Case Study: Death in Sewer Manhole

Case Study: Death in Sewer Manhole

The following article is taken from the Occupational Safety and Health Administration’s Fatal Facts newsletter. It proves that unauthorized entry into sewer manholes is always extremely dangerous and sometimes fatal.

A municipal construction foreman died from asphyxiation after entering a manhole with an uncontrolled hazardous atmosphere. Four workers were working in an inactive sewer system on a jobsite that was unoccupied for over a week. A few minutes after they started working, the crew noticed that the foreman was missing and a manhole cover was removed. While one worker called emergency services, a second worker entered the manhole to assist the foreman and found him unresponsive at the bottom of the deep manhole. When the second worker became disoriented inside the manhole, another worker used a fan to blow fresh air into the manhole and the worker was able to climb out. The foreman was retrieved by fire department personnel and was later pronounced dead due to asphyxiation.

Likely Causes of Incident

Although the manhole was newly constructed and not yet connected to an active sewer system at the time of this incident, it contained a hazardous atmosphere that resulted in asphyxiation. The employer did not ensure that atmospheric hazards were identified and precautions for safe operations implemented before starting work at the site.

  • Workers were not trained to recognize confined space hazards and to take appropriate protective measures.
  • The atmosphere in the manhole was not assessed to determine if conditions were acceptable before or during entry.
  • Proper ventilation was not used to control atmospheric hazards in the manhole.
  • Protective and emergency equipment was not provided at the worksite.
  • An attendant was not stationed outside the manhole to monitor the situation and call for emergency services.

Asphyxia occurred when the foreman did not get enough oxygen to sustain life, either because the amount of oxygen in the atmosphere was too low (i.e., oxygen deficiency) and/or a hazardous chemical concentration was too high (e.g., high carbon dioxide level). Although the exact atmospheric conditions in the manhole at the time of the incident are unknown, measurements taken after the incident indicated it was possible the oxygen levels were low enough and/or carbon dioxide levels were high enough to cause asphyxiation. When rescuers arrived the atmosphere contained 19.1% oxygen, however this measurement was taken after construction workers had used a fan to provide fresh air to the disoriented worker. One day after the incident, the atmosphere contained 16.5% oxygen and four days after the incident the oxygen concentration dropped to 7.7%. Hazardous atmospheres in OSHA’s Confined Spaces in Construction standard include atmospheres with less than 19.5% oxygen (29 CFR 1926.1202). Normal atmospheric oxygen level is between 20.8% and 21%. Four days after the incident, the carbon dioxide concentration was 6.5%. An atmosphere with 4% or more carbon dioxide is considered Immediately Dangerous to Life or Health according to the National Institute for Occupational Safety and Health. Changes in atmospheric conditions over time might be due to several causes, for example, organic matter (e.g., dead plants, animals or animal waste products) or decay in soil that uses up oxygen and produces hazardous gases (e.g., carbon dioxide, hydrogen sulfide). Although conditions within these spaces could seem acceptable during initial entry, atmospheric changes could occur over time and result in fatalities. 


Municipal employers must ensure each confined space in which workers may be assigned duties and each space that is a permit-required confined space (PRCS) is identified before starting work at a construction site (29 CFR 1926.1203(a))1.

A PRCS is a confined space with one or more of the following characteristics (29 CFR 1926.1202):

  • contains or has a potential to contain a hazardous atmosphere,
  • contains a material with potential for engulfment,
  • has an internal layout that could trap or asphyxiate an entrant, or
  • contains any other serious physical or health hazard.

Employers with workers who will enter one or more PRCS must implement a PRCS program for safe permit space entry operations (29 CFR 1926.1203(d), 29 CFR 1926.1204) including the following requirements:

  1. Provide training to workers at no cost to them in a language and vocabulary they understand, as required in 29 CFR 1926.1207, on how to safely perform permit space duties before their first assignment and as necessary.
  2. Prohibit entry into permit spaces until hazardous conditions (atmospheric and physical) present are identified, evaluated, and addressed (29 CFR 1926.1204(b)&(c)).
  3. Eliminate or control atmospheric hazards by ventilating, purging, inerting or flushing the permit space as necessary (29 CFR 1926.1204(c)(4)).
  4. Perform pre-entry testing for oxygen content, flammable gases and vapors, and potential toxic air contaminants (29 CFR 1926.1204(e)(3).
  5. Continuously monitor the permit space to verify that atmospheric conditions remain acceptable during entry (29 CFR 1926.1204(e)(1)(ii)).
  6. Provide essential equipment to workers with training on proper use, including:
    1. Personal protective equipment when necessary (29 CFR 1926.1204(d)(4)).
    2. Rescue and emergency equipment to authorized workers, or implement procedures for rescue and emergency services (29 CFR 1926.1204(d)(8)&(i), 29 CFR 1926.1211).
    3. Station at least one trained attendant outside a permit space to perform all attendant’s duties (29 CFR 1926.1204(f); 29 CFR 1926.1209). 
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