Anearby coworker using a torch to free up a stuck bolt accidentally ignites his shirtsleeve. as a trained first aid provider, you quickly put out the flames by throwing a nearby coat over his arm. when you remove the coat, you can see that his arm has been burned badly.

The first thing to do is activate EMS. Use cool or cold water to cool the burn for 10 minutes as soon as possible. ( Do not use ice or frozen compress.) Early cooling will minimize the depth of injury and decrease pain. Remove jewelry near the affected area. Cover the injury with dry,  clean pad loosely to protect it. Do not give food or drink to injured person. Keep him comfortable until EMS arrives and takes over.

the answer to your question is tooth decay.