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Aug. 7, 2024
163 - Fire Fundamentals pt 11 - Soot in Fire Safety Engineering
Soot is perhaps the most complex product of combustion, and at the same time one of the most profound for our everyday fire safety engineering. The topic of soot is not getting much love in the world of fire science, so I’ve chosen to give you a broad introduction to this subject. In this episode of fire fundamentals we will go through:
· Soot creation from chemical perspective;
· Soot creation from practical perspective;
· Soot effects on radiation, toxicity and obscuration;
· Extinction coefficient and specific extinction coefficient;
· Soot yield and surrogate value of soot yield for complex fuels.
If you would like to follow up on this episode with some reading, I highly recommend:
· Bart Merci and Tarek Beji book „Fluid Mechanics Aspects of Fire and Smoke Dynamics in Enclosures”
· Jose Torero lecture “Prof. Jose Torero - Fire: A Story of Fascination, Familiarity and Fear” available at https://www.youtube.com/watch?v=cIY0litILRA&t=2082s
· W. Węgrzyński and G. Vigne, Experimental and numerical evaluation of the influence of the soot yield on the visibility in smoke in CFD analysis – the paper with the source of our surrogate value of soot yield for complex fuels in fire safety engineering https://www.sciencedirect.com/science/article/abs/pii/S0379711217301327?via%3Dihub
· G. Mulholland, C. Croarkin Specific extinction coefficient of flame generated smoke https://onlinelibrary.wiley.com/doi/epdf/10.1002/1099-1018%28200009/10%2924%3A5%3C227%3A%3AAID-FAM742%3E3.0.CO%3B2-9
· W. Węgrzyński, P. Antosiewicz, J. Fangrat, Multi-Wavelength Densitometer for Experimental Research on the Optical Characteristics of Smoke Layers, https://link.springer.com/article/10.1007/s10694-021-01139-5
· K. Börger, A. Belt, T. Schultze, L. Arnold, Remote Sensing of the Light-Obscuring Smoke Properties in Real-Scale Fires Using a Photometric Measurement Method, https://link.springer.com/article/10.1007/s10694-023-01470-z
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Transcript
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Hello and welcome to the Fire Science Show.
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In fire science there are some topics and phenomena that sometimes are not very well discussed yet they are fundamental or at least very, very important to our everyday engineering and I have a feeling that the subject of today's episode, the soot, is one of those topics.
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We don't talk about it that much.
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I don't see that much research on soot formation in fires and soot effects in fires around, yet it is something very, very foundational to how we assess the fire safety in our buildings.
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And in this episode let me try to introduce you to the concept of the soot.
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And this also kind of ties in with the previous episode where we've discussed the historical experiments on visibility in smoke by gin.
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So suit formation and suit in fires is something you could even call a follow-up to that previous podcast episode.
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In this episode we'll try to answer some important and interesting questions related to soot.
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First, we're going to try to figure out where does the soot come from in fires and why in different types of combustion you'd find different amounts of soot as a result of that combustion.
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We're going to discuss what does it do once it appears in your fire.
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Where does it go?
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Which phenomena does it do once it appears in your fire?
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Where does it go?
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Which phenomena does it influence in the fires, and which of those are actually important to us for our everyday fire safety engineering?
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Then we're going to talk about how do we include soot in our design?
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Because, yes, we do.
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We actually do include that in our design and especially in our modeling.
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So a lot of important questions.
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There's going to be a bit of chemistry.
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There's going to be some stories in the episode.
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I hope you'll enjoy them.
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If you don't like soot at all, feel free to use a magical number for soothield of 0.1 gram per gram.
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You can consider that as almost conservative assumption for fuels with unknown composition and go on with your life as a fire safety engineer.
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If you want to understand why we've proposed this number as something that you could call conservative, then stay with me in the podcast episode, because I'm going to answer that question as well.
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So, yep, it's fire fundamentals.
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We're going to learn some fire science.
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Really happy to introduce you to the concepts of soot, let's spin the intro and jump into the episode.
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Welcome to the fire science show.
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My name is Wojciech Wigrzyń and I will be your host.
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This podcast is brought to you in collaboration with OFR Consultants.
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Ofr is the UK's leading fire risk consultancy.
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Its globally established team has developed a reputation for preeminent fire engineering expertise, with colleagues working across the world to help protect people, property and environment.
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Established in the UK in 2016 as a startup business of two highly experienced fire engineering consultants, the business has grown phenomenally in just seven years, with offices across the country in seven locations, from Edinburgh to Bath, and now employing more than a hundred professionals.
