Episode 3 Guests
David Pui Distinguished McKnight University Professor, University of Minnesota
This is the third episode of Dean Aaron Bobick’s new podcast: Engineering the Future.
How are dangerous nanoparticles formed, how do they impact our health, and how can we reduce those impacts?
From Washington University in St. Louis's School of Engineering and Applied Science, I'm your host Dean Aaron Bobick. And this is Engineering the Future, where we explore pressing problems of today in which engineering discovery, innovation and education can provide solutions for tomorrow.
Welcome to the second episode of our three-part series that we're doing on nanoparticles. Last time, we introduced nanoparticles: what they were, how some of them were produced, and just a bit about the properties that make them, well, frankly, weird and important. We learned that these particles can be created both as a byproduct of various processes such as combustion and also by design to produce new materials, or these are the so-called bad and good nanoparticles. In this episode, we'll dive a bit deeper on the environmental and health challenges of those particles produced by combustion and some of the technical elements in terms of how they interact in the atmosphere and with light. And then I'll save the more optimistic discussion for next time, where we'll explore the engineering uses of nanoparticles. Again joining me is Rajan Chakrabarty, Assistant Professor in Energy, Environment, & Chemical Engineering here at Wash U. And I realize, Rajan, you've been involved in so many of these, so let me thank you now for putting up with my requests to come to the studio.
So let us begin. To help us understand the nature of nanoparticles, I am thrilled to be joined today by two of my WashU Engineering colleagues, Srikanth Singamaneni, Professor in the Mechanical Engineering and Material Science Department, and Rajan Chakrabarty, Assistant Professor in Energy, Environmental, and Chemical Engineering. Welcome to you both, and thank you for making time to help our audience understand about the power and the challenges of nanoparticles.
Thank you, Aaron. Thank you for having me back again, and it's a pleasure to join you on your podcast.
So when we last talked, you described how one of the actually more interesting processes for producing nanoparticles was essentially fire and how the fire itself made these things happen. So my first question has to do with burning. We burn all sorts of stuff. We burn oil, coal, natural gas in power plants, gasoline and diesel fuel in cars. And in many parts of the world, they burn other fuels in their homes for cooking and heating. Do all of these processes typically produce nanoparticles?
In short, yes. And the reason being every combustion process is still incomplete, meaning if gas molecules get completely oxidized, you convert everything into CO2. But that's not what's happening when you are burning all these different fuels. So inadvertently, you produce these nanoparticles, which is an intermediary step in the process of combustion.
So that's kind of interesting because, at some other time, we'll talk about CO2. But basically, you're saying where you fail to produce CO2 because you haven't completed the combustion, what you get left with is a particulate, in particular, some of these nanoparticles.
Are some of these fuels much worse than others in terms of producing nanoparticles, given the way we typically burn them?
Yes. That depends on the extent of incomplete combustion taking place in the process itself. So for instance, if you are trying to go with solid fuels, like biomass or coal, you are going to produce more of these unwanted materials, which we call as aggregates of nanoparticles.
You said biomass. Everybody understands what it means to burn coal. We dig it out of the ground. We put it in a furnace of some sort. When you say biomass, what does that mean?
So biomass, you can think of it from two angles. One is for production of energy, like what happens in developing countries. You scavenge organic materials, and you cook a fire. So that particular category of biomass, we classify it as biofuels. Then you have the natural source of biomass combustion, what we typically see as forest fires.
Ah, got it. You and I have traveled to India together. The last time I was there, I was visiting Delhi and had the wonderful opportunity to drive out a couple of hours, about three hours or so, out to see the Taj Mahal. But along the way, the air was just awful. Where is all that particulate coming from?
Well, there are a number of sources identified. And in developing countries, such as India and parts of Africa, one of the major sources of these pollutants is cookstoves, biofuel combustion for heating purposes, for cooking purposes. That is one source. The other is traffic or gasoline combustion which takes place in very old vehicles. So these are some of the things which need to be controlled if you want to really reduce the amount of pollutants being emitted. And honestly, technology has to catch up in these countries.
So let me shift gears just a little bit. I know you spend a lot of time studying these particles in your lab. What are one or two of the fundamental things that you and your students do there?
Our primary focus is to characterize the pollutants emitted as a function of combustion efficiency from these different fires. We are currently focusing on studying the emissions from biofuels and biomass combustion. So when you talk about biomass combustion, we really have to cater to the problems which United States is facing in terms of wildfires. So we receive samples of wildland fuels from all over the country.
