Ammonium nitrate may be the more likely candidate in the explosion.

In small quantities, the white pellets won’t detonate, notes Ronald Willey, a chemical engineering professor at Northeastern University in Boston. Many drugstore cold packs use the compound because, when water is also put in the packs, the mixture absorbs heat from its surroundings.

But the compound begins to decompose into nitrous oxide and water when heated to temperatures above 150 degrees F. – a process that itself releases heat. If temperatures rise to about 400 degrees F. or higher, as in a fire, decomposition can become explosive.