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Question: Why exactly does it hurt when aluminum comes into contact with fillings in teeth?
The kind of tooth pain you're talking about comes from stimulation of the nerve that resides in the middle of the tooth. The nerve normally is encased in bone-like dentin, and the dentin in turn is protected by a layer of enamel. In the case of tooth decay, the enamel is worn away by acids produced by bacteria sticking to your teeth (plaque). The nerve is then accessible to the outside environment through pores in the unprotected dentin, and stimulation of the nerve through these pores by, for example, just blowing air on the dentin, causes pain.
Tooth decay is commonly treated by drilling out the decayed part of the enamel and dentin, and then filling the hole with a mixture mercury and silver, or with gold. These metals effectively seal off the nerve from the outside environment, preventing pain.
Aluminum and the metal filling form parts of a battery. Batteries that use metal work by transferring electrons (current) to or from metals. Without a wire connecting the plus and minus sides, the battery just has potential to transfer electrons. However, when the aluminum foil comes into contact with the metal filling in the tooth, the battery is shorted out, and current flows through the filling and the dentin, and to the nerve, giving the sensation of pain.
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