In setting environmental standards for industry and others to meet, it is inadvisable to require the best results that state-of-the-art technology can achieve  Current technology is able to detect and eliminate even extremely minute amounts of contaminants, but at a cost that is exorbitant relative to the improvement achieved
So it would be reasonable instead to set standards by taking into account all of the current and future risks involved
minute amounts of some contaminants can be poisonous