Not included in the official fatalities number was deaths that was recorded as pneumonia . Workers alleged that this diagnosis was a cover for death from carbon monoxide poisoning , brought on by the use of gasoline @-@ fueled vehicles in the diversion tunnels , and a classification used by Six Companies to avoid paying compensation claims . The site 's diversion tunnels frequently reached 140 ° F ( 60 ° C ) , enveloped in thick plumes of vehicle exhaust gases . A total of 42 workers was recorded as having died from pneumonia ; none was listed as having died from carbon monoxide poisoning . No deaths of non @-@ workers from pneumonia was recorded in Boulder City during the construction period .