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Colleagues are on a mission to continually explore the challenges that fire creates for clients and society, applying the best research, experience and diligence for effective, tailored fire safety solutions.
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In 2024, ofr will grow its team once more and is always keen to hear from industry professionals who would like to collaborate on fire safety futures.
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This year, get in touch at ofrconsultantscom.
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Okay, let's go soot in fires and smoke in fires.
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Long before I became a podcaster, and perhaps even long before I became a published author in scientific fire journals, I was already annoyed by the suit.
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You see, the reason I became a scientist is to seek answers to questions that were annoying to me, and most of those questions were related to kind of magic numbers and where does stuff come from and when you work as a fire safety engineer when you work as a smoke control engineer, actually, and you do a lot of CFD simulations you very quickly realize that visibility in smoke is the thing that you're playing with.
00:04:42.144 --> 00:04:47.855
Like I once put a claim that we should stop calling it fire safety engineering.
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We should call it visibility engineering, because, in the end, what we are doing is to try and build systems in our buildings that would maintain the visibility in smoke for the required duration of the time to escape the building.
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In essence, that's what we have been doing and that is what we largely still are doing, based on the assumption that we need to keep the evacuation conditions tenable.
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We have the tenability criteria related to visibility, temperature, heat flux and so on, so on.
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The visibility is always the first one that passes, and there's an episode of this podcast I think it's episode three with Gabriel Avigne, where we discussed this at large.
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Now, as you realize that your visibility in smoke is the driving factor, there are some ways to tune the visibility.
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Now, perhaps tune is the wrong word in here.
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There are ways to fiddle with visibility.
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You put a lot of stuff into your models and you put some numbers into your models that highly influence how much smoke you're going to have.
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One of those numbers is the soot yield factor, and that's the number that defines how much soot is produced in your fires.
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Now if you start playing with this, you immediately observe that the visibility in smoke changes.
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Basically, because you change the amount of smoke, it must alter the visibility.
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The effect is quite profound.
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Now, when you start doing your due diligence as an engineer and you want to give your best, what value are you supposed to put in your model?
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And as long as you can find values for very simple liquid fuels or gases methane, ethylene, methanol, propanol, stuff like that you can find suit yield values for that.
00:06:32.007 --> 00:06:37.949
Once you start venturing into complex fuels, you will soon find some crazy numbers.
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Like for polyurethane foam, you can find values up to, I think, 0.18.
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And yeah, the scatter is just tremendous and there's no suit-healed value for a vehicle or suit-healed value for a kiosk in a shopping mall.
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You have to figure out those values on your own and that's a heavy burden.
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I think that's a heavy burden for an engineer to come up with values of their own when scientific consensus does not exist in a specific field, like we don't have a scientifically sound value of soot yield for a very complex fuel package in a very odd architectural setting.
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We have values measured in a small scale laboratory apparatus for quite fewer fuels and yeah, because it's quite a heavy burden for an engineer, I decided that it's a science and I could do this scientifically and in fact I did.
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Then I've met Gabriel Levinier and he was also researching that.
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He's also an engineer.
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He also came to very similar conclusions, as I did with the challenges related to the use of soothed yield in modeling.
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He was doing sensitivity analysis for different parameters that go into the equations that yield the visibility in the end, and he found that soothed yield was one of the variables, or the amount of smoke that's a product of soothed yield was one of the key variables that changed the visibility value in the end in the biggest way.
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I saw his presentation in SFP Europe first conference in Copenhagen, I believe, and then we've talked, we've talked and then we've repeat his study together, experimentally and with CFD modeling, with FDS, with ANSYS, and we came to the same conclusion Suitability is extremely important, extremely impactful in our analysis.
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I'm going to continue the story later on in the episode when I'll introduce you to the value that we found as something that we believe is fairly safe for complex field packages for engineers to use.
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But before we get there, I want us all to better understand soot, to better understand what is the thing that we are talking about.
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What is the thing that we are dealing here about?
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So let's try and first answer the profound question what is soot?
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The profound question what is soot?
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So answering this question requires some chemistry, and let's start with chemistry of simple combustion.
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Take a simplest fuel.
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You can get like methane.
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When you write an equation for the combustion of methane, you have methane.
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You add oxygen, you receive carbon dioxide, you receive water.
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These are products of perfect combustion.