You mean like tree branches and things?
Tree branches, peat combustion. So like in Indonesia, we have these peat fires every summer. So our collaborators from Indonesia, they send us all these different peat samples to better characterize what is being emitted as a function of combustion temperature, and how do you characterize these particles.
This allows you to understand the nature of the particles that are being produced by these kinds of fires.
And what does understanding the nature of the particles produced by those fires-- why is that important?
So I'll give you two examples, and both of which are concerned with interaction of these particles with light. The first is, upon emission from a particular fire event, how do these particles interact with sunlight? And that has impacts on the Earth's energy balance in the atmosphere.
So in other words, how much, say, the sunlight will warm the atmosphere would be a function of these particles and their properties.
Yes. Their absorption and scattering properties. And the second one is what we call it in the community as the classical inverse problem. What I mean by classical inverse problem is you have all sorts of remote sensing tools now which involves shining a beam of laser or a light source, and you get a return signal after this beam of laser interacts with the population of particles. And then you have to extract information out of it.
So for example, we might have a high-altitude airplane or some other--
Or a satellite.
Or a satellite. And besides just taking passive imagery, where you just look at the imagery, you actually can project or aim a laser beam down.
Yes. Like a--
And then, presumably, you measure something about the scattering of that beam back to some sensor?
So all you get is a reflected beam of light. Now, from that particular reflected beam, you have to extract out information on the particulate properties. So essentially, it's an inverse problem.
So by getting a better understanding of what those properties are, you can do a better job of saying, "Given a particular scatter return, I can know about the particles. And that then let's me know about, say, the nature of the forest fires and dust and other kinds of things that are going on on the ground."
So fundamentally, by understanding how these particles interact with light, you produce much better remote sensing capabilities--
That is correct.
--which allow people to assess things about the environment or the ground or fires or the climate.
That is correct. So accurate can you invert that signal, that return signal, is dependent on how accurate is your algorithm which you're using all these remote sensing tools.
The better you understand the particles' interactions with light, the better you can take a light measurement and make an assessment about the particles.
That is correct.
In the first episode, we talked a little bit about these particles being very chemically active and that this might be really important with respect to what happens when, say, you breathe them into your lungs. I believe that you and your students also work on understanding those types of health impacts. Can you tell us a little bit more about that?
Yeah. So that's a secondary area of research in my lab, and we try to understand the impacts of these emitted particles on respiratory health. So just to give you an example, me and my students, we visited the villages of India, and we lived there for 10 days. And during the process, what we saw was that this incomplete combustion they emitted so much of nanoparticles. But these nanoparticles, they had a unique property, meaning they aggregated. And when you have a bunch of nanoparticles aggregate with each other, their effective density, they go down, which makes their aerodynamic property much higher.
So in other words, if I understand this right, so the particles themselves have a particular density. But when they aggregate, when they clump together, they clump even less dense than the actual particles themselves. So the overall aggregate is even less dense, still has lots and lots of surface area--
--stays suspended in the air. You breathe it in, and then these things become chemically reactive in your lungs.
Well, Rajan, you've told us a lot about the physical properties of what's going on here. Thank you so much for taking part. I'm sure I'll call on you six or seven times more [laughter].
Meanwhile, I have a couple of questions that I need to get answered, and I'll try to reach out to some other folks as well. But thanks again.
Thank you. It has been a pleasure. [music]
So after speaking to Rajan about studying the particles themselves and about their ability to stay suspended and enter the lungs, I really wanted to understand better the actual health effects of nanoparticles in the air. To get some insight, I was fortunate to be able to grab some time via computer will Bill Powderly, who is a dude with a bunch of titles. He is the Larry J. Shapiro Director of the Institute of Public Health, the J. William Campbell Professor of Medicine, and Co-Director of the Division of Infectious Diseases at the School of Medicine, all here at Wash U. Bill, thanks so much for making some time. So in talking with some of the folks in the chemical and environmental engineering groups here, they explain the production of nanoparticles through combustion. Now, I know that these are a real health issue, but I wanted to understand what is the actual problem? So to start with, what is the biggest health issue with these particles in the air?
So the biggest problem that the particles have is that they are very small and can get right down into the airways of the lung and damage the lung itself and also can be absorbed into the bloodstream and have indirect effects because they cause inflammation.
