镉是动物和植物的非必需元素，也是毒性最大的重金属之一。
Cadmium is one of the most toxic heavy metals and is not an essential nutrient element for plant, animal and human being.
汞是人体非必需元素，它可以多种物理和化学形态广泛存在于自然界中。
Mercury is an unnecessary element for human body, and it can exist in several physical and chemical forms in the environment.
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