Not your father’s house fire

Remember the good old days when our homes were built with only lumber, dry wall, and roofing? Me neither. However, we talk about ‘modern’ construction materials like this is a new phenomenon. The truth is ‘modern’ construction materials started sneaking into homes over 50 years ago. It’s not only construction materials that have changed: a century ago, we furnished our houses with wood, cloth, metal and glass. Today, it’s plastics, foams, and coatings.

Why does this matter? Well, in addition to behaving differently as they burn (rate of consumption, temperature, etc.), these materials have the potential to create different compounds than the typical products of combustion. Wood is primarily cellulose, which comprises carbon, hydrogen, and oxygen atoms. When it burns to completion, it forms CO₂ (carbon dioxide) and water vapor. Granted, incomplete combustion of wood will form CO (carbon monoxide, which is a deadly toxin), but that is the case for every carbon-containing compound on the planet!

Molecules matter

Let’s start with nitrogen-containing compounds, and there are a good many of those. A short list includes polyamides (nylons-carpeting), polyurethanes (used both as a coating and a foam cushion), urea-formaldehyde (insulation) and ABS (acrylonitrile-butadiene-styrene; a polymer that is easy to work with, cheap to produce and therefore is in many, many products). All of these compounds have a carbon atom bonded to a nitrogen atom. If during the process of combustion, a hydrogen atom joins this party, you end up with HCN, or hydrogen cyanide.

Another bad-actor in the nitrogen universe is nitrogen dioxide, NO₂. It is a toxic gas that is formed when nitrocellulose (found in fingernail polish, among other things) burns.  However, since there is nitrogen in the atmosphere (79% of the air you breathe is N₂), NO₂ can form when any combustion takes place, even natural gas and other ‘clean’ fuels.

There are other ‘bonus’ elements that show up in the materials all around us that go ugly early in a fire. Chlorine is one. PVC pipes can decompose during a fire and generate HCl, hydrochloric acid. Sulfur is another. Most common fertilizers contain sulfur, which will generate sulfur dioxide, SO₂, when it burns, which is also a toxic gas. HF, hydrogen fluoride has been shown to be generated when lithium-ion batteries burn.

In addition to the chemical make-up of these materials, it is also worth considering how they are treated before they enter the home. Much upholstered furniture is covered with ‘flame retardants’. This class of compounds is continually evolving, and scientists are constantly working to determine what they form when combusted. One last point to ponder is the ever-growing arsenal of different cleaning products that live under our kitchen sinks and in our garages/laundry rooms.

Figure 1:  What’s under your kitchen sink?

This is not to say that ‘modern’ is bad and we should stop using these new materials.  They have properties which make them valuable in their current usages. However, as fire investigators, it is imperative that we be aware of what compounds can be created when they burn.

Jennifer Morningstar, PE, CFEI, is a forensic engineer with The Warren Group, a forensic engineering and consulting firm. Her areas of expertise include chemical release & exposure, OSHA process safety management, industrial accident investigation, fires & explosions, and scope of damage/cost to repair analyses. Contact her at Jennifer@warrenforensics.com.