how do you keep the pollution in ocean water from contaminating sea salt?

Essentially, you can't, other than to hope the salt is harvested in an area not located near a pollution source, and thus the contaminants would be in a lower concentration. I suppose you could try to find a producer who treated or filtered the seawater, but I'

ve never heard of one.

do you know if it is ever tested to see what contaminants it contains, pcb's, dioxins etc.