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What were the gas vans? How did they work?
Gas vans were regular cargo trucks specially equipped with an air-tight metal cargo compartment. The exhaust system was designed so exhaust gases could be re-directed into the sealed cargo compartment, instead of flowing out the tailpipe and into the air. When the engine ran, the people locked in the back of the truck were poisoned by carbon monoxide, or suffocated due to lack of oxygen.
German mechanics secretly made the modifications that re-directed the engine exhaust gas into the cargo compartment. Harry Wentritt, a German workshop foreman, testified after the war about special modifications made in his shop: “A flexible exhaust pipe was installed at the truck’s exhaust, with a diameter of 58 to 60 millimeters [2.26 to 2.34 inches], and a hole of the same size was drilled in the van floor; a metal pipe was soldered into the hole from the outside to which the flexible exhaust pipe was fixed. When the various parts were connected, the truck engine was started and the exhaust fumes were channeled into the van, through the pipe leading from the exhaust to the hole in the van floor.”
Truck similar to gas van. Original uploader in the Russian Wikipedia was Zac Allan, and then Jaro.p [Public domain], via Wikimedia Commons.
What did they look like?
Walter Burmeister, a gas van driver at the Chelmno death camp, recalled the appearance of the gas vans: “The gas vans were large vehicles, four to five meters long and about 2.2 meters wide, with a cargo compartment about two meters high [approximately 13-16 feet long, 8 feet wide, and 6.5 feet high] . . . The vans’ cargo compartments had double doors at the rear, like those on a moving wan. They were painted a Wehrmacht gray and looked perfectly harmless from the outside. The inside was lined with sheets of zinc, and the metal floor was covered with a wooden grid. Under this grid was a pipe, pierced with holes, which led out to the front. At the front, attached to the exhaust, was a mechanism that allowed the gas to be directed through a metal spiral hose to the pipe . . . Inside the compartment was an electric light. When it was turned on it was possible to see into the van from the driver’s cab.”
A Polish mechanic, Bronislaw Falborski, who worked in a garage in Kolo, Poland, was ordered to repair a gas van. On its appearance, he recalled: “Once I was ordered to repair a vehicle which served to poison with gases . . . I think that it was in the summer of 1942. The vehicle was roughly 2.50 m [8 feet] high. Its length was 6 m [20 feet], but its [width] probably 2.50 m [8 feet]. The vehicle’s color was black and had the shape of a box. The roof was flat and rectangular to its walls. I believe that it was lined with sheet metal . . .”
Falborski explained the design: “I was entrusted with the repair. It consisted of replacing a part between the elastic part of the exhaust pipe and the part which led into the vehicle’s interior. I clarify that the exhaust pipe did not consist of one piece as in normal vehicles, but of three parts, where the middle part was elastic like a hose. Said middle part could either be connected to a pipe located in the floor of the vehicle—with the result that the exhaust gases flowed into the vehicle’s interior—or to the rear part of the exhaust pipe; in that case the exhaust gases flowed into the open like with a normal vehicle. When the vehicle was turned in for repairs, the middle part of the pipe was connected with the interior of the vehicle, but the part between these two parts was worn, and I was ordered to replace it.”
Where were they used?
Gas vans were used at the Chelmno death camp in Poland and by the Einsatzgruppen in the East.
 Matthias Beer, “The Development of the Gas-van in the Murder of the Jews” at http://www.jewishvirtuallibrary.org/jsource/Holocaust/vans.html.
 Shmuel Spector, “Gas Vans,” Holocaust Encyclopedia (Macmillan, 1990), V2, p. 541.
 Eugen Kogon, Hermann Langbein, and Adalbert Rückerl, editors, Nazi Mass Murder: A Documentary History of the Use of Poison Gas (Yale University Press, 1993), pp. 77, 86.
 Santiago Alvarez and Pierre Marais, The Gas Vans: A Critical Investigation (Barnes Review, 2011), p. 351.
 Santiago Alvarez and Pierre Marais, The Gas Vans: A Critical Investigation (Barnes Review, 2011), pp. 351-352.