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And the more complex fuels you put into those equation, you eventually, if it's a perfect combustion, it's going to end in the same way Carbon dioxide, water.
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If there were other species in your fuel, you're, of course, going to get other products.
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If you add nitrogen, you're going to get cyanide and so on, but at large, the products of perfect combustion are gases and water.
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Now, the combustion is not always perfect and sometimes you'll get byproducts of that.
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One of the byproducts is soot.
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But that's the short story.
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You can go here a little bit deeper.
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If you think about combustion, the first reaction equation I told you you put your fuel, you put your oxidizer, you receive the products.
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This is actually a lie.
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Well, it's not a lie.
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It's a simplification, a massive, massive simplification, because it's not a simple reaction from your basic fuel to your products.
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In reality, what happens is there are dozens, if not hundreds, of different chemical reactions ongoing in a combustion zone in which the compounds go to different transitions into radicals, they merge, they break, they change their forms.
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There's a ton of chemistry that's happening even for the simplest fuels.
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Actually, in some papers that are on the subject of chemical kinetics, and there's a paper on modeling studies of formation of aromatics in laminar, acetylene and ethylene flames, and these guys actually identify more than 500 different reactions that happen inside the flame.
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That's the complexity of the reactions that are happening in our flames.
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Now, another thing is those reactions happen with different probabilities and different speed.
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They don't happen infinitely fast, they take time to resolve and, as an effect of this complexity, it's not all molecules of your fuel that will go through the complete transition into the final products, into the last final step of that reaction that would yield you this carbon dioxide and water.
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Some of them will stop somewhere along the way.
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Some of the fuel may not react at all.
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Even there is, let's say, a competition between the reactions inside the flame, which of the reaction will occur, and, depending on the fuel that you put in and the conditions in which the combustion takes place, different reactions will complete before other ones and sometimes because of that you will receive an incomplete product.
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This is why a combustion of very simple fuel such as methane can be incomplete.
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It's not that we've stopped in the middle of the equation, it's just.
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The equation is a simplification of a much, much as methane can be incomplete.
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It's not that we've stopped in the middle of the equation, it's just the equation is a simplification of a much, much, much more complex phenomena that happen inside the flame.
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Now why this leads to formation of soot.
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So the reason is carbon.
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Of course, carbon is a molecule, is the most social of all molecules, I would say.
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Carbon loves to bond with everything, and carbon also loves to bond with itself.
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Now, another interesting aspect of carbon is that when it bonds with other carbons, they tend to create those circular structures, the rings.
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We call them aromatic substances.
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Those are rings that consist of six, sometimes five, carbons.
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They close a loop and that's a very stable chemical compound once it forms a ring.
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This is why those rings are so persistent.
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Now those rings can combine and form more complicated structures that consist of multiple rings of carbon.
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This is where we start calling them polycyclic aromatic hydrocarbons, pahs, and those substances are in a focal light because they're also connected with cancer and stuff like that.
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So they're pretty nasty stuff.
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Anyway, at some point point, if enough of those rings combine with each other, once enough pahs combine with each other, there are four molecules big enough that we start calling them soot.
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This is either true reactions between pahs between themselves.
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There's also a transition called the H-abstraction, c2h2 addition, haca mechanism Very complicated stuff, but there are models, chemical models, that explain us how those particles connect with each other to form a really large particle.
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That's a collection of those rings and those flakes are called the soot.
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This is exactly soot and this is how, chemically, the soot is born.
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Now, another thing that you must recognize is the fact that soot consists of carbon and carbon is fuel.
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Right, you can burn carbon.
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So why doesn't soot burn itself in a fire?
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In fact, it does In most or, in many cases, a lot of soot that's created within the flame itself is burned down in the flame.
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Well, outside flame is a very small region of space in which the chemical reactions and release of heat happens, but that's not the entirety of what we see.
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Then you have regions of very high temperature beside the chemical reaction region or above the chemical reaction region if you are under the gravity of Earth and buoyancy causes flame to go up.
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Now for different fuels.
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We have a property of the fuel which is called the laminar smoke point height, and this is basically a property, a measurement of how high the flame of a specific material or of a specific fuel can be before it starts emitting soot outside of the flame.
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So if your soot is created in a reaction but then it enters very hot region of the flame, it undergoes further reactions, it undergoes further transitions, it oxidates, it basically burns out, acts as a fuel and turns into new products which hopefully are the simplest carbon dioxide and water if the combustion is complete.
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Now for different types of fuels.
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This range is different.
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For methane that's almost 0.3 meters.
