P. pruinosus could be defined as a macroconcentrator of Cd, Zn, and Cu (BAF > 2) and as a deconcentrator of Pb (BAF < 1).

Source

Since isopods are food source of vertebrate and invertebrate predators, one threat is the transfer and potential accumulation of these contaminants at higher trophic levels. In a regional assessment of the Greater Washington-Baltimore Metropolitan area, Pouyat et al. (2015b) found a positive correlation between lead concentration of soil, isopod body, and blood of American robin (Turdusmigratorius Linnaeus, 1766) nestlings from the same residential yards.

Source

One way is just by keeping the heavy metals (Hg, Cu, Zn, Cd, etc) in their body. Any heavy metals they uptake will remain in their body, and while they’re alive they create more isopods to uptake more metal. But, if they get eaten it can accumulate in other animals or if they die it just returns to the soil. They’re also useful as indicators of soil contamination.