There are very few substances with noticeable effects on the human body in microgram doses, not to mention the ability to cross the skin barrier. I think there

There are very few substances with noticeable effects on the human body in microgram doses, not to mention the ability to cross the skin barrier. I think there is just maybe a dozen known toxins able to kill an adult in a dose below 1 milligram. I am not sure for those toxins the effective dosage was even widely known as cautionary tale, back then.