Well, the point being made on the number itself is that we really don't know what the biological end points would be for some chemicals. There are different biological end points for their impact on a biological process.
So whether it's 10-3, 10-5, depending on different chemicals, you will see different effects--also synergistic effects between chemicals. Whether we can dredge to 10-18 I highly doubt, depending on.... In answer to your question, I just don't know.
The point is that although detection limits have come down a long way from what we were able to do back in the early 1970s and 1980s, now that they're able to make these detections, the relationship between that and the effect on wildlife and on humans has not been.... We just haven't got to that point yet, although we can detect.