How do we know that cyanide was the poisinous gas used to kill Jews in Auschwitz?
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Executive summary
Contemporary forensic testing and documentary evidence identify hydrogen cyanide (Zyklon B) as the pesticide used both for delousing and for mass murder at Auschwitz; cyanide residues were found in delousing and gas‑chamber sites but not in living quarters, and multiple postwar forensic studies detected cyanide compounds in the ruins [1] [2] [3]. Claims that low cyanide readings disprove homicidal gassing rest on flawed sampling and methodology, a position thoroughly critiqued by forensic teams, historians and institutions such as the Auschwitz Memorial [4] [5] [3] [6].
1. Evidence from chemical tests: residue where expected
Analyses carried out after the war and in later forensic studies found cyanide residues in delousing chambers and in the locations identified as homicidal gas chambers, while control samples from living quarters showed none; that spatial pattern matches testimony and camp records that Zyklon B was used both for disinfection and for murder [1] [3] [7].
2. Why some tests showed "low" cyanide — sampling, chemistry and time
Reports such as the Leuchter study claimed lower cyanide in alleged homicidal chambers than in delousing rooms, but critics point out that Leuchter sampled brick and mortar incorrectly (grinding whole samples rather than surface layers), ignored the role of plaster and weathering that removed surface-bound cyanide, and misunderstood the differing exposure regimes required to kill lice versus humans [4] [5] [8].
3. Scientific rebuttals and repeat testing
Independent forensic work by the Institute of Forensic Research in Kraków and other laboratories redid microdiffusion and other micro‑analytical tests and found vestigial cyanide compounds preserved in the homicidal crematoria and gas‑chamber sites at levels above background — reinforcing the archaeology, documents and witness testimony [7] [3] [2].
4. The chemistry explains why "Prussian blue" is an unreliable single marker
Holocaust deniers have focused on the patchy presence of Prussian blue (the iron cyanide pigment) as proof of no mass killings; scientific analyses show Prussian blue forms only under particular cyanide concentrations, pH and iron conditions, so its absence in many places does not demonstrate Zyklon B was not used — the burden of proof lies with those asserting it must be present [8] [4].
5. Documentary, testimonial and material corroboration beyond residues
Physical traces of gas‑tight doors, ventilation shafts and Zyklon B inlet openings have been identified in crematoria ruins and in wartime plans and photos; camp administration documents, survivor testimony and perpetrator confessions together document the planned use of Zyklon B in gas chambers — a body of evidence institutions such as the Auschwitz Memorial and courts have cited [6] [7] [3].
6. How denial narratives use selective science and procedural mistakes
The Leuchter report and its successors repackaged methodological errors and misinterpretations into a conclusion that low cyanide readings disproved homicidal gassing; mainstream historians and chemists have exposed those reports as pseudoscientific, noting illegal sampling, lack of appropriate controls and misunderstanding of cyanide chemistry and exposure differences [4] [3] [8].
7. Institutional responses and legal context
Courts and memorial institutions have rejected denialist claims. For example, forensic evidence of cyanide residues was part of postwar trials, and later court cases and memorial authorities have emphasized that Zyklon B was used both for disinfestation and mass murder, citing forensic tests, documents and testimony [7] [1] [3].
8. Limits and what available sources do not mention
Available sources document cyanide testing, methodology disputes and documentary corroboration, but they do not provide every individual laboratory dataset or raw sampling logs in this collection; available sources do not mention unpublished raw analytical files beyond the cited forensic reports and critiques [2] [4].
Bottom line: the identification of hydrogen cyanide (Zyklon B) as a killing agent at Auschwitz rests on converging lines of forensic chemistry, material archaeology, Nazi documents and survivor and perpetrator testimony; challenges based on isolated low‑reading studies have been discredited on methodological and chemical grounds by forensic experts and institutions such as the Institute for Forensic Research and the Auschwitz Memorial [1] [2] [3] [6].