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Are Hot Particles Common Culprits of Wildland Fires?

Kevin H. Lewis, PE, CFI, CFEI

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Jul 2, 2019

While many of us are rejoicing the arrival of the hot weather, the summer months also bring an increased chance of wildland fires across the United States. Already, authorities are stating that this season could result in severe wildland fires.

It’s important to be aware of the possible causes of wildland fires. While we typically think of campfires, lightning and stray cigarettes as possible causes, hot metal particles are increasingly being blamed as possible ignition sources.

What is a Hot Metal Particle?

Unfortunately, it’s not a very well-defined term. There’s nothing about the size, shape, energy or ignition temperature from the National Fire Protection Association (NFPA) or National Wildfire Coordinating Group (NWCG). But, from our research and experience with investigations, we know that hot particles are:

  • Generally very small
  • Usually have a colored surface caused by oxidation referred to as “bluing”
  • Weigh less than 0.5 gram
  • Can reach temperatures up to 1000°F

Hot metal particles are usually created when moving machinery strikes a rock or other hard material. Typically, particles that are released are iron-based (steel) and are already at a very high temperature or can burn. Workers using machinery or logging equipment in a wildland area, or even railroads are a potential ignition source. Welding and cutting are common tasks that can emit hot metal particles.

Why Do Hot Metal Particles Matter?

Companies performing site work are frequently blamed for fires, but sometimes investigators gather little to no proof of the fire origin. In fact, some of the particles that are gathered don’t have the ability to ignite ground fuels, making them improbable ignition sources.

As we know, ignition is highly dependent on the ambient conditions. The weather is an important factor – the temperature, humidity, and wind speed all play a part. Conditions such as the moisture content, density and distribution of the fuel source (i.e., dead vegetation, and other grassy underbrush) are also critical. Finally, investigators should consider the size and temperature of the hot metal particle itself.

While wildland fire investigation teams regularly consider the weather and fuel conditions, less attention is given to the hot particle. We’ve researched the characteristics a hot particle needs to start a wildland fire and discovered that:

  • Most particles created from mechanical shearing will have a maximum temperature of less than 1000°F
  • A particle that has a bulk temperature of 1000°F would have to weigh approximately one gram or more to act as a competent ignition source for typical wildland fires if you are just considering heat transfer from the particle
  • Particles smaller than a gram can start fires, but the particle either must be burning or usually there are many of them. It’s difficult for an individual grinding particle to start a fire, but many of them directed on the same spot can be an ignition source
  • Metallurgical examination of the particle can sometimes reveal the “thermal history” of the particle
  • Empirical testing should reflect the actual fuels under consideration

Overall, our research found that it’s difficult for hot metal particles to ignite ground fuels – you need a large particle at a very high temperature (over 1000°F) to have high probability of ignition.

What if I Suspect a Fire Was Started by a Hot Particle?

When you find yourself in a situation where it’s anticipated that a hot particle may have started a fire, don’t rush to blame heavy machinery as the ignition source. Samples will have to be collected and analyzed to determine if they have the characteristics necessary to ignite the ground fuels present. When in doubt, collect the top layer of soil in the area of origin for further analysis.

Wildland fires are a complex phenomenon. Often, to get to the root cause of a wildland fire, a multidisciplinary team needs to be brought together to work together. A “dream team” including a vegetation management expert, origin and cause investigator, electrical engineer, meteorologist and metallurgical engineer all collaborating is the most effective way to determine how a wildland fire started and if hot particles are the culprits.

Learn about our wildland fire and fire science expertise, and how we assist in these investigations.