95. Because the N95s are in very short supply.
And the people who are on the front lines are putting their lives at risk to protect us. And we really owe it to them to allow them to have that supply. Now hopefully we can find ways to get higher quality masks available to everyone. The key feature in that is make the complex part of the mask something that will last and can be washed decontaminated and make the filtration part of it something that they can be installed, effectively, and securely and works well. That's doable.
Yeah. Alright, we have another question. They want to know, they said they've heard that if you put a ear loop surgical mask, and I assume that means, one of the pleated masks on. And then you put a cloth mask
over that, that the results are almost as good as an N95. What are your thoughts on that. The results will be better but only if they're actually sealed all the way around. The pleated masks.
The surgical style masks. Generally, perform much, much less well than less effective than what you would get with a mask, that is designed to seal. There are adapters that out on the market today that you could put over the mask and seal it all the way around. And at that point, they could be every bit as effective. All right. So, someone wanted to know earlier in the interview we, you mentioned that there, there's a pressure drop, when you wear a mask longer. So what do you mean
when you say that like the longer you wear a mask, the more of a pressure drop there is? As you deposit material on the mask. As you expose it to the humidity from your breath. The fibers may rearrange, in some way, the pressure drop in the mask is increasing we have not done the detailed microscopic analysis to figure out why, we've been focused on the big picture, do masks work, but we find is that after a mask has been worn for three or four days, the pressure drop about doubles. Okay, that makes the mask less comfortable. Okay, so the pressure drop is that that feeling when you exhale, and it feels like your mask is filling with air that's that's what you mean by pressure drop or when you inhale it's harder. Okay, so that's when you breathe in and the mask kind of sucks against your face. Now, you could do something that holds the mask better awake way from your face.
I've seen 3d printers used to make gadgets that you can fasten on a pleated mask and make it much easier to wear. Someone wants to know, are the results of your mask tests available for people to see anywhere. There are some results on one of the Caltech websites.
We have a paper that's just about to be submitted on that, where we have looked at quite a number of different masks. Doing measurements on a number of copies of different masks in order to find out. Do they perform consistently? And we are we are publishing that. We have been focusing so far on just getting the data. But now we're going to be getting that data out and available, we will have it on the website. And some of it already is as part of the Caltech COVID response website. Great.
So, and I have a question about, one of our viewers wants to know if you have any suggestions for active people. I think a lot of us still need to get exercise, and he wants to know if you have any advice for people that run or bike outside. He says that our discussion has made him realize that the cloth Gator he's been wearing is probably not very effective, and he wants to know what options are there for people who engage in exercise, as far as masks go. okay well as far as its as protecting you and protecting others. wearing a mask is very important.
I see people bicycling, there's a peloton that goes by my house every Sunday morning, large number of cyclists one right after the other, riding in each other's wake. Most without masks, they're breathing in, what the people in front of them are emitting. And when they're exercising hard, they're breathing hard and that's increasing the emission. So you're increasing your intake you're also increasing your output.
And when you see a large number of people going the probability that at least one of those is infected increases. So wearing a mask is important. You have to be able to breathe.
Having a good quality mask would make sense. Definitely you want something that's going to stop the, the strong wind in your face effect from increasing your intake of the big particles that are emitted. I would definitely wear a mask. Probably the KN95s are the way to go at this point in time, hopefully we can get something better. Alright, so this is a follow up question when we talked a little bit earlier about us mastering hazardous someone wants to know if masks become less hazardous if they if you leave them for a while, I think the science has shown that COVID-19 virus does not live indefinitely on surfaces, outside the human body so do we know anything about like, if you if you leave a mask aside for a while, does it become not dangerous at some point? If you leave it for a few years probably what what what is the time, I don't know. Okay, the
evidence from the tests that have been done, is that a day or a few days, makes a big difference. Yeah, exposing it to sunlight to heat may make a further difference, But I don't think that's really been quantified. So, another person asks, there's been a lot of talk and there's been a lot of advice for people to wash their hands regularly, but she says that she's not heard of anyone saying to wash your face regularly do you think that washing your face after being out in public. Is there any value in that? There are some pathways for contaminants, to get into your body, such as the tear ducts. Ways to expose yourself. Definitely.
