📌 3 Key Takeaways
- Manholes and sewer lines are classic vertical confined spaces where decaying sewage and sludge continuously generate hydrogen sulfide, methane, oxygen deficiency and carbon dioxide — and unlike open ground, these gases cannot escape, so concentrations can turn dangerous in an instant.
- Toxic-gas asphyxiation accidents during manhole work kept recurring in 2026 in Jinan and Incheon, and 7 workers died from manhole asphyxiation over the past year, prompting Korea's Ministry of Employment and Labor and local governments to tighten management.
- The EDW500 portable 5-gas detector measures the five gases (O₂·CO·H₂S·EX·CO₂) through the gap the moment a manhole cover is opened, and with ATEX·IECEx international explosion-proof certification it can be used safely even inside pipes where flammable gas has accumulated.
Just because a manhole looks odorless and clear when you open the cover does not mean it is safe. Inside a sewer line, organic matter in wastewater and sludge decays anaerobically, continuously building up hydrogen sulfide and methane, while microbial activity and pipe corrosion consume oxygen, commonly causing oxygen deficiency. Because manholes are narrow, deep vertical structures, gas easily settles at the bottom, and opening the cover can suddenly release a burst of hazardous gas due to the pressure difference. This guide covers why manhole and sewer work is dangerous, which gases must be measured, recent accident cases, and how to work safely with the WANDI EDW500 portable 5-gas detector.
Why Manhole & Sewer Work Is Dangerous — Gases and Causes
Manholes and sewer lines are vertical confined spaces with almost no airflow to the surface. As organic matter in wastewater decomposes without oxygen, hydrogen sulfide and methane form continuously, and gas generated upstream can travel through the pipe network and accumulate at high concentration in a particular manhole.
| Gas | Cause in manholes and sewer lines |
|---|---|
| H₂S (hydrogen sulfide) | Anaerobic decay of organic matter in wastewater and sludge (rotten-egg smell); inflow and buildup from upstream in the pipe network |
| O₂ deficiency | Sealed narrow space; oxygen consumed by microbial activity and corrosion of metal piping |
| CH₄ (methane, flammable) | Anaerobic decomposition of organics — explosion risk plus oxygen-displacing asphyxiation |
| CO₂ (carbon dioxide) | End product of organic decomposition, accumulates at the low point of the manhole |
| CO (carbon monoxide) | Exhaust from nearby construction generators or pumps flowing back into the pipe |
Because sewer lines connect many manholes together, gas generated at one point can travel through the pipe to another manhole. That means a manhole that was safe yesterday can be dangerous today — the rule is to measure fresh every single time.
Health Effects by Concentration — Cross These and It's Dangerous
Below are health effects by concentration per the Confined Space Work Manual (2014) and Korea's occupational exposure limits (MOEL notice).
| Gas | Reference | Effects by concentration |
|---|---|---|
| O₂ | Safe 18–23.5% | Below 18% deficiency · 16% headache/nausea · 10% fainting · 6% death within minutes |
| H₂S | TWA 10ppm | 10ppm irritation · 100–200ppm smell paralysis · 700ppm↑ loss of consciousness and death within seconds |
| CH₄ | LEL 5% | 5–15% explosive range · oxygen-displacing asphyxiation at high levels |
| CO₂ | TWA 5,000ppm | 3% headache/faster breathing · 10%↑ loss of consciousness |
| CO | TWA 30ppm | 200ppm headache · 800ppm↑ loss of consciousness/death |
※ Above 100ppm, hydrogen sulfide paralyzes the sense of smell — workers can wrongly assume "no smell means it's fine." Never judge by nose; always confirm with a detector. Sources: "Confined Space Work Manual" (2014); MOEL exposure-limit notice.
Real Accidents — Recurring Asphyxiation in Manholes and Sewer Lines
- June 2026, Jinan, Jeollabuk-do — four workers were poisoned by toxic gas during manhole work at a sewage-maintenance site, sustaining moderate to severe injuries (source: Kyunghyang Shinmun, 2026).
- June 2026, Cheongna-dong, Incheon — two workers in their 60s doing sewer-pipe maintenance in a 9-meter-deep manhole lost consciousness from toxic gas, with one in critical condition (source: Hankook Ilbo, 2026).
- Gyeyang-gu, Incheon — an asphyxiation accident occurred inside a manhole during sewage-pipe surveying work (source: Safety1stNews).
- Statistics — over the past year, manhole asphyxiation accidents killed 7 workers, prompting Korea's Ministry of Employment and Labor to strengthen confined-space asphyxiation management (source: Newspim, 2026). In response, Incheon City and the Ministry jointly declared a campaign to "eradicate manhole asphyxiation accidents" in August 2026 and piloted a pre-entry safety verification system that makes gas checks mandatory before manhole work (source: Ajunews, 2026).
What these accidents share: workers entered without measuring, assuming "I went in before so it's fine," and collapsed instantly. Secondary accidents also keep recurring, where a coworker who rushed in unprotected to rescue a collapsed colleague was injured or killed as well. Only a detector that checks gas in real time before entry and during work can prevent this.
EDW500 Portable 5-Gas Detector — Core of Manhole Work Safety
The WANDI EDW500 is a portable multi-gas detector that a worker uses through the gap before the manhole cover is fully opened, and carries for continuous monitoring after entry. It measures the five confined-space gases (O₂·CO·H₂S·EX·CO₂) from one unit.
- Simultaneous 5-gas measurement — oxygen, carbon monoxide, hydrogen sulfide, combustible (EX) and carbon dioxide in one unit, meeting Korea's confined-space measurement requirements
- ATEX·IECEx international explosion-proof certification — safe to use even inside sewer lines where flammable gas such as methane has accumulated
- Pump-linked pre-entry sampling — connect a dedicated pump (e.g. EP500) to draw and measure gas remotely through a hose inserted in the cover gap, without anyone entering the manhole
- Free IoT wireless (DCMS cloud) — a standby monitor on the surface checks the worker's readings remotely in real time, with zero data or subscription fees
- Light, sound and vibration alarms — immediate alerts at dangerous concentrations to prompt evacuation
Safe Manhole & Sewer Work Procedure (Measurement-Centered)
- Measure before fully opening the cover — open the manhole cover slightly and insert the EDW500 (or a connected pump) through the gap to check gas concentration first
- Ventilate — if unacceptable, ventilate thoroughly with a blower, then measure again
- Confirm acceptable air — enter only after confirming oxygen 18%+, H₂S ≤10ppm, combustible <10% LEL
- Continuous monitoring during work — carry the EDW500 to monitor throughout, and always station a standby monitor on the surface
- Rescue principle — never enter unprotected even if a coworker collapses; rescue only with a supplied-air respirator and rescue equipment, and call 119 immediately
Korea's MOEL also runs a One-Call service (1644-8595) that provides free pre-work gas measurement, safety training and equipment rental for confined-space work — apply by phone up to three days before work.
Closing
The danger inside a manhole or sewer line is invisible until the cover is opened. A manhole that was safe yesterday can become instantly lethal today from hydrogen sulfide or oxygen deficiency. The principle of measure before opening → ventilate → re-measure → monitor continuously, and the EDW500 portable 5-gas detector that carries it out, protect the lives of manhole and sewer workers. Contact WANDI (+82-31-340-6952) for EDW500 inquiries and consultation — we will propose the right configuration and quote for your site.