Hidden Hazards of Confined Space

TASA ID: 3404

Frequent headlines in breaking news stories convey the tragic results of yet another confined space fatality accident. With fatality statistics indicating that more than 60 percent of rescues in confined spaces result in a workplace fatality.  Since most of these cases are fully preventable, sadly they involve local firefighters responding to trapped workers injured on the job.

A common misunderstanding many people share is that local firefighters respond to emergencies fully equipped as technical rescuers.  Having experience in structural fire control or vehicle emergency response doesn't necessarily qualify one as a technical rescuer.  Comprehensive training, practical experience, and a working knowledge can adequately prepare responders to engage as rescue professionals, not before.

During a 2010 confined space incident in Middletown Ohio, three firefighters responded to a call to a city utility worker who fell into a manhole and perished.  Upon arrival at the scene, the firefighters (captain, fire marshal, and a firefighter) gazed into the deep manhole leading to a sanitary sewer. There they found the worker lying motionless at the bottom of the vertical access way.  After conducting a rescue size-up, they assumed the worker had only slipped and fell into the 20-30 foot deep manhole, as they tried to arouse the downed worker, who failed to respond, they decided to initiate rescue operations.  

Based on the type of entryway, they planned a rope rescue response, intent on securing the victim to a lowered stretcher, they would remove the victim to the surface level for care.  Hurriedly, they descended as a team into the dark space neglecting to perform the standard pre-entry air test, or even to use a self-contained breathing apparatus (SCBA).  During a fire department post incident investigation, it was discovered that the utility worker had removed the manhole cover, and immediately fainted falling headfirst into the space and died. The worker had not tested the air before removing the cover, and was overcome by a powerful gust of low air, consisting of oxygen depleted atmosphere, later reported to be only 2 percent of the normal 21 percent.

The three firemen, all 20 year department veterans, made an unprotected entry into the space completely unaware of the perilous and dangerous atmosphere awaiting them, as they descended into the untested manhole. While lowering themselves down, they were impacted by the low-oxygen levels, and passed out due to hypoxia; having to be rescued by other firefighters. The firefighters were taken to the hospital by ambulance, with one medevaced via helicopter to a critical care unit.  

Failure to follow established confined space air testing protocols resulted in one fatality and multiple firefighter injuries.  Federal OSHA and fire department protocol requires entrants and rescuers to conduct air testing before entering a confined space, even in emergency situations.  Due to potentials for toxic gas and low air, manhole entry into septic sewers or lift stations is inherently dangerous, as decomposition of bio-waste materials has the potential for oxygen displacement and accumulation of toxic gases.  John Rekus, an industrial hygienist, explains how the process works, "Fermentation is an enzymatically controlled chemical reaction in which microorganisms digest organic matter, producing nitrogen that displaces ambient oxygen."
Historically, hypoxia and asphyxiation account for the majority of documented fatalities in confined spaces.  According to a study by the National Institute for Occupational Safety and Health (NIOSH), of confined space fatality cases from 1984 to 1986, "Out of 188 deaths, 146 were from oxygen deficiency."  Testing air at the exterior of a confined space entry portal is an important first step to control injury and fatality.

Although a culture change for some organizations and teams, the importance of pre-entry air testing cannot be over emphasized. When considering the common misconception that confined space atmospheric hazards only exist inside the space, it's no wonder workers fail to test air at the approach to the entry portal. 

Testing atmospheric conditions in a concentric circular pattern allows us to detect presence of toxic or flammable vapors off-gassing from the confined space.   See Figure 1.

Perimeter Air Testing of Confined Spaces: (Vapor Plume)

Should toxic or flammable vapors and/or low air conditions emanate from a confined space, it will be most advantageous to determine their presence in advance of removing the cover, or opening the hatch or manway.
Note: Wind direction and speed will affect the detection of vapor plume and accuracy of test results.

Safe Perimeter Air Testing:

Since toxic or flammable gases may be present at the exterior of a confined space, initial air testing is advised for areas adjacent to a confined space entry portal.  Technicians testing air must concentrate on the identification of pre-entry atmospheric conditions being alert to conditions that could endanger the entry team.

Note: Always determine wind direction and insure that sampling instruments (gas monitor) are in a fully-charged state, recently calibrated; instruments must not be activated in contaminated atmospheres, always seek a fresh air source.

Upon determining the location of the confined space entry portal, always consider all potential sources of toxic gas generation (off-gas), use a direct read instrument to test air downwind from the designated source based on wind direction. (See figure 1.) Continue to sample in the direction of the source or entry portal. 

