Why is milk used as a first aid remedy for someone who has swallowed a heavy-metal poison?

The simple answer is -- it's not. Milk as a remedy or first aid for neavy metal poisoning is at best a wive's tale.

As there are a LOT of heavy metals, and dosage, route of exposure, size and weight of the victim, and many other factors apply to this question, there is no single first aid solution for this problem that applies generally (Source US Poison Control 1-800-222-1222, CDC, et al.).

Treatment is usually through administration of chelating agents that scavenge the metal from hemo-proteins.

AnswerProteins can be denatured (distorted) in shape) by heat, alcohol, acids, bases, or the salts of heavy metals...... Many well known poisons are salts of heavy metals like Mercury and silver; these denature the protein strands wherever they touch them. The common first aid antidote for swallowing a heavy metal poison is to drink milk. The poison then acts on the protein of the milk rather than on the protein sites and tissues of the mouth, esophagus, and stomach. Vomiting can be induced to expel the poison that has combined with the milk.

Source: Chapter 6 The Proteins and Amino Acids p.184 Nutrition - Concepts & Controversies Frances Sienkiewicz Sizer & Ellie Whitney 10th edition Copyright 2006 Thomson Wadsworth

Not any milk only healthy raw milk binds with mercury and then it expelles out of body, commercial pasteurized milk is useless since the proteins are damaged due to pasterization and dont react with mercury poison.