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I often read description of various compounds or elements as being poisonous. Now I know that through experience we have discovered the toxicity of various substances like lead, chlorine or phosgene etc. We know nitrogen isn't poisonous as its present in the atmosphere etc.

What I wanted to ask is whether there is a perfectly analytical or experimental way through which chemists determine whether a newly discovered substance is or isn't poisonous? Or is it always found out only by exposure to humans (which I don't think is always possible)? Or perhaps whether such an information can't always be deduced?

I think animal testing is a possible (but immoral) method but again some substances could be harmful to them but harmless to us.

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    $\begingroup$ For $\mathrm{LD}_{50}$ you get some rats and dose them. When the dose is large enough to kill 50% of them you have your answer. $\endgroup$ – MaxW Jun 21 '19 at 17:09
  • $\begingroup$ @MaxW But this clearly states a lack of validity in $LD_{50}$. $\endgroup$ – Sameer Thakur Jun 21 '19 at 17:24
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    $\begingroup$ Are you a strict vegan? Hypocritical to kill a cow to eat, but worry about killing some rats to advance science. $\endgroup$ – MaxW Jun 21 '19 at 17:40
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    $\begingroup$ The only way is to do some real tests. Drugs for people have to be rigorously tested before people are exposed. Since Thalidomide, we test animals for teratogenicity, for example, before use in people. (thalidomide is harmless to adults but causes terrible deformations in embryonic development). There is no simple short cut. $\endgroup$ – matt_black Jun 21 '19 at 21:47
  • $\begingroup$ @MaxW There is not killing like killing.Killing for food is fast. Killing by poison can be very nasty. Testing of drug candidates is inevitable, but not all testing is justified. $\endgroup$ – Poutnik Jun 22 '19 at 1:33
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No, there is no way to determine toxicity to humans with a "magical method".

Most of our knowledge about toxicity is extrapolated from animal testings and from reported accidental exposures in human.

Also, Nazis carried out a large number of human testing on Jews, and this is not limited to toxicity.

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