Abstract: On August 30, 2002, a five-man crew was installing a new branch sewer pipe, and was connecting it to the existing main sewer. The area where they worked included a manhole and excavation. The cover of the manhole was lifted, and the interior air was checked between 7:30 a.m. and 7:45 a.m. The foreman stated that the air was safe for entry. The crew did not install a blower to ventilate the manhole, nor did they install any rescue or recovery equipment above the manhole, or isolate it from the main. The crew then worked on the excavation that was adjacent to the manhole. The excavation was dug approximately 23 feet long, 11 feet wide and 11 feet deep. Water was present on the bottom of the excavation and the manhole. Employee #1 entered the manhole around 9:15 a.m. to remove the water with a bucket. Employee #1 collapsed due to an oxygen-deficient atmosphere. Employee #2 alerted the crew, and entered the manhole. Employee #3 entered the manhole to lend assistance to Employee #2 and help lift the collapsed employee from the manhole. Employee #2 and Employee #3 then collapsed. Emergency service personnel later retrieved all three employees. Apparently, hydrogen sulfide had entered the manhole from the existing sewer system. Another crew was working in a manhole approximately 0.5 mile away on a branch line that was connected to the main sewer line. They worked with a blower forcing air into its space. It was speculated that this forced ventilation might have pushed hydrogen sulfide in the system into the manhole space where the three collapsed employees were located. About three hours after the incident, the oxygen level in the manhole was measured to be approximately 16 percent by volume. Employees #1, #2, and #3 were hospitalized suffering from asphyxia.