Holy moly BuffloBob!!!
A nanogram is smaller than I can think!
Botulinum is the most acutely lethal toxin known, with an estimated human median lethal dose (LD50) of 1.3–2.1 ng/kg intravenously or intramuscularly and 10–13 ng/kg when inhaled.
https://en.m.wikipedia.org/wiki/Botulinum_toxin
LD50 of Polonium 210: estimated at 10 (inhaled) to 50 (ingested) nanograms in humans makes this one of the most toxic substances known.
Note: the polonium dose is for an average-sized human?
https://simple.wikipedia.org/wiki/LD50
It’s a rather large molecule. You don’t need very many therefore to kill a man.