Couldn't you still study populations with more or less plastic, even if "zero plastic" cant be found? Or is the amount of it more or less even across groups?
Throwing in my $0.02, a correlation study is technically an option, where you look at differences as microplastics increase in concentration. But. my instinct is you would see some really unfortunate covariance - that is, another variable that increases (or decreases) at the same time as microplastics and is known to impact your variable of interest, e.g. socioeconomic status.
I am having to admit that human kindness and empathy only extends to one and theirs and to all others only competition and hate, that is the hallmark of humanity. The greatest trick ever performed was convincing anyone that it was ever not thus.