Q+A

IN THIS ISSUE:

DR. OLIVER F. BISCHOF Senior Director, TSI Incorporated

can’t choose just one filter that is most efficient for everything. That doesn’t make any sense, because obviously, you don’t need the same efficiency for everything. There are also ways to incorporate pre-filters and more efficient filters subsequently. Still, there’s also the caveat that the more efficient the filter is, the more energy is required to pass the flow through it, particularly when it’s loaded heavily with particles. These concepts are fundamental and crucial to understand if you’re an engineer selecting and installing filters. It’s also essential to know that some filters contain electrically conductive materials, which means charging plays a role in particle loading. In humid conditions, the charging state of the particles can change, which presents additional challenges. If people know particle technology, they can do a better job of selecting and then installing those filters.

Al-Attar: With over 35 years of experience in the field, how do you think particle technology has evolved as a science? Additionally, how have advancements in instrumentation enhanced our ability to make better filter selections?

Bischof: Particle technology has been in existence for approximately 80 years, but it has gained momentum rapidly over the last couple of decades. It is much more prevalent today, and an increasing number of scientists are realizing that particles play a significant role in their fields. There are many users of particle technology, and filtration is more common today than ever before. If you think of filters, decades ago, we didn’t have exhaust filters in our cars, as the technology didn’t readily exist. Engineers then came up with a way to filter hot gas emissions very efficiently, while also maintaining a pressure drop that allowed the cars to operate as usual, so the user didn’t even realize that a filter was in place.

Rarely used until the COVID pandemic, another example is portable air cleaners. Most people didn’t think about cleaning the air in their indoor environments. With COVID, perceptions changed, and filtered air became a vast topic, with a massive number of companies manufacturing devices to keep up with the demand. COVID-19 also helped to refine many test standards, so that today we can purchase very efficient portable air cleaners that didn’t exist 20 years ago.

Al-Attar: How does the morphology and surface charge of airborne particles influence their capture across different filtration mechanisms?

Bischof: When the efficiency of a filter is measured, it is done in the lab under standard test conditions with very specific test particles so that an efficiency rating can be determined for these conditions. However, the efficiency of a given filter is not the same for all particles; it is specific to the test dust or particle type. In real life, morphology is a more complex theme because it encompasses not only the shape of the particle but also its entire structure. Just imagine that some particles are shaped more like snowflakes, but they are very loose particles. These particles exhibit a different behavior than particles that are an agglomerate, such as soot particles, which we try to filter because we want to avoid indoor combustion emissions, for instance.

Ultimately, real-life morphologically very different particles will behave differently compared to those used in standard efficiency tests. This difference impacts the efficiency of a given filter, and we do not test for it. We only test for what the standards dictate so that filters will behave differently in the real-world operation, and we don’t know how many morphologically different particles are present in whatever they’re filtering. That’s one of the big unknowns, unless you’re taking samples, for instance, upstream of the filter, and then repeat the efficiency tests with these particles, which most likely nobody does.

When I think about the morphology question, I mainly think of engineered particles that we’re using in industry, like silica fumes or something to make different materials. Still, there are also the particles that we encounter in real life that we want to keep out of our homes. Think of the Canadian wildfires, which were not only a local event, but also affected locations thousands of miles away because the emissions and dust from these wildfires were transported to locations thousands of miles away. We had Canadian wildfire episodes here in Europe, which made the ambient air incredibly dirty and you could see the color of the sky change. Obviously, wildfires produce combustion particles as trees and everything else have been burnt, and that’s not a clean burning process, so it generates something that is essentially dirty agglomerates. In addi- tion, the wildfire particles age over time during transport and there is atmospheric processing taking place, which is considered in atmospheric science. Anyway, these are not uniform or spherical particles once they get here, so if you want to keep them from entering our homes, then we have to take into account morphological differences.

Al-Attar: What advances in particle technology would you like to see happen? You’ve spoken lately about the low detection limit in terms of ultrafine particle or nanoparticles.