All right. Now, let's separate out these two. When you say damage the lung, what actual damage is being done?
So we see two problems with very small particles in the lung. The first is that high exposure to really high levels when there's a really bad smog, like it happens in a lot of cities in Asia.
Or like when I was driving from Delhi out to the Taj Mahal, and we could barely breathe.
Perfect example. That causes an acute lung injury. The lung just doesn't like being exposed to lots of toxins. The real problem from a health perspective is if you already have lung disease. So if you're a child with asthma or an adult with chronic pulmonary disease, you can get a very severe exacerbation, which could lead you to be hospitalized or even to have a death from it.
So in other words, there are segments of the population whose current health situation involves their pulmonary system, their airways, is such that it's really made much worse by being exposed to these particles.
Exactly. And those are people who have preexisting lung disease, preexisting heart disease, the very young because their adaptability is less, and the elderly because they're losing their adaptability.
Is this the type of thing that if you pull someone out of that environment after they've been there, say, six months or a year, and they go to some place where the air is cleaner, do the lungs generally recover?
So there is a reversibility to it, but there's also a permanent damage that can be created by prolonged exposure. And so that brings you to the second problem. And the second problem is prolonged, chronic exposure.
You mentioned also that when particles get this small, they can actually be absorbed into the bloodstream, and you said produce inflammation. What are the issues there?
So anytime anything gets into the bloodstream that's not supposed to be there, the body reacts to it. And the body reacts to it in different ways, depending on the chemical composition of it. But fundamentally, that reaction is good to get rid of it, but also has bad consequences. And particularly, if there's chronic exposure over time, those bad consequences accumulate, and they can result in further damage to organs. They can particularly cause things like atherosclerosis, which causes heart disease, stroke, and other bad outcomes. The degree and consistency to which you are exposed leads to exactly the same consequences you get from cigarette smoking. You get a higher risk of chronic lung disease. You get a higher risk of lung cancer, ischemic heart disease, and stroke. And now we're learning that it can also affect the risk of kidney disease and perhaps even neural development in young children.
So ironically, it was because of this health disaster with cigarettes that we have a much deeper understanding of the potential health effects of these particles produced by combustion.
So if there was any benefit to cigarette smoking, it might be that it accelerated the pace of medicine. Bill, thank you so much for explaining all of that. As I walked around the Delhi area and I saw people trying to filter their air, it became clear this was a real challenge. Thanks again.
You're more than welcome, Aaron. [music]
My conversations with Rajan painted a somewhat bleak picture of nanoparticles being produced at a fearsome rate by various fossil fuel energy systems, with the burning of coal being particularly concerning. In an earlier podcast, we learned that fossil fuel consumption is not likely to disappear quickly. The energy provided is inexpensive and easy to distribute in great quantities to arbitrary locations. And even with more efficient biofuel cookstoves and other devices in the developing world, there is the question of how to reduce the health impact. To answer that question, I am thrilled to be able to spend a few moments with David Pui, Distinguished McKnight University Professor from Minnesota, the L.M. Fingerson/TSI Chair in Mechanical Engineering, Director of the Particle Technology Laboratory, and last but not least, a member of the National Academy of Engineering. He is truly a pioneer in aerosol science and, in particular, the technologies of filtration. Let me just say thanks, David, welcome to the podcast.
Thanks, Aaron, for inviting me to share my experience.
So let me start with something basic. When I visited India last summer, you could taste the pollution in the air. And many people were wearing sort of thin paper masks on their faces, presumably, to reduce the health risk. Does that actually work?
No. Those are surgical masks which has less than 20% efficiency. So it's not adequate to protect people from exposure to PM2.5, those particles less than 2.5 micron.
Oh, that's right. And I know within the less than 2.5 microns, many of those are even less than, say, 500 nanometers. And those are the nanoparticles that we've been hearing about. So are there better masks that people wear?
Oh, yes. 3M each year make $1 billion of face masks for worker protection, and more recently, for protection against PM2.5. And those N95 has efficiency better than 95%.
I should interrupt and say we get no funding from 3M. But if they wanted to send us a check, that would be just fine [laughter]. So in other words, what you're saying is that you, in fact, can create these sort of-- and are they actually paper?
So it's a composite filter consisting of several different fiber. In particular, the electric filter media consists of fiber that are charged. So you can make them very porous, so that it's easy to breathe through it and still have very high efficiency because of the electrostatic charge.
