But the people working for that organization would all have to continue to work, no?

That's what I meant with being controlled for similar lifestyles. If factors like wealth, education, BMI, drug use, medical history and so on are held constant, and microplastics amount in blood scales with some type of disease, it could point us in the right direction, no? Without having to necessarily study populations without microplastics exposure.

Couldn't large scale studies pry apart some correlation between the amount of microplastics and the incidence of different diseases? If controlled sufficiently for similar lifestyles, this could potentially point us in the right direction, without needing to find a subpopulation with zero microplastics in their blood.