One is obligated to wash one's hands after a meal due to danger of "sodom salt" which is said to cause blindness.
R. Judah the son of R. Hiyya said: Why did [the Rabbis] say that it was a bounden duty to wash the hands after the meal? Because of a certain salt of Sodom which makes the eyes blind. Said Abaye. One grain of this is found in a kor of ordinary salt.
Chullin 105. The question is, what exactly is sodom salt that differentiates it from regular salt, yet is somehow mixed in with it? I found a chabad source which indicates that it refers to caustic chemicals found in natural salt, but it is unsourced and counter intuitive given the premium of natural salts and the apparent lack of danger in the widely used dead sea salts.