Confined space hazards and precautions

The confined space hazards and precautions blog covers the minimum requirements for Confined Space Entry to protect personnel from hazards such as oxygen deficiency, toxic materials, flammable substances and energy, or moving parts of power-driven equipment when working in confined spaces, as well as rescue requirements using Confined Space Entry Work Permit.

The protocol for confined space entry was established by OSHA (the United States Occupational Safety and Health Administration) 29CFR (United States Code of Federal Regulations) 1910.146 in April 1993. This protocol was developed to provide a work plan defined for confined space entry. Occupational health and safety professionals know that access to confined spaces is part of the daily routine in industrial plants.

A confined Space is any enclosed space that has a limited opening for access and egress and presents serious hazards to entrants. And the space is not intended for continuous human occupancy. The space is large enough and configured to allow entry to the performance of an assigned task. Confined spaces include, but are not limited to, storage tanks, process vessels, pipelines, sewer lines, ducts, manholes, wells, and excavations deeper than 1.2 meters.

To keep Confined space hazards and precautions up camera-controlled processes are used today. This process includes several aspects, such as camera control inside of confined space entrants, direct communication through the intercom system, and continuous monitoring of confined space conditions. In a centralized control room, one or more safety people may monitor multiple confined spaces.

The risk assessment is mandated to capture all relevant aspects (e.g. continuous entry control, number of entry/manhole attendants and their availability in a specific area, availability and deployment of a rescue team, and alarm method.

Confined space hazards and precautions
Confined space hazards and precautions

Before entering the confined space, a hazard assessment must have been completed. It is also essential to have the proper training to enable you to understand and identify the risks associated with this specific type of work, and then effectively mitigate these risks and hazards. Confined space hazards and precautions

Finally, before any work begins, the air quality should be tested to ensure conditions are suitable for entry. Let’s explore some of the steps to take before entering any confined space on your job site. Confined space hazards and precautions

Confine space main hazards

  1. Oxygen deficiency. 
  2. Presence of flammable gas. 
  3. Presence of toxic gas. 
  4. Improper arrangements of lighting and ventilation.
  5. Engulfment (OSHA). 

Confined space hazards and precautions – Gas Test

The gas tester should not enter the confined space for testing purposes unless the cross-section results (top, middle and bottom sections) indicate that the oxygen level is between 19.5% and 21%. Entry into a confined space is not permitted if the oxygen level is less than 19.5% or greater than 21%. Oxygen content above 23% can cause explosions or severe burns of flammable materials, including hair and/or clothing.

If entry is required for testing purposes, the gas tester must wear a self-contained breathing apparatus (SCBA) or supplied-air respirator with a man on standby outside the confined space.

Test the cross section (top, middle, and bottom sections) of the confined space’s internal atmosphere with a calibrated direct-reading instrument, before an employee enters the confined space, for the following conditions in the order given:

Confined space hazards and precautions

  • Stop venting/purging for at least 15 minutes, or based on risk assessment, before gas testing, to allow gas concentration to equalize.
  • Perform an oxygen test before entering a confined space.
  • Determine the nature of other gases when the percentage of oxygen is less than 19.5%. If it is an inert gas, the oxygen content will be raised by purging with air.
  • Perform gas testing in large spaces such as tanks, drums, towers, or excavations where gas may be trapped in nozzle dead spots, clogged corners, structural members, etc.
  • Perform gas tests at various levels in large containers, as some gases are heavier than air and tend to settle to the bottom and some, which are lighter, will rise to the top.

How to perform the gas test

Make sure toxic fumes are below OSHA’s Permissible Exposure Limit. The most commonly found toxic gases in a confined space might be hydrogen sulfide (H2S) and carbon monoxide (CO), but other toxic compounds can be found as well.

In a confined space it is important to sample the top, middle and bottom to locate the varying concentrations of gases and vapours. Some gases are lighter than air (for example, methane and other combustible gases) so they can be found at the top of a confined space. Others are heavier than air (for example, hydrogen sulfide) so they can settle near the bottom of a confined space. And other gases have the same weight as air (for example, carbon monoxide) and can be found throughout the confined space.

Take air samples from different levels within the confined space and continuously monitor the space as conditions can change rapidly. Perform gas tests considering the task to be performed. If gas testing is required for all of the below, then it will be performed in the order specified:

  • Oxygen
  • Flammable gases and vapours
  • Potential toxic substances
  • Record the results of the initial/periodic gas tests on the work permit.
  • When the gas detector indicates more than 0% of all flammable gas in any confined space, personnel must exit immediately
  • Stop hot work activities inside a confined space if the flammable gas concentration exceeds 0% LEL.
  • Use continuous monitoring instrumentation with alarms (such as a gas detector) that meet the appropriate electrical rating when specified in the permit.
  • At least one person at each confined space entrance must have a calibrated pocket oxygen meter with an alarm.
Confined space hazards and precautions
Confined space hazards and precautions


Atmospheric hazards that appear in a confined space are those that expose to a risk such as death, entrapment, injury or acute illness caused by one or more of the following causes: An atmospheric oxygen concentration of less than 19.5% (oxygen deficient) or greater than 23.5% (oxygen excess). Potential Effects of Oxygen-Deficient or Excessive Atmospheres

