I just want to add that the term "heavy metal" associated with toxicity is not really a good choice, even if it has been used for a long time.
The so called toxic "heavy metals" are not toxic because they are heavy. They are toxic because their ions have a large size (when compared with metals like magnesium or iron).
The large size of their ions makes them interfere with the functions of the ions having a similar large size and which have important uses in all living beings, i.e. calcium or potassium or ammonium or sodium. Large ionic size is also associated with good solubility, which enhances the chances of those ions to be absorbed after ingestion and enter living cells.
"Metals with large ions" is longer and more inconvenient to say than "heavy metals", but it would be much more accurate. Saying "heavy" does not make you understand why e.g. cadmium (same ionic size as calcium) is very toxic, while indium and tin, which are heavier (but which have smaller ions), have low toxicity.
Maybe they meant "heavy, metal particles", as in that are both metal and "heavy" relative to other materials (so somewhat redundant adjectives); as opposed to, "heavy-metal particles".