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So you can have 30 centimeters or 29 centimeters flame of methane and still do not see any soot coming out of that flame.
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For polystyrene, that's only one and a half centimeters the height at after which you're going to see soot coming out of that flame.
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For polystyrene, that's only one and a half centimeter the height at after which you're going to see soot.
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So if you burn polystyrene and your flame is higher than one and a half centimeter, you're going to see some soot escaping the flame and not oxidizing in time, not being completely oxidated, not being burned out by the flame, and it's released into the environment and when it escapes the flame we start calling that smoke.
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There are interesting stories actually about that.
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I love the story that Jose Torero gives about the candle, and there were multiple lectures by Jose.
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I'm going to link one that I like the most in the podcast show notes, so you're very welcome to watch it.
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It's really good.
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But long story short.
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Think about candle.
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Candle is is a product of engineering design, especially one particular part of candle and that is the wick.
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So, as I just told you, there's this length of a flame at which the soot will oxidate perfectly and will leave no smoke.
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So if you're using candles to light up your castle, you don't want smoke entering your castle because it's going to smell bad and it's going to get dirty and it's going to be problematic.
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So you don't want your candles to actually release soot.
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You can solve that by consulting with your nearby combustion specialist or attending a combustion symposium in the neighboring kingdom.
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But, assuming you're lighting your castle with candles, you probably don't have access to computational fluid modeling and the achievements of modern times.
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So you have to sort it in a different way.
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And they actually have sorted it out by observing the flames of the candle and realizing some basic properties of the flame.
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The fact that the candle releases or does not release soot is related to the flame height, and the flame height is related to how much fuel you put into the combustion.
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Now the fuel in a candle goes through wick.
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Basically, you have some sort of wax, some sort of solid fuel that melts.
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Then you have a piece of wick cloth that is drenched in this molten wax and because of capillary forces, the wax can travel upwards through the wick.
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Now it reaches a point where it's very hot, so it evaporates and vapors of wax burn out, creating your flame.
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So that's basically how a candle works.
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Now, if your wick is too large, you're going to take too much fuel from the wax and your flame is going to be too high, and if it's too high it's going to emit soot and you don't want that.
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So the way to deal with that is to manually trim the wick whenever it starts producing soot, but that would be annoying and a lot of work.
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So actually, a specific type of wick was invented, one that it's made from two weaves and those weaves are weaved together in such a way that the wick bends a bit and once it bends, a part of that wick is gonna burn off in the flame region, basically maintaining the same length over the time, because as it grows, as you burn off wax and more wick is exposed, it's going to bend more and it's going to burn out the tip of it.
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So, again, it's short enough to not cause sootiness in your flame.
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And you can explain this with chemistry.
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I just told you how it works.
00:20:04.205 --> 00:20:19.076
Basically, the soot is produced in the flame and it's oxidated before it can escape the flame, and as no soot escapes, then you have a beautiful source of light and you don't create much smoke, and your king is very happy with the cleanliness of his castle.
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Now you can explain that also with a thermophysical number.
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We call it the Damkohler number.
00:20:24.612 --> 00:20:29.957
Damkohler number is a ratio between the reaction rate and the transport rate.
00:20:29.957 --> 00:20:32.173
Basically, it's what I've just told you.
00:20:32.173 --> 00:20:42.891
So reactions, the transport is how quickly the particles fly through the flame before they leave the candle flame and they are not able to oxidize anymore.
00:20:42.891 --> 00:20:45.016
Reaction rate is how quickly they burn.
00:20:45.016 --> 00:20:59.871
So if those two things are in perfect alignment, if the reactions take just this much time as it takes for the particles to be transported through your flame, then you're not going to get any byproducts that you don't want in your reaction.
00:20:59.871 --> 00:21:08.653
It's funny because the medieval engineers or Renaissance engineers who were working on the candle wick, they didn't know Damkohler number.
00:21:08.653 --> 00:21:13.432
Damkohler was not alive yet at that point, but still they figured out.
00:21:14.045 --> 00:21:17.971
I've also learned about a very interesting thing that's happening in Japan.
00:21:17.971 --> 00:21:27.156
So they have those very expensive inks traditional, extremely expensive inks for calligraphy.
00:21:27.156 --> 00:21:36.028
Calligraphy is a very popular thing in the Far East and there is a very specific black ink that they would sell for a large amount of money.
00:21:36.028 --> 00:21:42.132
And where that ink comes from is also a story of soot, and actually it's also a story of a candle.