Potential Effects of Oxygen-Deficient or Excessive Atmospheres

oxygen content

(% in Vol)

Effects and symptoms (at atmospheric pressure)
> 23.5%Excess oxygen, extreme fire hazard
20.9%The normal concentration of oxygen in the air
19.5%Minimum allowable oxygen level
15-19%Decreases in the ability to work stubbornly can affect coordination and can cause early symptoms of heart, lung, or circulatory problems
10-12%Breathing increases in speed and depth; poor judgment, blue lips
8-10%Mental failure, fainting, unconsciousness, pale complexion, nausea and vomiting
6-8%Recovery is still possible after four to five minutes. 50% fatality after six minutes
4-6%Coma within 40 seconds, seizures, respiratory arrest, death

Flammable Gas Test:

The LEL test shall be performed by the authorized gas tester before any hot work permit is issued for any location where there is a possibility that flammables may be present. The test will be carried out a maximum of 2 hours before the permit is issued. A fresh gas test must be performed before restarting work. 

The Permit Issuer shall specify repeated gas testing at defined intervals or continuous monitoring in the Permit to Work for locations or conditions where there is a higher probability of gas release. Standard gas tests every 2 hours. But the issuer can specify the gas test <2 hours (for example, every 1 hour).

If the gas detector indicates more than 0% LEL, do not do hot work. If additional purging efforts fail to remove all traces of flammable gas, the permit issuer shall authorize hot work to continue only if the following conditions are met:

  • The source of the flammable material is identified.
  • It is determined that the concentration cannot exceed 0.5% of the LEL.

The LEL for methane is 5% by volume, and the UEL is 17% by volume. When 2.5% by volume of methane is reached in a confined space, this value would correspond to 50% of LEL. (5% methane by volume would be 100% LEL). Between 5 and 17% by volume, a spark would cause an explosion.

Different gases have different concentrations in % by volume to reach 100% LEL. Some examples are: the LEL of pentane is 1.5% by volume; the LEL of hexane is 1.1% by volume; the LEL of propane is 2.1% by volume and the LEL for regular gasoline is 1.4% by volume. 

Toxic Substances Test:

Entry into confined spaces or work will not be permitted in the presence of toxic gases such as H2S, CO2, CO etc. Specify continuous gas testing throughout the job or at specific intervals in some cases, such as hot work or confined spaces where conditions could change.

An atmospheric concentration of any toxic compound that is above the OSHA Maximum Allowable Concentration (MAC). Below are some examples of the toxic gases most commonly found in a confined space.

Ammonia25ppm35ppm500 ppm
Carbon monoxide25ppm200ppm1,500ppm
Hydrogen sulfide10ppm15ppm100 ppm
Sulfur dioxide2ppm5ppm100ppm
Hydrogen cyanide4.7ppm50ppm
Nitric oxide25ppm100ppm


Effects of exposure to carbon monoxide
ppmExposure timeEffects and symptoms
358 hoursPermissible exposure level
2003 hoursMild headache and discomfort
4002 hoursheadache,
6001 hourheadache,
1,000-2,0002 hoursconfusion, discomfort
1,000-2,0001/2 – 1 hourtendency to wobble
1,000-2,0001/2 hourslight palpitations
2,000-2,5001/2 hourUnconsciousness
4,000> 1 hourFatality


Effects of Exposure to Hydrogen Sulfide
ppmExposure timeEffects and symptoms
108 hoursPermissible exposure level
50-1001 hourSlight conjunctivitis (“gas eye”) and respiratory tract irritation after 1 hour. May cause digestive upset and loss of appetite.
1001 hourCoughing, eye irritation, loss of smell after 2-15 minutes (olfactory fatigue). Altered breathing, and drowsiness after 15-30 minutes. Throat irritation after 1 hour. Gradual increase in severity of symptoms over several hours. Death may occur after 48 hours.



1/2 – 1 hourMarked conjunctivitis and respiratory tract irritation after 1 hour. Pulmonary oedema may occur from prolonged exposure.

The staggering, collapse in 5 minutes. Serious damage to the eyes in 30 minutes. Death after 30-60 minutes.

Rapid unconsciousness, “knockdown” or immediate collapse within 1 to 2 breaths, breathing stops, death within minutes.

Standby Man

Standby Man who is trained/qualified, certified and interviewed by the EHSS Department to monitor the entry site and seek assistance from emergency to ensure the safety of personnel. Standby Man will be required for all confined space entry permits. Entry into confined spaces will not be permitted without a standby man.

