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Atmospheric Hazards



Many confined space accidents occur because the workers did not realize the dangers or potential dangers within or nearby the space. Workers may not take into account the new hazards and other conditions created during work in confined spaces. Thus, it is crucial to carefully identify all confined space hazards before entering a space.

Confined space hazard categories

Usually, confined space incidents are caused by multiple factors. There are two primary categories of hazards:

  1. Atmospheric, or those that involve problems with the air in the space (lack of oxygen, the presence of other gases in the space, etc.) and
  2. Non-atmospheric, physical, or those hazards that are caused either by equipment (rotors, sparks, etc.) or by other dangerous conditions (slippery surfaces, heat, etc.).

It is critical you identify all the hazards in a space and determine how they can impact the health and safety of workers who enter this space.

1. What are the two basic types of confined space hazards?

a. Ergonomic and physical
b. Atmospheric and non-atmospheric
c. Falls and overexposure
d. Mechanical and dangerous surfaces

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Hazardous atmospheres

A hazardous atmosphere is any atmosphere that may incapacitate, injure, or impair an employee's self-rescue or lead to acute illness or death to workers and rescuers who enter confined spaces.

The following are examples of hazardous atmospheres within a confined space:

  • Flammable or explosive gas, vapor, or mist in a concentration greater than 10 percent of its lower flammable limit (LFL) or lower explosive limit (LEL).
  • Combustible dust suspended in air, which obscures vision at a distance of five feet or less.
  • Atmospheric oxygen concentration levels below 19.5 percent or above 23.5 percent at sea level.
  • Atmospheric concentration of any substance with an acutely toxic effect above its PEL, and any other atmospheric condition that is immediately dangerous to life or health (IDLH).

This does not include atmospheric concentrations of substances that are not capable of causing death, incapacitation, impairment of ability to self-rescue, injury, or acute illness.

Acceptable atmospheric conditions

For guidance, refer to sources of information that comply with published scientific and industry information, such as a Safety Data Sheet (SDS), and National Consensus Standards from organizations such as the American Conference of Governmental Industrial Hygienists (ACGIH) and the National Institute for Occupational Safety and Health (NIOSH).

2. Which of the following is an example of a hazardous atmosphere in a confined space?

a. Combustible dust
b. The temperature at 4 degrees Celsius
c. The oxygen level is at 21%
d. The concentration of toxic gas is below its PEL

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Oxygen deficiency

The normal atmosphere is composed approximately of 20.9% oxygen, 78.1% nitrogen, and 1% argon with small amounts of various other gases.

  • Oxygen deprivation is one form of asphyxiation. While it is desirable to maintain the atmospheric oxygen level at 21% by volume, the body can tolerate deviation from this ideal.
  • When the oxygen level falls to 17%, the first sign of hypoxia is a deterioration of night vision which is not noticeable until a normal oxygen concentration is restored. The physiologic effects are increased breathing volume and accelerated heartbeat.
  • Between 14-16%, the physiologic effects are increased breathing volume, accelerated heartbeat, very poor muscular coordination, rapid fatigue, and intermittent respiration.
  • Between 6-10%, the effects are nausea, vomiting, inability to perform, and unconsciousness.
  • Less than 6%, spasmodic breathing, convulsive movements, and death in minutes.

Reduction of oxygen in a confined space may be the result of either consumption or displacement.

The consumption of oxygen takes place during combustion of flammable substances, as in welding, heating, cutting, and brazing. A more subtle consumption of oxygen occurs during bacterial action, as in the fermentation process. Oxygen may also be consumed during chemical reactions as in the formation of rust on the exposed surface of the confined space (iron oxide). The number of people working in a confined space and the amount of their physical activity will also influence the oxygen consumption rate.

A second factor in oxygen deficiency is displacement by another gas. Examples of gases that are used to displace air, and therefore reduce the oxygen level, are helium, argon, and nitrogen. Carbon dioxide may also be used to displace air and can occur naturally in sewers, storage bins, wells, tunnels, wine vats, and grain elevators.