00:21:42.132 --> 00:21:51.355
So I've learned that the materials that are used to produce this expensive ink is actually soot from candle flames.
00:21:51.355 --> 00:22:13.027
So basically, they have this soot factory, which consists of hundreds and hundreds of candles, and those candles are built in a very specific way so that the wick is a little longer than necessary, a little longer than necessary for the complete combustion or complete oxidation of the soot within the flame range.
00:22:13.027 --> 00:22:39.093
What happens then is that those candles start to produce a tiny amount of soot, so a little bit of smoke escapes the flame from those candles, and above those candles there are some surfaces at which this soot would deposit, and the people would actually gather that soot that deposited on those plates, and then this soot is used to create the ink.
00:22:39.513 --> 00:22:45.382
So I assume that this soot has some properties that are very attractive to the ink makers.
00:22:45.382 --> 00:22:58.678
Most likely it has a very specific particle size distribution that works very well when mixed with other substances to create the ink and gives the blackness that they look for with ink.
00:22:58.678 --> 00:23:24.717
However, from my perspective, from the perspective of a fire safety engineer and someone who's passionate about combustion and all of those stuff, what these guys did actually is they manually trimmed the wicks to get a damkuller number at which a very specific particle distribution is produced, like they don't even know, most likely, what damkuller number is and they have no clue.
00:23:24.717 --> 00:23:32.326
They've done that but through years of experience and this is some generation to generation art of creating of that ink.
00:23:32.326 --> 00:23:35.653
So the method to do that is very, very old.
00:23:35.653 --> 00:23:42.230
They found a number the ratio between the reaction and transport that produces soot that they like.
00:23:42.652 --> 00:23:49.817
Now the funny thing is they do it with candles and I find it quite dangerous because people are walking through a sooty environment.
00:23:49.817 --> 00:23:52.691
That's really horrible and we're going to talk about that in a moment.
00:23:52.691 --> 00:24:06.944
But I think with fire science, knowledge of thermodynamics and chemical reactions, we could just create a simple burner that would have this particular dump color number and would produce the soot that they need at an industry scale.
00:24:06.944 --> 00:24:12.417
So they would not have to collect tiny amount of soot from their candles.
00:24:12.417 --> 00:24:21.990
However, yeah, it's a craft, it's a handcraft and I guess this gives some magic and gives a justification to the price point for the ink.
00:24:22.652 --> 00:24:33.557
So here you have learned the story of soot creation from a chemical perspective, from thermodynamic perspective and simply from observing the real world.
00:24:33.557 --> 00:24:36.730
This is how soot comes into light In fires.
00:24:36.730 --> 00:24:37.531
It's the same.
00:24:37.531 --> 00:24:41.048
You also have chemical reactions in the flame, hundreds of them.
00:24:41.048 --> 00:24:44.557
The more complex the fuel, the more complex the reactions will be.
00:24:44.557 --> 00:24:47.112
A lot of those reactions are competing with each other.
00:24:47.112 --> 00:24:48.516
Not all of them will complete.
00:24:48.516 --> 00:24:58.701
Then the incomplete combustion products will start merging to form soot particles and those soot particles will transition into outer regions of your flames.
00:24:58.701 --> 00:25:14.220
In those outer regions, if there still is enough oxygen left and some heat that allows for oxidation, they will oxidize and disappear, and if there's not, they're going to transport into your building, creating problems with visibility in smoke that we have to deal with as engineers.
00:25:15.022 --> 00:25:17.317
One question that you could ask is there soot in the flame?
00:25:17.317 --> 00:25:19.470
And that could be a very interesting question.
00:25:19.470 --> 00:25:26.132
In fact, yes, there is, and in fact that's a really really important scientific problem how much soot there is in the flame.
00:25:26.132 --> 00:25:34.376
If you consider effects of the soot, one of the most profound effects of the soot is related to the combustion science.
00:25:34.376 --> 00:25:43.232
People who study the flame itself or the combustion processes itself, the sootiness in the flame changes the color of their flame.
00:25:43.232 --> 00:25:47.971
So if there's no soot in the flame, the flame is going to be blue or almost invisible.
00:25:47.971 --> 00:25:50.805
That's when you have minimal amount of soot in there.
00:25:50.805 --> 00:25:54.374
That's why methanol flames are almost invisible.
00:25:54.374 --> 00:26:04.395
There's this famous YouTube video of a Formula One car burning and the driver is escaping and he's shaking a flame from himself and you cannot see the flame.
