Mine comes off my shed roof which has a nice clean piece of Perspex
on it which I clean regularly as well as the guttering I have aquarium sponge
in the down pipe and muslin type cloth strapped over the opening.
In a good down pour ive had as low as 2ppm but average around 11ppm
the water doesn't sit there long enough to accumulate crap and its clear
I definitely wouldn't drink it though