The manhole was filled with hydrogen sulfide and methane gas created from years of rotted vegetation. The manhole was 15 ft deep, and its diameter was just enough to fit a body. The workers, who were not of the utility department (they were outsourced workers), probably, had no training and, they went inside without safety measures.
According to US Occupational Safety and Health Administration (OSHA), While moderate concentrations of hydrogen sulfide can cause more severe eye and respiratory irritation (including coughing, difficulty breathing, accumulation of fluid in the lungs), headache, dizziness, nausea, vomiting, staggering and excitability, high concentrations can cause shock, convulsions, inability to breathe, extremely rapid unconsciousness, coma and death. Effects can occur within a few breaths, and possibly a single breath.
While these accidents are common in poor developing countries, it is a bit surprising that the incident occurred in a developed country.
It is essential that the sanitary workers are taught about the safety measures, before they descend in manholes.