First of all, let me clarify that the act actually refers to the precautionary principle in four places, not just in the preamble. It imposes an administrative duty on the government in section 2 to implement the precautionary principle throughout the act. So the departments have an obligation to account for the precautionary principle in every decision they make under the act, including administrative decisions. Indeed, CEPA is the only federal statute in Canada that has that kind of obligation. There are a number that refer to the precautionary principle in their preambles, but none of them imposes an actual duty. So it informs every decision.
One way to think about the impact of the precautionary principle is to say that it enables certain decisions to be made that would otherwise not be made. So, many decisions that are made under CEPA probably could have been made, and would have been made, without those provisions in the act, or without having an obligation to consider the precautionary principle. What the precautionary principle does is move the threshold of acceptable decisions.
To exactly where is not clear, and exactly what kinds of decisions should be made is not clear. That's left wide open to the discretion of decision-makers, with one exception, and that is the exception of persistent bioaccumulative and inherently toxic substances. When the departments find that a substance is a PBIT, to use Mr. Benevides' acronym, then the ministers must recommend that the substance be subject to virtual elimination. That's one example of where the act actually does prescribe a certain type of action, but Mr. Stoffman is absolutely correct that it doesn't prescribe similar types of actions for other groups or classes of substances, based on the kind of harm that is found.
So the answer to your question is that the departments account for the principle in every decision. I am fairly confident that if industry had agreed to come, at least some would have argued that Environment Canada and Health Canada have been overzealous in applying the precautionary principle. It's a value judgment as to how precautionary we ought to be.