Confined space hazards and precautions
Confined space hazards and precautions

Primary Responsibilities of standby man:

  • To monitor the entry site.
  • Keep a log of all persons entering/exiting the confined space at each entrance or exit on the back of the confined space permit. 
  • Wear a coloured vest for identification. 
  • Terminate entry in the event of any site condition change per the confined space permit.
  • Notify the Fire Watch/Permit Issuer/Permit Receiver if entry to a confined space is cancelled due to premature termination of the action, improper equipment and procedure failure, or medical or other emergency requiring immediate exit from the confined space.
  • Standby men will not enter confined spaces under any circumstances and will not leave the site. 
  • Monitor the entry site and seek emergency assistance to ensure the safety of personnel present within the confined space. 
  • Maintain visual/verbal contact and maintain eye contact with workers inside the confined space.
  • Stop work if conditions deteriorate. 
  • Sound the air horn alarm to notify everyone that the confined space must be evacuated.
  • Use of portable radio for communication or air horn.
  • Ask all participants to leave the confined space immediately in the event of an emergency and headcount to ensure everyone is safe and no one is left.
  • Assist in the evacuation of personnel during the emergency without entering the confined space. 
  • Maintain eye contact with the participants as much as possible. In certain cases, eye contact and visibility for those entering confined spaces may be impeded due to equipment size or configurations.
  • Familiarize yourself with the emergency alarm manual call point/mounting area.
  • Each time the Standby Man leaves the activity area, the work must be stopped or endorsed to another qualified Standby Man.

Rescue plan

In addition to these preventive measures, the possibility of an emergency must be considered, and therefore the way to act must be planned, prohibiting the performance of work alone, and maintaining surveillance personnel abroad.

And with continuous communication between the inside and outside to report any possible anomaly that could seriously endanger the health or safety of workers. And Have all the materials necessary to carry out rescue operations and first aid.

A Rescue Plan is used to rescue people from the Confined Space in an Emergency. The rescue plan must be approved and all actions must be taken before the work permit. The Rescue Plan will be prepared following the formal Job Safety Analysis (JSA) and will need to be approved by all stakeholders before initial entry.

General Guidelines for Confined Space Entry Permit

The owner of the respective area shall identify all confined spaces by posting signs stating “DANGER – CONFINED SPACE – ENTER ONLY WITH PERMISSION” at entry points in case of entry. A Confined Space Entry Permit will only be issued to enter a CONFINED SPACE to perform their activities with the following arrangements:

  • Blind list with process equipment to attach to the LOTO.
  • P&IDs. 
  • Rescue plan. 
  • Ventilation Plan. 

Risk assessment/formal JSA

Every time a confined space is to be entered, the risks to that space should be assessed and checked for new hazards due to work being done or events taking place around the confined space. This reassessment ensures that workers entering the confined space are adequately protected, based on current conditions.

Conditions often change and therefore this assessment needs to be constantly updated. This can include a variety of issues, such as unsafe levels of gas requiring respiratory protection, the presence of flammable substances, loud noises requiring hearing protection, and control of all energy sources (lockout/tagout).

This part of the planning should also include a pre-enter briefing. During this step of preparation, the necessary emergency protocols should be reviewed so that everyone agrees.

Ventilation and lighting

Before entering a confined space, air tests should be performed. The atmosphere in the confined space may be contaminated or it will not be safe to breathe. Gas detection instruments should be checked to ensure they are working properly, according to the product’s instructions for use.

Most confined space air monitoring is done with a four-gas analyzer. This instrument monitors the concentration of oxygen in the atmosphere and identifies the presence of various hazardous fumes, gases, vapours and particles. Depending on your risk assessment, specific gas monitoring may be required to determine lower-level concentrations that may be present.

Ventilation is one of the fundamental preventive measures to ensure the safety of the inner atmosphere of the confined space, both before the performance of the work (in case of a contaminated environment), and during the work (because it requires a renewal of the inner environment).

The objective of ventilation is, therefore, an inner atmosphere with a concentration of contaminants or flammable substances present lower than their respective exposure and flammability limits, while an appropriate oxygen concentration.

Ventilation is a simple and effective control technique, which can be applied naturally (opening entries and exits of the enclosure), or forced (using equipment that insufflates or extracts air from the space). For this, the air is extracted from the space or fresh air is introduced. Forced ventilation is usually used.

Make sure the 24-volt explosion-proof lighting is available before entry.

Personal protective equipment

Last but not least, we have to consider the Personal Protection Equipment for carrying out work in confined spaces:

  • Safety Helmet protects against falling objects on the head or blows with fixed or mobile elements.
  • Safety glasses protect against projections of particles with speed or liquid splashes.
  • Safety gloves, protect against mechanical risks -eg, cuts, blows-, against contact with chemical products, against the biological risk…
  • Safety footwear with a non-slip sole, against perforation, and contact with chemical products.
  • Harness and its accessories protect against falls at different levels.
  • Self-filtering masks against particles or toxic gases or vapours.

Health and safety-related topics:

  1. What does NFPA 704 mean
  2. Rescue plan 2021
  3. Excavation hazards control measures
  4. Safety Job Analysis
  5. Training for emergency preparedness



One response to “Confined space hazards and precautions”

  1. Jessie Holloway Avatar

    Thanks so much for the step-by-step advice on how to ensure a confined space is safe and accessible when needed. My cousin works for a small construction company and their Wearhouse has some small areas that they want to ensure workers are safe around. They’ve been looking at knowing rescue service companies to ensure they can help an employee as quickly as possible.

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