Bischof: Particle technology is a broad field that encompasses various areas, including particle manufacturing, transport, collection, and measurement. However, it also encompasses a wide range of industries, each with distinct challenges for the measurement side, including the limit of detection, accuracy, and comparability of measurements, all of which are critical factors. Specifically, the most difficult to measure particles, whether they are ultrafines or nanoparticles, are often the most important ones. This is where the detection limit of the measuring instruments, their resolution, and accuracy come into play. Furthermore, there is a need to have more measurement sites, which also involve lower-cost devices. Getting more data points is key to getting better spatial resolution that could reveal the locations of pollution sources, the substances to which people are exposed, and how particles are transported.

Al-Attar: In some countries today, liquid filtration technologies have helped countries make their tap water drinkable. What role do we want global governments to play in making our tap air fit-for-purpose, and what role can particle technology instruments play in that regard?

Bischof: It’s a very interesting analogy to consider both the water we drink and the air we breathe. And I’m very fortunate to live in a country that has excellent quality tap water. My dad, who was a food chemist for the city of Berlin, used to say that our water over here is the best controlled foodstuff we have. Everybody realizes that they can simply open a tap and drink the water, so why don’t we expect the same from our air, particularly considering the number of liters of air we inhale daily, compared to the little amount of water we drink? It is impressive how water quality is well-controlled, from appropriate purification mechanisms to transportation. The fact of the matter is that there is so little we do about the poor urban air quality in cities and appropriate air filtration in buildings where people spend most of their time. More work is needed to make available filtered air that is fit for purpose to all communities and building occupants, and not only in the developed world.

Al-Attar: Given that people spend much of their lives indoors and indoor air can be more polluted than outdoor air, how can advancements in particle technology instrumentation (like real-time, low-cost sensors for various particle sizes and compositions) help governments shape public policy for better indoor air quality?

Bischof: That’s a loaded question. There are some spaces where it’s easier to do that, specifically public buildings, such as government buildings, convention centers, shopping malls, or even indoor sports arenas. Those are probably relatively easy to control because there’s money involved, and the fact that people are made aware of what air quality they are exposed to, which can be potentially not very clean. Many of the efforts in North America these days focus on filtration and ventilation, as well as monitoring the air quality introduced into buildings. It becomes more complicated when addressing personal spaces, such as homes, because in many parts of the world, there is neither filtration nor air conditioning. Evidently, we need to educate people that poor outdoor air can impact their health. What happens when I open the windows and let my living room naturally ventilate? I have an air quality sensor in my home because I want to monitor the air quality when I use my fireplace, which I do very rarely now, as I’m very aware of what even very efficient stoves still emit into the air. I actually completely stopped burning candles in my home because my particle sensor goes berserk due to the high particle concentration they emit. It all starts with awareness and educating people on how to improve the air in their own homes.

Al-Attar: What do you want governments to do for us?

Bischof: We can both agree that we would like governments globally to make more efforts to clean up the air. However, what we see these days is the impact ambient air monitoring can have, spe- cifically on source control. There are local regulations in many countries that require fence-line monitoring when people start building activities. Companies that dig up the soil and create dust by doing so need to monitor what is being emitted to the immediate environment, which governments and legislation drive. The same is true for sources like incinerators, power plant stacks, or factories. There are continuous emission monitoring systems, and then there are filtration mechanisms. Huge industrial filters, which are beyond most people’s imagination, consisting of multi-stage bag filters, are, for instance, used in the cement industry to comply with ambient air quality standards. Another example is vehicle exhaust filtration to comply with exhaust emissions legislation, which is already implemented in many countries. However, we can all agree that it shouldn’t stop there; we need to tighten the standards and source control while enhancing filtration technologies to achieve tighter limits and ultimately better air quality.

Al-Attar: Embracing sustainability involves responding to environmental challenges linked to fossil fuel use. We need to address source control to prevent particle emissions and minimize impact. What can we do individually and at a state level to achieve this?

Bischof: First of all we need to listen and pay attention to science! That might be an easy statement to make because science is complicated, it exists in niches, there are various experts, and they might not even all agree, etc. – but the combined body of science has taught us a lot of things. Unfortunately, it’s often not understood, ignored or even denied. That being said, in the past 10 years, we’ve made a lot of progress by paying attention to science, with concrete measures that have been taken to improve both outdoor and indoor air quality. If I had a wish, it would be to get things to governments and through parliaments more quickly. Let’s not be hesitant. If there’s something that benefits the population, improves our lives, keeps people safe, and saves the planet, then we shouldn’t be that hesitant. On a personal level, I think we all have a role to play as humanity consists of individuals, and every individual has to challenge themselves in their role. We can all make an impact.

