If it ain't publicly known and reproducible then it ain't science. No public policy or regulation should be based in reasons that are not subject to examination and validation. This is pretty simple.
This is ridiculous. Real after me: It is impossible to scientifically prove that environmental damage is bad, or should be illegal. It is impossible to scientifically prove that killing innocent people is wrong. It is impossible to scientifically prove that theft is wrong. It is impossible to scientifically prove that the risk of particulate pollution above level X creates an intolerable hazard, but below the level does not, and lacking that level of predication, rule making becomes impossible.
If you create the standard that a law must be justified by science, no law could be sufficiently justified. The EPA makes regulations based on science, but also on things like risk assessments, ethics, moral attitudes on the value of human life, and popular democratic demands. Risk assessments and ethics aren't science and never can be, making them into science is scientism. If the people vote for clean air or dirty air, or their legislators demand it, they should get clean or dirty air, science be damned.