The National Institute for Occupational Safety and Health (NIOSH) definition of a confined space is “a space which by design has limited openings for entry and exit; unfavorable natural ventilation which could contain or produce dangerous air contaminants, and which is not intended for continuous employee occupancy.”

NIOSH estimates that millions of workers may be exposed to hazards in confined spaces each year. The institute’s investigations of confined space injuries and fatalities indicate that workers usually do not recognize that they are working in a confined space, and they may encounter unforeseen hazards. Testing and monitoring of the atmosphere is not performed, and rescue procedures are seldom planned.

Recognition of potential confined space hazards is not always easy. Confined spaces such as manholes, sewers, boilers, silos, vessels, vats, pipelines, tunnels, storage tanks, ship compartments and underground vaults are often easily identified. However, often overlooked are potential confined space hazards such as open-topped water and degreaser tanks, open pits and enclosures with bottom access. The first step to reducing the risk associated with confined space is recognition of the hazard.

Atmospheric hazards
Confined spaces alone may not be hazardous, but they can become a hazardous in a very short span of time. When coupled with common atmospheric hazards, confined spaces can be life threatening.

Common atmospheric hazards found in the work environment include:
  • Oxygen deficiency: Exposure to atmospheres containing 12 percent or less oxygen can bring about unconsciousness without warning and so quickly that the individual cannot help or protect himself.

  • Hydrogen sulfide exposure: Although the foul odor (rotten eggs) accompanies H2S at low concentrations, it is an unreliable warning because the gas rapidly desensitizes the olfactory (sense of smell) nerves and leads to a false sense of security. In high concentrations of H2S, a worker may collapse with little or no warning.

  • Carbon monoxide exposure: Carbon monoxide is an odorless, colorless gas that may build up in a confined space. In high concentrations of carbon monoxide, a worker may collapse with little or no warning and thus be unable to aid himself.

  • Combustible gases within the LEL (lower explosive limit) and UEL (upper explosive limit): When fuel, oxygen and a source of ignition are present at the same time and in the right mixture, a serious explosion or fire is the result. If a combustible gas or vapor and air mixture is trapped in a confined space, only a source of ignition is necessary to create an explosion. Welding, sparking tools or even static electricity can easily satisfy this requirement.
Assessing the risks
Identifying atmospheric hazards is vital to maintaining safe work environments. Fixed and portable gas detection instruments are commonly used to identify the presence of atmospheric hazards. Instruments are typically used to detect the presence of toxic gases such as H2S and CO. For combustible gases such as methane or propane, instruments are typically used to detect concentrations of gas up to the LEL. A gas-air mixture becomes explosive at the LEL, but the monitoring range of most instruments (0-100 percent of the LEL) is below this limit to provide advanced warning. Alarm levels may be set at 10 percent of the LEL to forewarn workers of a potentially hazardous combustible atmosphere.

Atmospheric hazards can originate from a variety of sources within the work environment, so it is important to understand common sources in order to develop appropriate safety plans and emergency response plans. Common sources of atmospheric hazards include: leaking gases and liquids; decomposing organic matter; combustion (welding, cutting, brazing) or oxidation (rusting); cleaning processes; oxygen enrichment; oxygen absorption; and combustible dust concentrations.

To maintain the safest possible work environment, you should always preplan your work by using your company’s confined space entry permit as a guideline. In addition, below are additional items to be considered when working in confined spaces.
  • Atmospheric testing and monitoring: 1) Fully understand the operation and calibration of your fixed and portable gas detection instruments; 2) Zero your instrument in known fresh air prior to sampling for gases or vapors; 3) Sample at various levels of a confined space — hazardous gases can settle at different heights based on weight; and 4) While working, sample continuously — conditions can change quickly.

  • Procedures: 1) Develop an initial work plan; 2) Identify a standby person or attendant; 3) Maintain constant communications/observation; 4) Develop a rescue plan; 5) Follow all established work processes and procedures.

  • Preparation: 1) Isolate/lockout/tag; 2) Purge and ventilate; 3) Follow all established cleaning processes; 4) Follow all requirements for special equipment/ tools; 5) Follow all labeling and posting procedures as required; and 6) Identify appropriate respiratory protection, gas detection and personal protection equipment for the environment.

  • Rescue equipment: 1) Make available required rescue equipment per company policy and your rescue plan; 2) Inspect and maintain all rescue equipment per the requirements of your company safety plan
If questions arise, contact your site environment health and safety manager or consult your company’s safety manual.