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.
The argument given concerning the reasonable way to set standards presupposes that there are effective ways to take into account all of the relevant risks posed by allowing different levels of contaminants.