I have not seen that article but I would guess from the title of
"particles washed" they are referring to those tiny pieces of gravel
and stones that the shingles have on the surface and those tiny stones
are never a threat to plants. I would guess that as the gutters of
roofs with asphalt shingles are mostly the accumulation of those tiny
stones that deteriorate from the shingles and accumulate in the gutter.
And I would guess that the report is because those tiny stones present
a contaminant in the urban environment.

Can Dano post a paragraph about that article so that we can see what
specifically is the contaminant.

Archimedes Plutonium
www.iw.net/~a_plutonium
whole entire Universe is just one big atom
where dots of the electron-dot-cloud are galaxies