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Confined Space Basics

OSHA's Confined Space Standard

Do Not Enter sign

OSHA’s standard for confined spaces (29 CFR 1910.146) contains the requirements for practices and procedures to protect employees in the general industry from the hazards of entering permit spaces.

General industry employers must evaluate their workplaces to determine if spaces are permit spaces. If a workplace contains permit spaces, the employer must inform exposed employees of their existence, location and the hazards they pose. This can be done by posting danger signs such as "DANGER-PERMIT-REQUIRED CONFINED SPACE-AUTHORIZED ENTRANTS ONLY" or using an equally effective means.

If employees don’t need to enter and work in permit spaces, employers must take effective measures to prevent them from entering these spaces. An employer is required to block or securely seal off the confined space so employees cannot enter the area. If employees are expected to enter permit spaces, the employer must develop a written permit space program and make it available to employees or their representatives.

What is a confined space?

OSHA’s definition of a confined space is a space that:

Sample Flow Chart
(Click to enlarge)
  • is large enough and configured so an employee can enter and perform assigned work;
  • has limited or restricted means for entry or exit;
  • and, is not designed for continuous employee occupancy.

Permit-Required Confined Spaces (PRCS)

Some confined spaces require a permit to enter. By definition, a permit-required confined space is a space that meets the criteria for a confined space and has one or more of the following characteristics:

  1. It contains or has the potential to contain a hazardous atmosphere;
  2. It contains a material with the potential to engulf someone who enters the space;
  3. It has an internal configuration that might cause an entrant to be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross section; or
  4. It contains any other recognized serious safety or health hazards.

OSHA requires employers to determine if a permit is required when entering a confined space. To do that, they can use OSHA's Confined Space Entry Decision Flow Chart to the right

Here are some examples of spaces that usually meet the criteria for confined spaces:

  • Tanks
  • Manholes
  • Boilers
  • Furnaces
  • Sewers
  • Silos
  • Hoppers
  • Vaults
  • Pipes
  • Trenches
  • Tunnels
  • Ducts
  • Bins
  • Pits

Why Confined Spaces are Hazardous


Confined spaces are deceiving. A confined space often appears to be harmless; no danger signs are apparent and the space may have been entered on prior occasions without incident. However, a worker cannot assume conditions have not changed and the space is safe for entry each time. Some materials may pose an immediate threat to the life and health of the worker entering the space. However, the dangerous materials, such as hydrogen fluoride gas, may cause a sudden or fatal employee collapse 12-72 hours after exposure. The victim "feels normal" after recovery from transient effects until collapse. Such materials in hazardous quantities are considered to be "immediately" dangerous to life or health.

Below is a list of other hazards that could be encountered while working in a confined space:

  • Space configurations such as small openings and inwardly converging walls, which can trap an entrant, restrict easy entry and exit, or impede rescue.
  • Atmospheric hazards such as gasoline tank vapors, combined with limited ventilation. Such conditions can cause asphyxiation or explosion. Oxygen levels may be hazardous and cause asphyxiation (less than 19.5%) or fire (over 23.5%).
  • Physical hazards, such as unstable grain contained in silos, which can engulf a worker.
  • All other serious hazards associated with general industry, such as electrical equipment, moving machinery, falling objects, and wet or slippery surfaces.

Below is a list of potential hazards to be aware of when working in a confined space:

  • No ventilation (pits and vaults seldom opened).
  • Leaking chlorine gas (which is heavier than air) can accumulate in low-lying spaces.
  • Oxygen depletion; which can be caused by:
    • Rotting vegetation and decaying dead animals.
    • Corroding or rusting machinery.

Real-Life Accident

A worker suffocated inside of a chemical tank at the Port of New Orleans. The Occupational Safety and Health Administration (OSHA) said the company sent the worker and two others into a tank without first testing the air quality of providing them lifelines. As a result, the worker was killed and the others were hospitalized.


OSHA proposed $226,000 in fines against the company in connection with nine safety violations. Two of the nine proposed violations were classified as "willful," which is the most serious category. This category is reserved for situations where an employer knows of an unsafe condition or practice, but does nothing about it.

Before the worker's death, OSHA cited the company for almost 30 violations. Many of those violations were related to failure to take proper precautions when sending workers into confined spaces filled with dangerous chemicals.

Non-Permit Required Confined Space

This is a confined space that does NOT contain or have the potential to contain any hazard capable of causing death or serious physical harm.

Here are some examples of a non-permit required confined space:

  • Drop ceiling
  • Motor control cabinets

Location of Confined Spaces

Confined spaces are found not only in industrial settings but also in public places such as shopping malls and large public swimming pools. Waterfalls and water fountain displays used in malls for beautification may have pump vaults or valve pits that are seldom entered. Some swimming pool pumps are placed in vaults below ground. There have been reports of maintenance employees entering these areas and losing consciousness.

What is Confined Space "Entry"?

Entry occurs when any part of the body crosses the plane of the opening.

A confined space entry is considered to have occurred when any part of a person's body crosses the plane of an opening into the space. Each employer should ask these two questions at the onset of each project:

  1. Is confined space entry always necessary for this task?
  2. Is it possible to complete the task from the outside?

If possible, avoid entering a confined space. Every consideration should be given to completing the task from the outside.

Using Alternative Entry Procedures

Under certain conditions, you may use alternate procedures for worker entry into a permit space. For example, if you can demonstrate with monitoring and inspection data that the only hazard is an actual or potential hazardous atmosphere that can be made safe for entry using continuous forced air ventilation, you may be exempted from some entry requirements, such as permits and attendants.

However, even in these circumstances, you must test the internal atmosphere of the space for oxygen content, flammable gases and vapors, and the potential for toxic air contaminants before any employee enters it. You must also provide continuous ventilation and verify that the required measurements are performed before entry.


In this short video, Frank Quarato, from the Center For Safety And Environmental Management, gives a good presentation about the differences between a confined space and permit-required confined space (PRCS) .


Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. What must an employer do if workers need to enter permit confined spaces?

2. Which of the following does NOT meet one of the three criteria for a confined space?

3. Dangerous materials in a confined space may cause a sudden or fatal collapse _____ after exposure.

4. When does confined space entry occur?

5. In which of the following situations may alternative procedures be considered to enter a confined space?

Have a great day!

Important! You will receive an "error" message unless all questions are answered.