Al-Attar: How can we relate air filter performance and indoor air quality to the characteristics of challenging particles? Is this too much to ask from a particle technology and filtration perspective?

Bischof: That is an excellent and relevant question. It brings together particle technology, filtration technology, and building technology by examining various parameters. From an instrumentation standpoint, the capability is there as scientists have the tools to measure airborne particles. Let’s start upstream of the filter. Whatever is upstream can be like the layers of an onion. You could examine the ambient environment of a specific location. Then you could look at the air intake of a building and sampling ports from different locations of the HVAC system to determine whether it is really clean air that is being brought into indoor environments. Measurements within a given room with human occupants are also of interest, and it’s possible to do so when we move our measurement equipment there. The problem is that these are typically spot measurements for research, not continuous, as it’s not practical and an affordability issue.

A daily routine tool is what’s required, so that every building can have access to a continuous monitoring solution. It should also not require a scientist to operate it and interpret the data obtained, which must accurately represent the current air quality status. I see that as a major challenge. Providing IAQ or IEQ solutions for office buildings and public spaces that are accurate requires careful consideration of all factors. We already have IAQ monitors that continuously measure several parameters to understand if indoor air is healthy for occupants. Still, adoption will be slow unless there are concrete guidelines and requirements for it.

Al-Attar: What role can AI play when everything surrounding particle technology is based on measurement and a scientific basis, and the physics of particles? What is left for AI tools to play?

Bischof: AI is a topic that is very interesting these days because it continues to grow in terms of its capabilities. You are certainly right to question what AI can do because it is neither going to change the fundamentals of particle technology nor change particle kinetics. It will for instance not change how filters remove particles. However, there are instances where AI will play a crucial role. The first one is in selecting appropriate tools for specific requirements. Consider this from the perspective that we may be experts in one field, but nobody can be an expert in all of them.

Ultimately, AI can collectively look at the subjective holistically and come up with suggestions that we should consider as a more efficient way of combining knowledge in one place for review and consideration. I would still advise every- one who is using AI not to simply accept, but to question it. But AI can save time by providing more knowledge in selecting the right tool for a given job, and by tool, I mean anything. It could be a specific particle technology process to be employed, or it could be a device used for metrology to measure and monitor outdoor air. AI could also help by selecting the right filter for a specific application, providing a rationale for the choice. It is a process of devising solutions while considering numerous parameters that impact your decisions. AI can also play an essential role in interpreting and presenting data, and putting it into a wider context.

Al-Attar: Can AI, as a tool, help in air performance prediction, given the pollution profile of specific individuals or sources?

Bischof: The most significant benefit that I see for AI is in analyzing, processing and using data because that’s ultimately what AI can do. It cannot replace the actual measurements. The measurement will still need to be done by instruments, but once their data becomes available, you need to interpret the data. We need to use the data and draw conclusions from it, which is where the power of AI lies. It’s able to do that in very short amounts of time and by questioning whatever we’re getting from AI, by refining it, so we come up with very credible and functional analysis.

It then allows us to address pollution sources. Imagine that you have a specific source or a certain production environment, and you know when an emission takes place and how it spreads. AI can simulate it for you and provide concrete areas, and you can then use that information to automatically trigger your ventilation and filtration system to address this pollution. That’s the type of role that I can imagine AI can play, and I’m keeping it to the simple example, but you can come up with a lot more complicated examples that we as humans would struggle with conceiving. AI, by its very nature, will come up with proposals that we can then take into account and make fit our needs.

Al-Attar: How should particle technology inform global policy frameworks that aim to harmonize indoor air quality, occupational health, and climate resilience?

