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

Heavy metal poison specifically due to lead are removed by intake of milk (as it is a source of calcium)

Explanation:

Toxicity produced by heavy metal specifically lead can be removed by intake of calcium metal which is predominantly found in milk. The interaction between lead-calcium subsides the toxicity of lead.  

Both calcium and lead ion i.e Pb(2+) and Ca2+ compete with each other for transportation at the plasma membrane but due to the presence of Ca2+ channels, and the Ca2+ pumps, calcium get preference over lead and hence it is removed from the nervous system.

the statement is true

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