Caltech Science Exchange Presents Conversations on COVID 19 Why Masks Work

Show video

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.

2020-12-30

Show video