Abstract: On September 12, 2000, Employee #1 entered the manhole of an inactive sewer to take a measurement when he started breathing heavily and came back out. A short time later he re-entered the manhole to complete the measurements, but again started to breathe heavily. Employee #1 lost consciousness before making it to the top and he fell back into the hole. Neither he, Employee #2, nor the crew chief recognized labored breathing as a symptom of insufficient oxygen. While the chief left to call 911, Employee #2 apparently entered the manhole to rescue his coworker. When rescue personnel arrived, they also found Employee #2 in the hole and unresponsive. Both employees died of "environmental suffocation" or asphyxia. The fire department tested the air in the manhole when they first arrived and found the oxygen level to be 12.7 percent. With the help of the health response team, investigators collected air samples to analyze for possible contaminants. The manhole where the employees died was found to have between 3.1 to 4.7 percent oxygen and 11,000 ppm carbon dioxide at depths between 12 and 18 ft. The pump shaft next to the manhole also showed low levels of oxygen and high levels of carbon dioxide. Other inactive sewer manholes in the area had air levels similar to ambient air: 20 to 21 percent oxygen and 300 to 400 ppm carbon dioxide. Members of the health response team hypothesized that the low oxygen levels were the result of the manhole's proximity to swamp and marsh lands. The decomposition of simple organic matter by aerobic bacteria depletes oxygen and generates carbon dioxide. There had been no confined space training, no testing of oxygen or carbon dioxide levels, no ventilation, and no available rescue equipment or even an emergency cell phone. Employees #1 and #2 and the crew chief believed that inactive sewer manholes were always safe from atmospheric hazards.