Bischof: That’s a loaded question! I think the easiest thing is to go away from this part of the world and look at a different one. If you look back to what happened in China when industrialization first took place and production ramped up very, very quickly, the air in China suddenly became very poor. China had one of the poorest air qualities, and people were exposed to it for 10 or more years. The Chinese science community and in their wake also legislators learned that this is very negative for what they were trying to do in the country and for their population. So, they took measures.

China is actually an example where legislators stepped in, as they banned the use of combustion vehicles on certain days. They also electrified the vehicle fleet, they fitted stacks with filtration mechanisms like ESPs and whatever was deemed to be relevant. Through all of these steps they drastically managed to cut air pollution.

Now we’re looking at India, and India still has a long way to go. If you’re looking at any publication or comparison table that shows you the most polluted places on earth, then typically six out of 10 are in India and then two are in neighboring Pakistan. This is closely related to industrialization in the country, which has led to significant traffic emissions.

However, it’s also some common practices that are still prevalent in India, like open fires or the burning of crops in the fields. Farmers still do that because it’s a cheaper way of doing it. Some of these problems are probably easier to govern from a legislative point of view than others, but it is something that the Indian research community and also the Indian government are paying increasing attention to. So maybe five years from now India will have made the same improvements that we’ve seen in China, and even China still has ways to go to reach the air quality that we have in much of Western Europe.

Al-Attar: When COVID-19 hit in 2019, we needed to address knowledge gaps about virus transmission and particle technology. Are we now better equipped, given our advancements in understanding these areas over the past six years?

Bischof: Complex question to answer on the COVID side. Particle technology continues to improve, and the significant difference from pre-COVID times is that we’ve bridged the gap between the medical community’s understanding of an aerosol and the understanding of the aerosol particle science community.

As I’m on the particle technology front, many of these concepts were understood, but without having knowledge of the virus itself. How the virus impacts human health from a medical perspective required knowledge of the virus, which was surprisingly quickly understood, considering how quickly it led to the development of effective vaccines.

Whatever was missing was a good understanding of the profound difference in the knowledge of aerosol kinetics, because the medical understanding was that aerosols are (very) large droplets that fall to the ground quickly, making them no longer inhalable. Therefore, much time was spent on surface decontamination and the importance of not touching surfaces, which was driven by the medical community. There was a lack of understanding the kinetics of small particles and small aerosol droplets containing the viruses.

My understanding is that COVID-19 is transmitted as part of an airborne droplet that contains the virus, which has a size that determines how long it stays in the air, how long it remains in a room, how it can be effectively prevented from entering the human body, and how it can be removed by different mechanisms such as air cleaners and effective, well-fitting respirators. So, we have learned a lot about social distancing, avoiding crowded indoor spaces, proper ventilation, filtration, and face masks. Not a single one of these measures alone would prevent us from having another COVID pandemic, but a combination of many of them, starting with appropriate vaccines and the right masking approach, as well as air filtration, ventilation, and spending time outdoors versus indoors, would help us to be in a very different position these days. The global science community has certainly learned a great deal by coming together and combining its knowledge and tools.

How much of a role can particle technology help urban planners make our cities more fit to occupy with their buildings and building envelopes?

Bischof: A lot of that is the understanding we have in the use of particle technology, as well as in building sciences. And there are ways of doing it alone and how we are building e.g., in terms of how air moves through our cities, how clean air even comes to our cities, how we keep places well-ventilated and cool, how we use plants in cities to remove pollution, how we bring plants close to roadsides, how we avoid busy roads in cities – all of these are questions that are related to outdoor air. Again, excellent research is being conducted in this field, and it is well-documented in the literature. The other aspect is not the city itself or the urban planning side of it, but rather the transport of air through cities. However, it’s really the buildings themselves that’ve been a focus, and again, we’ve learned a lot during the COVID pandemic in terms of the need for ventilation and filtration, particularly in public buildings where people spend a lot of time, and are not necessarily in control of their own destiny. How to actively manage air quality in schools, offices, buildings, hospitals, hotels, and other similar settings, and what tools are available to mitigate poor air quality indoors, must lead to a very different understanding than we had pre-COVID.

