Saying that accidents recur does not assert an annual accident count
The phrase “recur every year” in this title does not invent an unverified annual accident count. In an August 2026 press release, Korea’s Ministry of Employment and Labor stated that hazardous-gas poisonings and secondary casualties during rescue attempts had recurred during sewer and manhole work commissioned by local governments and during work for public institutions. The same material lists measuring oxygen and hazardous gases before work, adequate ventilation, respiratory protection, calling 119 first in an emergency, and prohibiting direct rescue as basic rules. The essential cause of recurrence is not that hazardous substances are being discovered for the first time, but that established procedures are omitted from short, small jobs.
Opening a manhole cover to inspect a valve or repairing a pump inside a septic tank may take only a few minutes. This makes it easy for both the party assigning the work and the field worker to think, “I can just go down briefly and come back.” Yet a space with restricted entry, inadequate natural ventilation, and no design for continuous occupancy retains the characteristics of a confined space even when the job is brief. An open cover also does not mean that the internal atmosphere has been safely replaced.
Four atmospheric hazards interact in one space
In manholes and septic tanks, microorganisms can break down organic matter in sewage and sludge and generate several gases. Other gases may also enter through inlet pipes, pumps, chemicals, adjacent processes, or the soil. Concentrations change with temperature, water level, retention time, rainfall, agitation, and pumping conditions. Judging the atmosphere under the single label “sewage smell” therefore misses distinct hazards.
Hydrogen sulfide, H₂S, is an acutely toxic gas that can form where sewage and human waste are present under oxygen-deficient conditions. It irritates the eyes and respiratory tract and can cause rapid loss of consciousness at high concentrations. Although its rotten-egg odor is well known, NIOSH explains that the sense of smell fatigues rapidly and cannot be trusted to warn of its continuing presence. Because this relatively heavy gas may also collect at the bottom, smelling at the entrance or briefly placing a detector there is not a safety determination.
Methane, CH₄, can also form through anaerobic decomposition. Its principal hazards are flammability and explosibility. If its concentration in air enters the flammable range and encounters an ignition source, it can explode; by displacing oxygen, it also creates a simple-asphyxiant hazard. The absence of a toxic-gas alarm is no reason to omit the methane channel or measurement of the lower explosive limit, LEL, for flammable gases. Lighting, power tools, phone chargers, and sparks whose explosion protection has not been confirmed must also be controlled during permitting.
Carbon dioxide, CO₂, is colorless and odorless, making it difficult to detect with the senses. It forms during organic decomposition and fermentation, can displace oxygen, and at high concentrations itself burdens respiration and bodily functions. The Korean Occupational Safety and Health Agency’s domestic range for “adequate air” includes oxygen at 18% or more but below 23.5%, hydrogen sulfide below 10 ppm, carbon monoxide below 30 ppm, and carbon dioxide below 1.5%. These values show the minimum items that must be measured, but do not replace task-specific risk assessment or stricter applicable criteria.
Oxygen, O₂, must be treated as a separate channel from the other gases. Microbial activity and oxidation can consume oxygen, while gases such as methane and carbon dioxide also lower its concentration by displacing air. Normal oxygen does not mean hydrogen sulfide is safe, and low hydrogen sulfide does not eliminate methane explosion risk. OSHA and NIOSH advise testing the atmosphere in the order of oxygen, flammable gases and vapors, and toxic gases and vapors. The starting point is not to let one normal reading stand for the entire atmosphere.
The atmosphere when the cover opens differs from the atmosphere during work
A pre-entry measurement is a snapshot of one location at one time. Agitating septic-tank sludge, operating a pump, changes in sewage flow, or movement of a ventilation duct can release trapped gas. A worker descending may also obstruct airflow. In deep, interconnected spaces such as manholes, concentrations can differ between upper, middle, and lower levels and at the work location.
Begin measurement from outside the space with direct-reading equipment whose calibration status has been confirmed. With pumped equipment, allow for response delay caused by hose length, then test the upper, middle, and lower levels and the work point in sequence. An instrument displaying oxygen, LEL, H₂S, and CO does not necessarily measure CO₂. Confirm that the sensor configuration matches the expected hazards, and record the pre-use functional test, sensor life, battery, and pump status. Putting only the tester’s head through the opening may itself constitute entry and must be prohibited.
Once work begins, a system is needed to monitor concentrations continuously with personal or work-position detectors. OSHA’s construction standard requires continuous monitoring of conditions in the area where workers are located when entry is permitted into a continuous system, such as a sewer, that cannot be fully isolated. As a rule, atmospheric hazards in other confined spaces should also be monitored continuously; periodic monitoring at a suitable frequency is allowed only in limited cases where continuous-monitoring equipment is unavailable or periodic measurement has adequately demonstrated that safe levels can be maintained. At Korean worksites too, changes during work and the actions linked to alarms matter more than a single normal reading.
When an alarm sounds, do not go deeper to investigate the cause. Stop work immediately, exit by a route on which power has been safely isolated, and then reassess ventilation, inflow, and process conditions. Fan stoppage, a disconnected duct, sensor failure, and an over-range reading should be defined as equivalent stop-work conditions. Silencing the alarm or leaving the detector outside the space does not remove the atmospheric hazard.
“A quick job” leads people to bypass the work permit
A work permit is not merely a sheet of paper bearing signatures; it is evidence of pre-entry decisions. Identify the space as confined, first consider whether the work can be performed from outside, and permit only unavoidable entry. Before authorization, isolate and lock out sewage and sludge inflows, pumps and agitators, electricity, pressure, and chemical piping, and verify residual energy. A pump switch simply turned off must be distinguished from physical isolation.
