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.
Urinary fractional excretion potassium is high or inappropriately normal in the context of hypokalaemia, and high levels of urinary sodium and chloride are observed. Other clinical indicators include elevated serum renin and aldosterone in the bloodstream, and metabolic alkalosis. The symptomatic features of this syndrome are highly variable ranging from asymptomatic to mild manifestations (weakness, cramps) to severe symptoms (tetany, paralysis, rhabdomyolysis). Symptom severity is multi-factorial, with phenotypic expression varying amongst individuals within the same family. Genetic testing is another measure of identifying the underlying mutations which cause the pathologic symptoms of the disease. This mode of testing is available at select laboratories.Work-up to exclude the differential diagnosis of the electrolyte abnormalities is key. In Gitelman syndrome hypocalciuria is present, and a urine calcium:creatinine ratio may help distinguish it from Bartter syndrome as the two disorders can be clinically indistinguishable. Additionally in Bartter syndrome maximal urine concentrating ability is lost. Laxative abuse can mimic the serum electrolyte abnormalities, but fractional excretion of potassium will be low
Diuretic abuse could be suspected if urinary chloride excretion varies by time of day but may require a diuretic assay to detect
Surreptitious vomiting can cause metabolic alkalosis and hypokalaemia, but urinary chloride levels will be low
Medication history; Proton-pump inhibitors can cause an isolated hypomagnesaemia phenotype, and aminoglycosides such as gentamicin can cause a transient metabolic alkalosis with hypokalaemia and hypomagnesaemia that resolves 2–6 weeks after drug termination.