You are taking too long so I looked it up myself. I understand why you're having issues finding this information, I am having more difficulty that I expected.

According to Wikipedia it is 28-83 milligrams per cubic meter of air.

Though oddly Wikipedia states that hydrogen cyanide is lethal at approximately 2,860 milligrams. That is less than the 10-20 grams per cubic meter of air you listed, but is perhaps in a more concentrated form than Zyklon B.

But still, it seems that it takes much less Sarin gas to kill a person than it takes hydrogen cyanide. Is Wikipedia wrong?