 As part of the Holocaust during World War II , German Nazis used gas vans at Chelmno camp and elsewhere to kill an estimated over 700 @,@ 000 prisoners by carbon monoxide poisoning . This method was also used in the gas chambers of several death camps such as Treblinka , Sobibor and Belzec . Gassing with carbon monoxide started in action T4 , the euthanasia programme developed by the Nazis in Germany to murder the mentally ill and disabled people before the war started in earnest . Many key personnel were recruited to murder much larger numbers of people in the gas vans and the special gas chambers used in the death camps such as Treblinka . Exhaust fumes from tank engines for example , were used to supply the gas to the chambers . 
