One of the gases emitted by a lithium battery fire is hydrogen fluoride. (HF)
Hydrogen fluoride is highly corrosive and a powerful contact poison. Exposure requires immediate medical attention. It can cause blindness by rapid destruction of the corneas. Breathing in hydrogen fluoride at high levels or in combination with skin contact can cause death from an irregular heartbeat or from pulmonary edema (fluid buildup in the lungs).
Hydrogen fluoride is an extremely dangerous gas, forming corrosive and penetrating hydrofluoric acid upon contact with moisture.
So, when the FD attempts to extinguish the battery fire with water, they're producing hydrofluoric acid in large quantities.
HF is a bone seeker, it loves calcium so much so that the steals calcium from the nerves, so you don't even feel a burn until it gets to the bones. Then with all that extra calcium you REALLY feel it!
People have died from a burn no bigger that the palm of a hand.
The heart doesn't beat very well when the nerves are calcium depleted.
Translated:
Too dangerous for racing, but Ok for public transportation.
/s