Aside from the natural development of these gases, or their use in the chemical process, certain gases are also used as inerting agents to displace flammable substances and retard pyrophoric reactions. Gases such as nitrogen, argon, helium, and carbon dioxide, are frequently referred to as non-toxic inert gases but have claimed many lives.

The use of nitrogen to inert a confined space has claimed more lives than carbon dioxide. The total displacement of oxygen by nitrogen will cause immediate collapse and death. Carbon dioxide and argon, with specific gravities greater than air, may lie in a tank or manhole for hours or days after opening. Since these gases are colorless and odorless, they pose an immediate hazard to health unless appropriate oxygen measurements and ventilation are adequately carried out.

3. At what percentage of oxygen do accelerated heartbeat and poor muscular coordination occur?

a. Less than 6%
b. Falls below 5%
c. Between 14-16%
d. Between 6-10%

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Oxygen Enrichment

Oxygen enrichment refers to air containing more than 23.5 percent oxygen. This dangerous condition is an extreme fire hazard in which static electricity from materials such as hair or clothing can provide the ignition source needed to start a fire. This environment also allows any fire to burn more readily. Oxygen enrichment does not occur naturally and should be investigated.

Oxygen enrichment can be caused by leaking oxygen cylinders or hoses that have been brought into or near the space. Always ventilate confined spaces with normal, ambient air. Never use pure oxygen.

Combustible and Flammable Gases

The Fire Triangle
(Click to enlarge)

Atmospheres containing combustible or flammable gases or vapors can be dangerous because of the threat of fire and explosion.

Three ingredients are necessary for an atmosphere to become flammable or explosive:

  1. an ignition source (heat or flame),
  2. fuel (combustible gas or vapor), and
  3. oxygen.

However, the proportions of fuel and oxygen in a mixture must be within the flammable range for this mixture to be readily ignitable.

4. What are the three ingredients necessary for an atmosphere to become flammable or explosive?

a. Proper sequence, gas, fuel
b. Fuel, gas, ignition source
c. Ignition source, fuel, oxygen
d. Oxygen, ignition source, proper sequence

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Lower and Upper Explosive/Flammability Limits

Lower Explosive/Flammability Limits (LEL/LFL)

The lower explosive limit, or LEL, is the lowest atmospheric concentration of fuel in the fuel-air mixture at which a gas or vapor can explode. The lower flammable limit, or LFL, is the lowest concentration at which the gas or vapor will burn. Fuel concentrations below the LEL and LFL will not explode or burn because there is not enough fuel in the mixture and is considered too "lean".

For example, the lean flammability limit for Jet A (aviation kerosene) in air at sea level is a concentration (by volume or partial pressure) of about 0.7%. The rich flammability limit is about 4.8% by volume or partial pressure. Flammability limits are not absolute, but depend on the type and strength of the ignition source.

Upper Explosive/Flammability Limits (UEL/UFL)

The highest atmospheric concentration of a gas or vapor in the fuel-air mixture that can explode is called the upper explosive limit, or UEL. The upper flammability limit, or UFL, is the maximum fuel concentration above which the mixture will not burn. Above this concentration, the mixture will not explode or burn because it has too much fuel and is considered too "rich".

The composition of a fuel vapor and air mixture can change over time and may fluctuate within a confined space. Fluctuations occur because the fuel-air mixture moves around the space, particularly when people or other things create air currents that disturb the atmosphere. Consequently, the mixture is not uniformly distributed within the space.

Flammable Range

Vapor Pressure and Temperature Diagram
(Click to enlarge)

Gases or vapors can only be explosive or flammable between their LEL/LFL and UEL/UFL. This is called the explosive/flammable range. Substances with a wide explosive/flammable range are considered to be more hazardous since they are readily ignitable over a wider range. However, any concentration of combustible gas or vapor should be of serious concern in a confined space. Workers should be especially careful when ventilating a space containing a gas or vapor above its UEL/UFL. In order to reduce the concentration below the LEL, this procedure will first bring the gas or vapor within its explosive/flammable range. When it does, the possibility of an explosion or fire exists.

