How an ordinary pesticide became the Nazi's most deadly killer in the Holocaust

German chemical companies developed Zyklon B in the 1920s in response to rising demand for reliable fumigation methods in the period after World War I, when food shortages and falling public health placed new demands on farming and transport sectors.

Hydrogen cyanide had already proven its usefulness in the United States as a fumigant in citrus groves, and German scientists modified the method by mixing the gas with a stabiliser and a porous carrier that allowed it to be stored in pellet form.

The compound released hydrogen cyanide (HCN) gas when exposed to air, which allowed it to kill vermin efficiently in enclosed spaces.

The active ingredient was absorbed onto porous carriers such as diatomaceous earth or gypsum and stored in sealed metal canisters. 

Fritz Haber, a Nobel Prize-winning chemist known for both the Haber-Bosch process and his role in chemical warfare during World War I, laid much of the foundational work that made Zyklon B possible, although he later fled Germany due to his Jewish heritage.

Metal canisters held the pellets securely until opened, which allowed safe handling, storage over time, and controlled use. 

Degesch was the company responsible for commercial production, and they promoted Zyklon B as a modern chemical answer to long-standing pest problems.

State agencies also openly supported its use, especially in situations where disease prevention became a matter of public concern.

In seaports, train stations, warehouses, and border towns, officials relied on Zyklon B to disinfect clothing, cargo, and facilities.

Because it acted quickly and left no permanent residue, the chemical gained a reputation as an effective and efficient tool.

Throughout the 1920s and 1930s, advertisements and government orders, along with health policies treated Zyklon B as a proper and even advanced success of modern science.

By the time Nazi authorities began searching for industrial methods of extermination, the chemical had already secured its place in public and professional life as a trusted product. 

The SS began to explore chemical alternatives to mass shootings during the invasion of the Soviet Union in 1941, after frontline officers reported that the mental effect of executing civilians at close range had begun to affect their troops' morale.

Nazi leaders, who had already implemented carbon monoxide gas vans in occupied Poland, broadened their search for faster solutions when faced with the scale of the Final Solution.

On September 3, 1941, at Auschwitz, SS doctors and camp administrators tested Zyklon B on Soviet prisoners of war in the basement of Block 11, where they saw its deadly effects under controlled conditions.

The results convinced them that the gas could provide a faster and more reliable method of mass execution. 

Engineers who were at Auschwitz-Birkenau built gas chambers based on the success of these early trials, fitting them with sealed doors and roof vents designed to receive the Zyklon B pellets.

SS guards herded victims, most of whom had arrived on deportation trains under the pretext of resettlement, into large chambers masqueraded as communal shower blocks.

Once sealed inside, the victims received no warning before camp staff dropped canisters of Zyklon B through the openings above.

As the pellets released their gas in response to the surrounding air, those inside began to suffocate, usually within ten to fifteen minutes.

SS medics observed the process and declared the area safe only after the noise had stopped and the gas had cleared. 

Other camps, including Majdanek, adopted similar methods based on the Auschwitz model.

Although Zyklon B’s use at Mauthausen is less conclusively documented, the camp employed similar gas chambers for killing.

The use of Zyklon B allowed Nazi officials to process thousands of victims per day without engaging directly in physical violence, which enabled a mechanical system of murder that could be operated with emotional detachment. 

Two companies held the primary responsibility for manufacturing and distributing Zyklon B: Degesch, which handled production, and its subsidiary Tesch & Stabenow, which oversaw commercial distribution to extermination camps.

These firms operated under civilian leadership, but they worked in close coordination with the SS and other Nazi departments.

Gerhard Peters, director of Degesch, personally authorised bulk shipments of Zyklon B to Auschwitz and other camps, despite receiving reports that the gas had been diverted from public health use to support extermination operations.

Shipment records show that over 7,000 kilograms of Zyklon B were delivered to Auschwitz alone between 1942 and 1944. 

At Tesch & Stabenow, company director Bruno Tesch and deputy Karl Weinbacher managed contracts with the SS and instructed personnel in the proper use of the gas at extermination sites.

