Today, in episode four of "Dehaan On Fire", Dr. John DeHaan discusses the issues and practices that change when the bodies of fire victims are still on scene during an investigation.
- As I said in the introduction on the channel here, we're going to have some longer videos talking about past cases and discussion about bigger issues, but we also want to hear questions and receive questions from people who are watching the videos and today is a day when we have one of those questions. So I will just launch right into it and ask you. If someone throws a match into a pool of gasoline, what is the material first ignited?
- Well, material first ignited is the vapors in a layer above the pool of liquid gasoline. It isn't the gasoline itself, it's the vapors generated by the evaporation of the gasoline.
- And why is it that the match is not the first material ignited?
- Well the matches is just an ignition source. I mean, it's a critical part of recreation identification of that, you know, that mechanism. But, I don't think I've ever encountered a fire scene in which the match was the, you know, in entire fire, the match was always there to light a much larger, more significant fuel load and that's what carried the damage and then made the fire worth investigating.
- There's something that I wonder as a layman, why is it important to identify the first material ignited?
- Well, you're trying to recreate the, a good investigator's trying to recreate the entire event and if you're trying to establish how the fire started, you have to understand what fuels might be available to be ignited and then you need that information to test possible ignition sources. And for instance, a glowing cigarette, despite all of the dramatic scenes in movies in television, a smoldering cigarette will not ignite gasoline vapors or natural gas or much of anything else in the gaseous phase. In fact, there's only four or five gasses that a smoldering cigarette will ignite. You almost never encounter those in non-industrial settings. It's things like hydrogen and acetylene and things like that, that are capable of being ignited by a smoldering cigarette.
- Okay, so I think a lot of people will find that interesting to learn that. Why is it that the cigarette will not ignite gasoline?
- Well, there's a number of reasons. One is it's a very low-energy ignition source, so you have to have contact between the burning tobacco inside the cigarette and theoretically that's hot enough. But the problem is getting that heat transferred to the fuel while the fuel is in an inappropriate state to be ignited. And if you draw on a cigarette for instance, you get an increase in the temperature of that coal and that's helpful. But unfortunately it accelerates the combustion of the tobacco to the point where, it's an oxygen deficient, in fact, oxygen is completely consumed in the vicinity of that coal. So there isn't any oxygen to support combustion. The contact time for gasoline vapor, even under ideal conditions between the vapor itself and an appropriate ignition source and effective ignition source is about 20 milliseconds. Which is really fast until you realize that when you're drawing on a cigarette, the contact time as any kind of gasoline vapor molecules come into contact with that coal, the contact time is about one millisecond. And so there's no oxygen, it's a carbon dioxide rich-environment, you don't have enough contact time to get the heat transferred and therefore you don't have ignition
- And that's drawing on a cigarette. So you can't really just take a cigarette and flick it and get vengeance on your enemies.
- No. If you're really lucky, or your target is really unlucky, if you throw it, in very rare instances, you can get the, enough ventilation actually ignite very momentarily a fragment of paper on the outside of the cigarette and you'll see that, you know, once in awhile if you throw a cigarette, you can get that little brief flicker. But even that's in most cases too short a duration to actually ignite the gasoline. So, we've had, you know, we used to do this as part of the training classes that we'd pour out a pool of gasoline and when I could afford it, back in the days when a pack of cigarettes was two bucks instead of seven, I would challenge the class to anybody that can ignite the gasoline with this, you know, a smoldering cigarette I'd buy him a case of beer. Well, did that a lot of times I had never had to buy anybody any beer cause they would go through the whole pack of cigarettes and throwing it and placing it and all kinds of stuff and it just doesn't work.
- Well, on behalf of everyone watching, thank you for wrecking some of my favorite movie scenes in our memories.
- Well, if you look real closely, in some of those scenes, the ignition point is of course the igniter, the pyrotechnic igniter that the special effects guy is put in the scene, hidden in the scenery somewhere. And in some cases they see the cigarette go one way and the ignition occurs from the other side of the, you know, the car or whatever you go, Oops!