I see. So I guess this also relates back to these particles, very small, hardly any volume but lots of surface area, that their surface area is very sensitive to charge manipulation. And so the electrostatic charges on the filter can be used to make the particles stick to those filters.
And also, it could attract the particles to the fiber so that it can collect the particle more efficiently.
Got it. So those masks that we were talking about, those are individual. Presumably, they last for a certain period of time. Do you clean them? Or once they're used up, they're used up?
No. You typically throw them away after a few days.
Okay. So those are sort of in the small individual. You got lots of really polluted air, and an individual can wear something to reduce the health effect. Now, I know that you've been involved in building-scale filtration, which seems pretty audacious. Tell us a little bit about what that is and what the goal is.
Recently, we have constructed two of the large-scale air-cleaning systems for urban air. And the first one was built in Xi'an, China, which has a base of 60 meter by 40 meter, with a tower height of 60 meter. And there are four sides to it, and each side, we place a different type of filter element so that we can compare efficiency through the different sides equipped with different filters.
So this is a building, right? This is almost like 200 feet by 130 feet by-- how high did you say it was?
60 meters. So--
So another 200 feet?
So this is a building-sized filter?
How do you move the air? I mean, from a layman's perspective, in order for a filter to work, I got to push air through the filter.
So how do you do that?
Yeah. Thank you for asking that. We make use of solar energy. In fact, it's a greenhouse consisting of glass panels on all four sides. And sunlight will warm the air below the glass panels. And then the warm air will rise due to buoyancy towards the middle, where we place the filter in there to remove the particles.
Got it. So we have this 150-, 200-foot-square building. Is this to filter the air for a plaza, for a town, for a--? How big an area is the air now cleaner because you're running this filter?
This unit that you call it large, it's not at all large. It's a pilot scale. The large-scale system that I wrote a couple of papers on is a kilometer in diameter. And--
Yes. And we modeled that. If we place eight of them around Beijing city, and in about 30 hours, you could reduce the PM2.5 by 15%.
So we're basically talking about city-scale atmospheric engineering, where by building these structures - and they're probably not very expensive to build, I would imagine, because there's not a lot of moving parts; it's basically the material - that you could perhaps insulate a city-- or remove from a whole city the 2.5 elements?
Yeah. That's our original intention, but that's what I call a first-generation unit. The second-generation unit was built in Yancheng, Jiangsu Province of China, where we include a water shower. It's a filter to scrub out the PM2.5. And for that unit, during the summertime, when PM2.5 level is low, we can put in sodium hydroxide solution to scrub out CO2. So this will then reduce the global warming, climate change associated with it.
That is remarkable. And actually, that's a great place for us to conclude because this also relates back to our previous conversation that I had in an earlier episode about some of the challenges with fossil fuel. All right. So we've talked about the very small. You've talked about a second generation that was big enough to do a city, which seems relatively ambitious. Anything that's sort of in a more intermediate-level scale?
We actually now have a third generation. This is smaller unit with a 20 meter by 20 meter by maybe 16 meter tower. And that is intended to place within some apartment complex. In fact, we have already performed modeling, and there are interests of placing this in the middle of an apartment complex. And the air recirculation causes the whole apartment complex concentration to reduce to a very, very clean level. And I work with economists, and they feel that if the apartment resident is willing to pay maybe 10% more in the price of the apartment, it will be sufficient to build a unit and operate it for 10 years.
Well, that's an interesting thought. The idea that part of your rent goes to making sure you have clean air is both a positive and a negative. It's concerning that one would have an air environment where you need to pay to clean the air. But it's nice to know that there are technologies on the horizon that would make that possible. Well, we're about out of time. So, David, let me thank you again for stopping by. It's always great to be able to sort of balance technologies involved in production of various energy and then other technologies involved in helping make that production clean. So thanks for your time.
Thank you, Aaron. [music]
That concludes the second of our three-part series on nanoparticles. This episode was focused on the so-called bad nanoparticles, those produced by various processes which can harm both the environment and health. The final nanoparticle segment will be about the good particles, those explicitly designed and produced for engineering uses, such as creating new materials or to enable new delivery mechanisms for drugs targeting specific tissues. Hopefully, it'll be a tad more uplifting. Until then, this is Aaron Bobick at Wash U for Engineering the Future. [music]
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