Carfentanil - "A dose as small as a grain of sand can kill you"

From a report in The Guardian:

Developed in the 1970s as a tranquilizer for large animals such as elephants and bears, the synthetic opioid has also been studied as a potential chemical weapon by countries including the US, China and Israel. It is thought to have been deployed with disastrous effects when Russian special forces attempted to rescue hundreds of hostages from a Moscow theatre in 2002.

But it only burst into public view last year after officials across North America began to warn that it was being cut with heroin and other illicit drugs, leaving a rash of overdoses and deaths in its wake.

“An amount as small as a grain of sand can kill you,” Dr Karen Grimsrud, Alberta’s chief medical officer, told reporters after traces of carfentanil were found in the bodies of two men who had overdosed. “Carfentanil is about 100 times more toxic than fentanyl and about 10,000 times more toxic than morphine.”..

The remarks came after Canadian police – protected by hazmat suits and oxygen containers – seized one kilogram of carfentanil hidden inside cartridges labelled as printer ink and which had been shipped to Vancouver from China.

Given the purity of the substance seized, police estimated that the package could contain as many as 50m lethal doses – enough to wipe out the entire population of the country.

Scary.  And I bet it's not hard to synthesize.  Could be aerosolized via drones in a city or at a stadium.