Published on November 1, 2018.


Readers Respond: Nacho Issues

To the editor:

I am a forensic engineer and certified fire and explosion investigator in practice for nearly 40 years and a longtime member of NFPA. I read your recent “Dispatches” article on boxes of ground up tortilla chips that had spontaneously combusted in a series of fires in Texas [“Nacho Typical Fire,” September/October], and I was appalled to read that the incidents were described as combustible dust fires. These were clearly not combustible dust fires—rather, they were caused by spontaneous oxidation of the frying oil with which the tortilla chips were saturated.

Frying oil oxidizes naturally, which generates heat. In chip form, that heat readily dissipated into the air. When those chips were pulverized and boxed, however, the mass of chip dust acted as insulation, retaining the heat and allowing the temperature within the mass to gradually increase. The higher the temperature, the faster the oxidation, a vicious cycle that culminated in ignition of the cardboard cartons containing the powder.

The tortilla dust was confined, not suspended in air—the article correctly states that suspension in air is one of the conditions necessary for combustible dust to ignite. The quote from the NFPA technical expert that “maybe [the boxes] had been shaken up” is clearly a contrived explanation to support the preconceived explanation of cause. The question is not about the role of combustible dust in these fires, but rather how the boxes became so hot.

Because of the importance of proper handling of waste cooking oil and other combustible oils, including rags containing such oils, I suggest that a clarification be published in the next issue of NFPA Journal.


Merion, Pennsylvania

Guy R. Colonna, PE, senior director of Engineering at NFPA, responds:

The writer is correct to question how a suspended dust cloud could have formed in this example, which raises questions about this being a combustible dust incident or just a finely divided fuel in dust form that underwent an alternative ignition sequence. While the pulverized chip waste created a combustible dust, there were no means, according to published descriptions of the incident, where that dust could have been suspended, concentrated, and ignited. The formation of the dust merely provided a more easily ignited fuel, since dust is more susceptible to ignition sources than whole chips or chip waste might be. The NFPA response in the article would have been more complete if it had addressed the spontaneous combustion aspect in more detail and made a clearer link to the frying oils and their role in the ignition of the stored material.

The writer is correct to point to the high edible-oil content typically found in tortilla chips. Some years ago at NFPA, we encountered a number of fires in commercial laundries. The culprit turned out to be the cooking oil used in restaurants that had accumulated on linens, including aprons, table cloths, and napkins. The linens were collected by the laundry company, washed, and dried in commercial dryers, then folded and stored overnight for loading into trucks and delivery back to the restaurants the following morning. Because the linens had not been sufficiently cooled before they were folded and stored, however, they were able to retain heat that reacted with the oil residue that had not been fully laundered out. The self-heating process continued until ignition occurred.

For a related report on spontaneous combustion by NFPA’s Research, Data and Analytics Division, visit us online.