Al-Attar: Thank you so much for having me in Germany at the TSI’s European headquarters. This has been an insightful journey in learning not only how to measure particles, but also to embrace sustainability. Thank you for addressing the correlation between particle characteristics and filter performance, as well as the associated boundary conditions, which constitute a viable and reproducible dataset. Your explanation of data interpretation and how AI can aid in understanding the complex science of particle technology is insightful.

Bischof: Likewise, Dr. Iyad, thank you for taking the time to come and interview me today. It’s a real pleasure and an honor to do this for International Filtration News. I consider myself a passionate engineer and atmospheric researcher, as you may have noticed during this interview. There’s a significant role we can play as engineers, researchers, and scientists in helping to improve the lives of our communities. Making a difference lies in addressing issues at the early stages; it’s often in the planning side.

How do we design our cities, and how do we manage production processes? Some of it involves understanding what is happening in advance. Such an understanding centers on the research component, and many people are working diligently to advance the knowledge of particle technology. I genuinely respect it, and it gives me pleasure to see it in peerreviewed literature, conferences, and so forth. I encourage everybody to embrace that research side and its contribution to advancing our knowledge. Particle technology has a promising future and will help us combat the harmful particles that impact our climate, which have adverse effects on human health, reduce life expectancy, and cause visibility issues.

There are also beneficial particles that we can consciously utilize, for instance, to create new materials, better semiconductor wafers, and new sensors. From a personal perspective, I have dedicated a significant part of my career to the measurement side, focusing on measuring and monitoring particles. We have recently made substantial advances, and it is fantastic to see how new guidelines to regulate particles, and especially ultrafines, have been embraced by the WHO and even the European Commission in this part of the world. A wider implementation of more advanced measurements is a necessary but promising step. Still, there is a need to continue down that road to improve the quality of the air we breathe.

Al-Attar: [Note: Interviewing Dr. Oliver Bischof felt like drinking from a fire hydrant. His deep knowledge of particle technology measurement and the underlying physics offers immense value to audiences and learners alike. A key takeaway is the potential of specific technologies to save lives, allowing more time with loved ones.]

Dr. Bischof, you highlight that although particle technology and filtration have been around for over a century, their recognition and rapid advancement have surged recently. Yet, the field remains “overlooked” in air filtration despite its critical importance. Please comment.

Bischof: When I said particle technology is a broad field, I have to think back to the late Professor Sheldon Friedlander from Caltech in the United States. He talked about the good particles and the bad, and that’s something that really stuck with me. In fact, there are different reasons why we are surrounded by particles, and the good ones are the ones that we are creating deliberately. There is particle syntheses, in which we are engineering them to create new materials, such as my tennis racket, which incorporates carbon nanoparticles.

These are tiny particles to improve its strength and stability. Consider microchips, as the leading semiconductor manufacturers can now produce wafers with much smaller feature sizes. These tiny structures are created by using nanodroplet, that then form more powerful transistors.

Many materials use particle technology that people may not even consider, such as sunscreen and pharmaceuticals. Think of medical sprays that can be tailored to be inhaled to reach even the lowest region of the human lung.

Harmful particles are those we neither want to inhale nor have around us, such as from combustion sources, like vehicle emissions in cities and those emitted from industries through huge stacks into the atmosphere, but there are many other sources as well. Also, natural sources like the wildfires and Saharan dust from sandstorms that bring particles into our air even in cities far away from where they occurred. I believe that the concept of good and bad particles is an important one to remember, as it also incorporates the need we have for applying particle technology.

Dr. Al-Attar is IFN’s Global Correspondent, Technology and Innovation, with insight as a mechanical engineer and an independent air filtration consultant. He is a Visiting Academic Fellow in the School of Aerospace, Transport, and Manu facturing at Cranfield University, consulting for air quality and filter performance relevant to land-based gas turbines. His expertise is on the design/performance of high-efficiency filters for HVAC and land-based gas turbine applications, focusing on chemical and physical characterization of airborne pollutants. Dr. Al-Attar is also the strategic director, instructor, and advisory board member of the Waterloo Filtration Institute. In 2020, Eurovent Middle East appointed Dr. Al-Attar as the first associated consultant for air filtration, as well as an Indoor Air Quality (IAQ) patron for EUROVENT.

Compiled By Dr. Iyad Al-Attar