Note: Should high levels be detected, immediately evacuate the area and don personal protective equipment (PPE) before continuing air testing.  Never enter a hazardous environment alone, use the buddy system and always follow OSHA regulations and onsite safety rules.

When approaching the entry portal, sample the air in a circular pattern probing for toxic or flammable vapors and/or low air conditions.  Should the presence of toxic or flammable gas vapors be suspected, air testing should be performed before removing a manhole cover or hatch.

Should unacceptable levels of flammable/explosive/toxic vapors be present (methane gas or hydrogen sulfide), it may be necessary to reduce ambient levels to a safe level, prior to allowing the entry team to assemble.  The same holds for low air (oxygen deficiency) conditions, as illustrated in the case of the city utility worker.  Many persons are unaware that low air (oxygen-deficiency) gust can emit from an entry portal, directly impacting workers or entrants. There is a common misbelief that atmospheric hazards only exist inside a confined space.

Note: Adequate safety training must be provided to all confined space entrants, including advanced instruction (Confined Space Attendant/Supervisor), and specialized rescue training for emergency responders.

Remote Air Sampling:

When exterior air sampling is complete, the next step is to test air levels inside the confined space. As a precautionary measure, OSHA requires that acceptable entry conditions exist before workers are allowed entry.  Warning: DO NOT enter a confined space to perform air testing.

Testing should proceed from top to bottom, sampled from the top, midway, three quarters, to the bottom.  Since each instrument has a different sample pump velocity, speed or strength, the length of the probe tubing must be considered.  Sampling must proceed in a patient, methodical and deliberate manner to locate toxic gas, including pockets of hidden gas within a confined space.  The longer the sampling tube, the more time it will take to draw air from the space to cross each sensor in the instrument. Air sampling conducted in a reckless or hasty manner may produce spurious results endangering the health and safety of the entry team, resulting in serious injury or fatality.  Focusing on the digital readout, the gas testing technician should patiently read each readout and accurately log data to the confined space permit or sampling log.

Instrument Selection / Recommendation:

Most gas testing technicians use instruments that contain internal motorized sample drawing pumps; other models also have detachable pumps.  It is not uncommon to see workers lowering personal sampling instruments into a confined space. These types of units do not have onboard sample drawing pumps. Although designed to be worn by confined space entrants, the principle problem with using a personal sampling instrument is the technician is unable to read or see the digital readout without entering the confined space. 

Non-Absorbing Probe Tubing:

Remote air sampling instruments use fixed probes or flexible tubing connected directly to the monitor (with a sample drawing pump), to safely facilitate the task. The most frequently recommended extension tubing for gas detection is: Tygon®, Teflon®, or metal tubing which are non-absorbing.

There is good reason to ensure that non-absorbing tubing is used, because cheap, clear, vinyl tubing used as an alternative can have negative results; when the instrument registers a false-negative, as vinyl tubing can absorb many solvent fumes.  Closing words of caution, "Never substitute the manufacturer's recommended sample tubing for a lesser quality alternative. Although you may save a few dollars, it may be your ultimate confined space."

Five-Deadly Sins of Confined Space Entry:

According to Colleen Eubanks, a retired OSHA compliance officer, common errors found on confined space projects include:

1. Failure to monitor the confined space prior to, and during, entry

2. Failure to remove hazards from a confined space

3. Failure to account for hazards brought into a confined space

4. Failure to have trained rescue personnel onsite

5. Failure to use a mechanical means of rescue

In conclusion, educating workers on the hidden hazards of confined space, and insuring proper air testing protocols are used will help control or reduce serious injury and illness, ultimately lowering the fatal consequences of entering and working in confined space. 


Baker J., Morse J., May 7, 2010 (, "City worker killed, three firefighters hospitalized in Middletown,"

Rekus, J, 1994, Complete Confined Spaces Handbook, pgs. 28, 29.

National Institute for Occupational Safety and Health (NIOSH), January 1994, "Worker Deaths in Confined Spaces"
online,  Summary of NIOSH Surveillance & Investigative Findings.

SAFETRAN Confined Space Safety Training, (Level 1), Air Testing Protocol, Training for Industry, February ‎17, ‎2006

O'Connell, D, April 2, 2007, Confined Space Case Files, Industrial Safety and Hygiene News (ISHN), online 

Eubanks, C. 2001, "Five Deadly Sins of Confined Space Entry", Industrial Safety Hygiene News (ISHN), 5/01


This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

This article may not be duplicated, altered, distributed, saved, incorporated into another document or website, or otherwise modified without the permission of TASA.

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