The permit must record the following with actual values and names:
The exact space and scope of work, and permit start and end times
Entrants, the outside attendant, the entry supervisor, and each person’s role
Isolated equipment, piping, and energy, and the method used to verify isolation
Measurement equipment, functional-test and calibration status, and results by location and time for O₂, LEL, H₂S, CO₂, and other hazards
Locations of the ventilation fan and duct, and actions if power is lost
Alarm values and the conditions for stopping work, exiting, and reauthorizing entry
Communications, personnel accountability, the rescue method, and how to contact rescue services
Required respiratory protection, full-body harnesses, lifelines, tripods, winches, and explosion-protected equipment
Permit conditions must be reviewed again when shifts change, after breaks or rainfall, when the water level changes, ventilation stops, the work method changes, or another contractor begins simultaneous work. Do not reuse yesterday’s permit or readings from the next manhole. Korea’s Ministry of Employment and Labor describes the employer’s duties to provide measurement equipment and record oxygen and hazardous-gas concentrations as key provisions of amended rules that took effect in December 2025. Records should not be decoration used to assign blame after an accident, but a control showing that pre-entry conditions were actually verified.
Ventilation is an action that maintains airflow, not just dilution
The Korean Occupational Safety and Health Agency identifies pre-work measurement of oxygen and hazardous gases, running ventilation fans before and during work, wearing a supplied-air mask or self-contained breathing apparatus during rescue, and prohibiting unauthorized entry as core procedures. Merely placing a fan beside the manhole is insufficient. Confirm that the clean-air intake is separated from vehicle exhaust and sewer outlets, insert the duct close to the work point, and design a flow that actually removes contaminated air. Ventilation fans and electrical equipment must be suitable for the anticipated flammable atmosphere.
Do not stop measuring while ventilating. Blind spots can form depending on fan capacity, duct bends, manhole depth, and connected pipelines, and concentrations rise again if the generation rate exceeds the ventilation rate. If ventilation alone cannot maintain permit conditions, perform the task from outside the space or postpone it; for unavoidable entry, a qualified person must redesign it to include supplied-air respiratory protection and IDLH procedures. An air-purifying gas respirator does not supply oxygen, so it cannot resolve an oxygen-deficient atmosphere or one of unknown concentration.
An improvised rescue attempt to save a colleague creates the second accident
When a worker collapses, the nearest colleague instinctively wants to descend. But the atmosphere that incapacitated the first worker remains. NIOSH expressly says that people without training, appropriate tools, and protective equipment must not attempt a confined-space rescue. Korea’s Ministry of Employment and Labor also emphasizes calling 119 first and prohibiting direct rescue in manhole incidents. The outside attendant is not someone who leaves the station and follows an entrant inside; the attendant continually accounts for entrants, maintains communication, orders evacuation when something is wrong, and activates the prepared rescue system.
Where feasible and where it does not increase the hazard, non-entry rescue takes priority. Before entry, connect a full-body harness and lifeline to a suitable retrieval point and install lifting equipment such as a tripod and winch for vertical spaces. OSHA requires non-entry rescue unless it would increase the overall risk or would not contribute to rescue; when entry rescue is necessary, it requires prior selection of a rescue team trained and equipped for the space’s dimensions, configuration, and hazards. Merely writing “call 119 if there is an accident” is not enough. Communicate the location, access time, manhole configuration, required rescue equipment, and hazardous-substance information in advance, and confirm feasibility through drills.
Procedures break down at small businesses and across contracting chains
Manhole and septic-tank work may be divided among the facility owner, client, principal contractor, maintenance firm, and subcontracted workers. The party controlling the space may know prior gas readings, piping connections, possible inflows, and accident history, yet the entry contractor may receive only an instruction to “check the pump once.” Conversely, cleaning-agent vapor or agitation hazards created by the entry contractor may not be shared with the facility operator. At small firms with limited equipment and dedicated staff, detector rental, securing a rescue team, and training are especially likely to be deferred until after work begins. This does not mean that smaller scale brings less risk; it means that safety functions are more likely to depend on an individual’s experience.
OSHA’s construction confined-space standard anticipates such multi-employer sites. The facility owner and the controlling contractor must convey known confined-space hazards and previous entry information to the entry contractor, and coordinate operations when several contractors enter simultaneously or nearby work can create hazards inside. Hazards discovered after work must also be shared. At Korean sites, regardless of the legal titles and relationships among parties, the commissioning stage should state in the scope whether confined spaces are present and specify the costs and responsibilities for investigation, measurement, ventilation, monitoring, and rescue; the field manager should verify evidence of implementation. A lowest-price bid or same-day callout must not become a reason to omit the permit procedure.
Breaking the cycle requires four connected checks
First, do not assume every manhole and septic tank is safe; identify the possibility of entry and atmospheric hazards in advance. Second, measure expected hazards such as O₂, LEL, H₂S, and CO₂ at multiple locations and monitor them continuously during work. Third, connect isolation and ventilation, the outside attendant, communications, and exit criteria in the permit as actual field conditions. Fourth, prohibit improvised entry rescue and prepare non-entry retrieval and a trained rescue team before work.
One detector, one ventilation fan, and one signed permit do not form an accident-prevention system when each exists in isolation. Measurements must determine whether entry is permitted, continuous monitoring must verify ventilation and changes in the work, and an alarm must trigger immediate exit and the rescue plan. The test for breaking the cycle of asphyxiation accidents is not whether equipment is owned, but whether these connections actually work during short jobs and at small contractors’ sites.