can anyone explain what threshold level of toxicity means? also, what it may be for a specific species?

Answer 1

This can also be called the LC0 or LD0 for the analyte. This is the concentration above which some number of the species may die, or below which none will die. If you want to look at some values , look at the EPA references given.

Answer 2

In some context it means when toxic effects start to show. It varies depending on species and toxin.

Answer 3

OSHA has already established that levels.
for instance the first alarm point for H2S is 10 ppm. u could live and work at this level or below for 8 hr.
CO is 35 ppm for first alarm level and u could stay at that level or less for 8 hr.