we touch our face far too often. Every time we do that we're transferring stuff from surfaces we've contacted to our to our faces. Yes, it would make sense to wash your face frequently. Yeah, I've touched my face, several times in this interview already so, yeah, I think it probably everyone does that all the time without even thinking about it. Um, we have another question. Are portable air purifiers with HEPA filters, effective for removing COVID-19 virus and the air of say your home? If they can process the air fast enough, they can do a good job. But you have to look at the
whole volume of the space, you're trying to clean and say how long is it going to take for the air to pass through that filter several times, because that's what's required to get a dramatic reduction in the particle concentration. If you do that, Yes. You may not need the most efficient filter to do that it may be the running more air through the filter will have a bigger effect than having the higher pressure drop filters that's harder to pump through that is extremely efficient but can't process the whole volume of the house. Yeah, we got another question here that says the air in my mask becomes hot and sweaty as I wear it. If my mask is wet from perspiration exhalation Does it still work? Has your lab tested the effect of humidity on masks.
We have not tested the effect of humidity on masks. Yet. Okay. To date, our focus has been on characterizing the masks, helping make rational decisions as to what masks to buy. Right. Um, someone wants to know you mentioned earlier that when you're on an airplane if you have someone coughing you would wear a mask and someone wants to know if you have had a chance to look at the US Transcom, I don't know. I don't know if that's US transportation committee or something. The US trans
com study on aerosol transmission aboard aircraft and what you think of those results. I have not looked at that particular report. I have seen some reports on that. The filtration media used on aircraft have been very efficient filters for a long time, but how effectively they're pulling the air from the entire space of the aircraft is a question I have, I have not looked into it in detail. But I don't know, know the answer.
We have time for just one more question, and I think this sounds like a good one. She says, I noticed many people wearing masks no longer feel they need to social distance. How can you assess the risk of exposure? The social distancing... Within a small distance from a person if I'm standing a couple feet away from someone and talking. I'm emitting aerosol if I don't have a mask, the other person is if, even if both have mask we're both emitting masks are not perfectly sealed.
So, some of the aerosols that's emitted is going out unperturbed what fraction we have to assess yet. There's a concentrated region right around your body. Anyone who is in the plume if there's any air motion that will be moving at the speed of the wind outside or the speed of the ventilation, air motion inside. You'll still be exposed you can be many feet away.
We've seen reports of viable virus 20 feet away from a patient in in a hospital, but six feet is a number that relates to the cough spread projectile sprays, the ballistic droplets that you emit when you cough or sneeze. Okay, that's where that comes from. There's also a concentration dilution that occurs, just with a gentler motion, as well.
But there's a plume, and that plume could take it could take it a long ways. An analogy. If you're at a restaurant, you're sitting out doors at a restaurant. There's someone at a table 20 feet away and they light up a smelly cigar. Do you smell it?
If the answer is yes, then, if they're emitting you're exposed. That makes sense. Alright, it looks like we actually have time for one, maybe one or two more questions left I'm being told. So, since you just mentioned that six foot spray. There's a question here about not not not face masks but face shields, can you talk a little bit about the effectiveness of wearing a face mask and then wearing a face shield above the face mask? If you look at someone who is, you know, in the line of fire, someone your health care workers.
Having that that face shield does two things one is, it protects the mask. So the mask will remain effective longer. It also protects deposition on the face. And for those other routes of contamination. That, that can be important. It's not going to stop the particles that can move with the air
motion and anything below 10 or 20 microns can move pretty well with the air motion. The face shield is a bit of extra protection, but it is not sufficient. Same thing goes with the plastic shields in the supermarket, and they will stop the projectile motion. But the stuff that you emit that can get carried with the gentle airflow will get carried with it. All right.
I think that we are out of time. So, um, with that said, I want to thank you so much Professor Flagan this was great, and I want to thank everyone who joined us today. We're going to be doing more of these, so be sure to sign up for our next one, it's going to be January 11 with my colleague Lori Dajose, and Professor Pamela Bjorkman, they'll be talking about vaccines.
In the meantime, if you're interested in exploring more of the science about viruses and COVID-19, please check out the Caltech science exchange, which launched last spring to provide trustworthy answers clear explanations and fact-driven conversations on critical topics in science and technology. You can find it at science exchange dot caltech dot edu. Thanks again. It's been wonderful having you.