The preliminary reasons given indicate the importance of exhaust systems in chemical plants or in areas where toxic gases can accumulate (like in confined spaces, mines, etc).
The way it looks, risk assessment, and safety measures for protection appear to be inadequate. Power failure for about 90-120 minutes caused the work area to be filled with toxic gases. This indicates that workers were working in such hazardous environments with dependence on the functioning of exhaust systems. This plant area seems to be the most dangerous area with continuous generation and exhaust of toxic gases to maintain a breathable atmosphere. If this is the case, entry to such areas should be with breathing sets and for a short duration. These areas are not fit for long-duration works. There should have been online gas detectors with audio-visual alarms with emergency power backup.