The proper response to this inevitable scientific uncertainty is to avoid practices that have the potential to cause severe damage, even in cases in which we do not have scientific proof of harm. This is the precautionary principle, familiar to Rachel's readers. However, Thornton points out, the precautionary principle does not tell us what kind of action to take. Their persistence tells us that nature does not have means for handling them. The point of clean production is to seek out, and adopt, the least harmful alternatives.
Reverse onus is a new way of evaluating chemicals.
Using the principle of reverse onus, the burden of proof, which now rests with society to prove that a chemical will cause harm, is shifted to those who want to produce or use a novel chemical. Such people must demonstrate in advance that their actions are not likely to pose a significant hazard. Chemicals currently in use that cannot meet this criterion will be phased out in favor of less damaging alternatives. The result is the current permissive, laissez faire system in which anything goes until someone can prove to a scientific certainty that significant damage has occurred. Under the risk paradigm, chemicals are produced and used without any restrictions.