Do mushrooms really contain heavy metals?

Yes, mushrooms, like other plants, contain elements that they draw into themselves from the surrounding soil. Compared to other plants, however, mushrooms can absorb a greater number of different elements. This is thought to be caused by the mycelium, a tangle of fine filaments that are in the ground and spread over a large area (the largest mycelium is said to be several square kilometres in area).

Different elements can be stored in larger quantities in mushrooms thanks to their porosity — a large number of different pores and tubes for the same volume (weight) significantly increases the surface of the mushroom, and therefore also the space where the mushroom can store unnecessary materials that it pulled from the ground.

The mass of elements (not only heavy metals) that the mushroom contains depends on the place where it grows. If it grows in a place with environmental stress or near a road, there are much more harmful elements in the ground than in a mushroom that we pick in the forest in the wild.

I even read of a case in which (edible) mushrooms pulled up so much arsenic from the subsoil that it caused poisoning — the concentration in the mushrooms was much greater than the concentration in the soil in which they were growing.