(4) In lawsuits, the regulator should have to prove that the substance in question is fatal. The EPA will have published a list ahead of time and and if challenged it will be up to a court whether the EPA has correctly determined the lethality of the substance.
(a) Radioactive pollutants, cadmium, arsenic, and polycyclic organic matter Not later than one year after August 7, 1977 (two years for radioactive pollutants) and after notice and opportunity for public hearing, the Administrator shall review all available relevant information and determine whether or not emissions of radioactive pollutants (including source material, special nuclear material, and byproduct material), cadmium, arsenic and polycyclic organic matter into the ambient air will cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health. If the Administrator makes an affirmative determination with respect to any such substance, he shall simultaneously with such determination include such substance in the list published under section 7408(a)(1) or 7412(b)(1)(A) 1 of this title (in the case of a substance which, in the judgment of the Administrator, causes, or contributes to, air pollution which may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness), or shall include each category of stationary sources emitting such substance in significant amounts in the list published under section 7411(b)(1)(A) of this title, or take any combination of such actions.
(b) Revision authority Nothing in subsection (a) shall be construed to affect the authority of the Administrator to revise any list referred to in subsection (a) with respect to any substance (whether or not enumerated in subsection (a)).
That leaves us with some options, such as these ones which I'm ordering from "most reasonable" to "most insane":
(1) In lawsuits, courts should generally assume that the lawmakers have given the EPA permission to create a formal list and judgement criteria for what counts.
(2) In lawsuits, courts should assume the list is totally empty unless a federal lawsuit has happened where both sides have called in "chemical experts" to testify and then a federal judge decides which chemicals are deadly and which are not.
(3) The law is totally meaningless until congress amends it with another bill that inserts a full list of every possible chemical composition and configuration required concentration-level, and anything not explicitly included on the list is exempt.