00:26:04.395 --> 00:26:09.717
That's because he was covered in methanol and methanol produces almost no soot, so the flame is invisible.
00:26:09.717 --> 00:26:19.955
Once you start adding soot to the flame, it becomes more and more yellow and actually the yellow color of the flame is a product of soot being present in there.
00:26:20.365 --> 00:26:23.112
Now why it's important for our combustion colleagues?
00:26:23.112 --> 00:26:25.597
Because soot is related to radiation.
00:26:25.597 --> 00:26:28.670
Clean air has emissivity of zero.
00:26:28.670 --> 00:26:37.134
It's clean, it's transparent radiation, which you can test experimentally by going outdoors to a sun and you feel heat on your skin.
00:26:37.134 --> 00:26:58.794
This is because the heat can transport through the atmosphere a little bit of it, being taken by carbon dioxide and methane in the atmosphere, thanks to which we have global warming, but most of it passes through the atmosphere and reaches your skin, depending on the region of the world you are in, causing a pleasant warmth or perhaps even burns.
00:26:59.295 --> 00:27:11.132
Now, once you start adding soot to your gas, to your air, soot has emissivity of almost one because it's black carbon, so it's almost a perfect black poly.
00:27:11.132 --> 00:27:17.836
Once you start adding soot to the air, you start getting a mixture that would have some emissivity.
00:27:17.836 --> 00:27:19.403
What emissivity?
00:27:19.403 --> 00:27:23.679
That depends, of course, on the amount of soot you add, and we'll reach that in a few moments.
00:27:23.679 --> 00:27:27.950
But you basically added a new property to your air.
00:27:27.950 --> 00:27:32.925
It starts participating in radiant heat transfer and in flame.
00:27:32.925 --> 00:27:48.019
If you have soot in your flame, that flame starts emitting heat, starts radiating heat outside of the flame and this heat goes somewhere, can heat up new molecules, can heat up new fuel, can influence the reactions that are happening within the flame.
00:27:48.019 --> 00:28:09.035
So actually, the fact that soot creates radiation, this changes everything in the combustion regime, changes everything in the combustion regime and when you study combustion, when you really try to model those chemical reactions that are happening inside of the flames, the presence of soot changes a lot and is a very, very important condition that you need to consider.
00:28:09.035 --> 00:28:11.246
What other thing it influences?
00:28:11.246 --> 00:28:13.652
From our perspective it influences toxicity for sure.
00:28:14.032 --> 00:28:24.508
So one thing is that soot oxidation would lead to carbon monoxide production and there is an observation that with increased soot production there's an increased carbon monoxide production.
00:28:24.508 --> 00:28:32.414
You can read on that in Bart Mercier's and Tarek Beji's book the Fluid Mechanic Aspects of Fire and Smoke Dynamics in Enclosures.
00:28:32.414 --> 00:28:40.098
Actually that's the book I've read when preparing to this episode a very good read and I always recommend this book.
00:28:40.098 --> 00:28:45.676
So it affects the carbon monoxide production but also the soot itself.
00:28:45.676 --> 00:28:48.855
You can consider it as a vessel.
00:28:48.855 --> 00:28:50.958
It's a very large particle.
00:28:50.958 --> 00:28:58.618
It's a very large particle of carbon that is in the flame is surrounded by other products of incomplete combustion.
00:28:58.618 --> 00:29:05.346
If we assume that soot is created from polycyclic aromatic hydrocarbons, you cannot assume that all of them form soot.
00:29:05.346 --> 00:29:17.758
Some of them remain in their primal forms and those PAHs do actually sit on the soot particles, so some of them will just land on the soot particle and fly with the soot particle.
00:29:18.204 --> 00:29:20.614
Those particles are connected with cancer.
00:29:20.614 --> 00:29:28.857
Those are connected with serious health issues, so the soot itself is a very toxic or hazardous compound.
00:29:28.857 --> 00:29:32.816
Another aspect of that comes from the size of those particles.
00:29:32.816 --> 00:29:36.630
So they have a really nasty size distribution.
00:29:36.630 --> 00:29:43.400
Most of them would be in the range of, let's say, less than a few micrometers.
00:29:43.400 --> 00:29:46.273
For most of the smokes, less than one micrometer.
00:29:47.226 --> 00:29:57.892
And this is a really, really nasty size that you get, because if the particles are less than 100 micrometers, you can inhale them, so they're going to pass through your nose.
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