Documentation recovered after the war showed that these men received bonuses based on profits from their largest clients, which included Auschwitz and Majdanek.

Though they claimed ignorance during postwar trials, evidence clearly survived that showed that they understood the scale and purpose of the deliveries, especially as quantities exceeded what would have been needed for disinfection. 

Also, transport authorities ensured that Zyklon B shipments received priority rail status, and local officials facilitated deliveries directly to the camps.

Chemists advised on storage conditions, and plant workers maintained the equipment needed to keep the gas stable and ready for deployment.

Each company and individual involved in the process became part of the logistical network that sustained genocide.

They masked their actions behind legal contracts and technical language and attempted to separate themselves from the consequences of the product they supplied. 

More than one million people died in gas chambers using Zyklon B at Auschwitz-Birkenau, where the system operated as the centrepiece of the Nazi killing programme from mid-1942 until late 1944.

Most victims arrived from ghettos across German-occupied Europe, packed into freight cars with no knowledge of where they were going or what awaited them.

Upon arrival, SS doctors selected the young and the strong for forced labour and also assessed the able-bodied for the same purpose for forced labour, while the old, the sick, and almost all women with children went directly to the gas chambers. 

Inside the chambers, panic broke out almost immediately once the doors closed and the lights went out.

Those closest to the vents died first, while others scrambled for air and piled atop one another in a futile attempt to escape.

Witness accounts from members of the Sonderkommando, such as Filip Müller and Henryk Tauber, described the tangled mass of bodies and the terrible expressions frozen on their faces.

After the gas dissipated, prisoners had to strip the corpses of valuables and prepare them for cremation.

The process ran day and night during peak periods, leaving behind mountains of ash and bone fragments. 

Zyklon B made it possible to systematise killing on a scale never seen before. Its effectiveness, ease of use, and availability allowed Nazi officials to kill rapidly, with minimal disruption to other wartime operations.

Victims became statistics, recorded in transport lists and tally sheets, their deaths processed by a management that hid its cruelty behind charts and timetables. 

After Germany's defeat, Allied prosecutors placed particular focus on the use of Zyklon B during the Nuremberg Trials, where the gas chambers at Auschwitz-Birkenau featured prominently in the charges brought against Nazi leaders.

Evidence included survivor testimonies, captured documents, shipment records, and forensic inspections of the remaining crematoria.

Photographs of canisters and diagrams of gas chambers helped demonstrate the planned and organised way of the killings. 

In 1946, the British military tribunal in Hamburg held a specific proceeding known as the Zyklon B Trial, where Bruno Tesch, Karl Weinbacher, and Joachim Drosihn faced charges of supplying poison gas to the SS with knowledge that it would be used to kill civilians.

The court convicted Tesch and Weinbacher and sentenced them to death by hanging.

Drosihn, who was a technician, was acquitted. During the trial, witnesses testified that Tesch had personally instructed SS personnel on-site, while Weinbacher had signed off on shipments to extermination camps.

Their convictions would be one of the first cases in which civilian industrialists were held legally accountable for crimes committed using the products they supplied. 

Gerhard Peters, the director of Degesch, later stood trial in West Germany and was eventually convicted in 1955.

He received a five-year sentence but was released after serving two years. He avoided earlier prosecution and resumed work in the private sector after completing his term.

Many other individuals connected to the Zyklon B supply chain escaped prosecution altogether, especially those in supporting roles within rail transport and chemical engineering. 

Regardless, the postwar trials established a legal example that set responsibility on those who gave orders or pulled triggers and on those who manufactured, sold, and delivered the tools of genocide.

Zyklon B showed how ordinary industrial processes, when placed in the hands of a totalitarian state, could facilitate mass murder under a veil of 'business operations'.

Today, original canisters of Zyklon B, along with technical documents and photographs, are displayed in memorial museums such as Yad Vashem and the Auschwitz-Birkenau State Museum, where they are evidence of the bureaucratic machinery of death and as warnings against moral indifference.