What is halogen? How it affects environment and human?

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Yesterday, i got maan email regarding an audit for halogen-free compliant which is a requirement for ROHS (Restriction of Hazardous Substance) for manufacturing firms. I am thinking what halogen is, how does it looks like. How can i determine if the item contains halogen. What is its hazard to the environment, and how does it affect human health. Would you like to share?