The last typically occurs through one of three mechanisms:

The gas is directly released from canisters (e.g. as part of a pesticide, insecticide, or Zyklon B). It is generated on site by reacting potassium cyanide or sodium cyanide with sulfuric acid (e.g. in a modern American gas chamber). Fumes arise during a building fire or any similar scenario involving the burning of polyurethane, vinyl or other polymer products that required nitriles in their production.As potential contributing factors, cyanide is present in:

Tobacco smoke. Many seeds or kernels such as those of almonds, apricots, apples, oranges, and flaxseed. Foods including cassava (also known as tapioca, yuca or manioc) and bamboo shoots.As a potential harm-reduction factor, Vitamin B12, in the form of hydroxocobalamin (also spelled hydroxycobalamin), might reduce the negative effects of chronic exposure; whereas, a deficiency might worsen negative health effects following exposure to cyanide. Mechanism
Cyanide is a potent cytochrome c oxidase (COX, a.k.a Complex IV) inhibitor. As such, cyanide poisoning is a form of histotoxic hypoxia, because it interferes with oxidative phosphorylation. : 1475 Specifically, cyanide binds to the heme a3-CuB binuclear center of COX (and thus is a non-competitive inhibitor of it). This prevents electrons passing through COX from being transferred to O2, which not only blocks the mitochondrial electron transport chain but also interferes with the pumping of a proton out of the mitochondrial matrix which would otherwise occur at this stage. Therefore, cyanide interferes not only with aerobic respiration but also with the ATP synthesis pathway it facilitates, owing to the close relationship between those two processes.
A total of 909 individuals died in Jonestown, many from apparent cyanide poisoning, in an event termed "revolutionary suicide" by Jones and some members on an audio tape of the event and in prior discussions. The poisonings in Jonestown followed the murder of five others by Temple members at Port Kaituma, including United States Congressman Leo Ryan, an act that Jones ordered. Four other Temple members committed murder-suicide in Georgetown at Jones command. On 6 June 1985, serial killer Leonard Lake died in custody after having ingested cyanide pills he had sewn into his clothes. On 28 June 2012, Wall Street trader Michael Marin ingested a cyanide pill seconds after a guilty verdict was read in his arson trial in Phoenix, AZ; he died minutes after. On 22 June 2015, John B. McLemore, a horologist and the central figure of the podcast S-Town, died after ingesting cyanide. On 29 November 2017, Slobodan Praljak died from drinking potassium cyanide, after being convicted of war crimes by the International Criminal Tribunal for the former Yugoslavia. Mining and industrial
In 1993, an illegal spill resulted in the death of seven people in Avellaneda, Argentina. In their memory, the National Environmental Conscious Day (Día Nacional de la Conciencia Ambiental) was established. In 2000, a spill at Baia Mare, Romania, resulted in the worst environmental disaster in Europe since Chernobyl. In 2000, Allen Elias, CEO of Evergreen Resources was convicted of knowing endangerment for his role in the cyanide poisoning of employee Scott Dominguez.