The DOT regulation at 49 CFR 174.450 previously stipulated that when fire occurs in a domestic rail shipment of charcoal in transit, water should not be used if it is practicable to locate and remove the burning charcoal. Any charcoal that becomes wet while a fire is being extinguished must be removed from the car and not reshipped, and the remaining charcoal must be held under observation in a dry place for at least 5 days before forwarding. What is the most likely reason DOT regulates the domestic transportation of charcoal in this fashion? What chemical reaction is involved here?