How about streets? Or industry? City means pollution and this can be relatively high even in residential areas. I sometimes use twigs from the yard, but leaves I wouldn't even get from an average suburb.I don’t live near all the office buildings and sky rises but I don’t live in the country
Provided they are from a low pollution area:
As biofilm is nothing bad per se it should be enough to take some boiling water and soak the leaves for a few hours.