Not sure if you don't a smoke much ,best way,i found is to keep drinking water flush it out your body,i had cannabis a show up,when I worked in Germany after 4 months of been clean of it,and from that I lost my job has a chippy on site.
Has for the drug cheat test there a rip of they will make you look a fool,im not sure if you could get away with saying you have been in a room where people was smoking though,but you was not smoking yourself