The diagram on the right shows the relationship among the lower and upper explosive/flammability limits, explosive/flammable and non-explosive/flammable regions, flash points, and the vapor pressure curve. It also reveals what happens to a vapor/air mixture as concentrations and temperatures vary.

5. The explosive/flammable range exists _____.

a. above the LEL/UFL
b. below the UEL/UFL
c. between the LEL/LFL and UEL/UFL
d. below the LEL/LFL and above the UEL/UFL

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Combustible Dust Atmospheres

Finely powdered dust from combustible materials such as wood, metal, or grain can be fuel for powerful explosions. Dust clouds can develop as result of handling dusty materials or when solid materials are reduced to smaller particles from processes such as grinding, drilling, or crushing.

Airborne combustible dust at an explosive concentration would obscure vision at a distance of five feet (1.52 meters) or less. A direct reading instrument may be used to measure actual dust concentrations.

Toxic atmospheres

Substances regarded as toxic in a confined space can cover the entire spectrum of gases, vapors, and finely-divided airborne dust in industry. Toxic gases may be present in a confined space because:

  • toxic substances are used as part of the production process, (for example, in producing polyvinyl chloride, hydrogen chloride is used as will as vinyl chloride monomer, which is carcinogenic).
  • the biological and chemical "breakdown" of the product being stored in a tank, or
  • maintenance activities (welding) being performed in the confined space.

Click on the button to see the four common types of toxic gases encountered in confined spaces.

  1. Hydrogen Sulfide (H2S) - or "sewer gas," is a colorless gas with the odor of rotten eggs. Excessive exposure has been linked to many confined space deaths. Hydrogen sulfide causes a loss of our sense of smell, causing people to mistakenly think that the gas has left the space. Hydrogen sulfide inhibits the exchange of oxygen on the cellular level and causes asphyxiation.
  2. Carbon monoxide (CO) - is an odorless, colorless gas that is formed by burning carbon based fuels (gas, wood). Carbon monoxide inhibits the body's ability to transport oxygen to all parts of the body.
  3. Methane (CH4) - is a natural gas produced from the decay of organic matter. It is a flammable, explosive, colorless, and odorless gas. It can displace oxygen to the point of oxygen deficiency in a confined space, causing dizziness, unconsciousness, and asphyxiation.
  4. Solvents - many solvents, such as kerosene, gasoline, paint strippers, degreasers, etc. are not only flammable, but if inhaled at high concentrations can cause central nervous system (CNS) effects. CNS effect can include dizziness, drowsiness, lack of concentration, confusion, headaches, coma and death. (Source: University of South Carolina)

Remember, atmospheric changes may occur due to the work procedure, the product stored, or a nearby gas line leak. The atmosphere may be safe upon entry, but can change very quickly.

  • The work performed within the confined space (such as welding, degreasing, painting, or sanding) may produce toxic atmospheres.
  • Toxic gases and vapors from adjacent areas can migrate to and collect in the confined space.
  • Vapors may be released from the sludges on the bottom or scales on walls of emptied confined spaces, such as storage tanks, that previously contained flammable or toxic chemicals. Vapor release may be accelerated by wall scraping and sludge removal from confined spaces.

Confined spaces prevent toxic substances from escaping, diluting, or readily dissipating. Instead, substances can become trapped and a buildup occurs, whereby the concentrations of toxic substances reach dangerous levels.

The atmosphere inside a confined space can change rapidly and unexpectedly. Also, any ignition source (such as sparks from grinding or welding equipment, static electricity, or unapproved electrical equipment that is not non-sparking or even smoking) can initiate an explosion.

6. Which of the following gases inhibits the body's ability to transport oxygen to all parts of the body.

a. Methane
b. Carbon monoxide
c. Hydrogen sulfide
d. Nitrogen

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Irritant/Corrosive Atmospheres

Irritant or corrosive atmospheres can be divided into primary and secondary groups.

  1. The primary irritants exert no systemic toxic effects (effects on the entire body). Examples of primary irritants are chlorine, ozone, hydrochloric acid, hydrofluoric acid, sulfuric acid, nitrogen dioxide, ammonia, and sulfur dioxide.
  2. A secondary irritant is one that may produce systemic toxic effects in addition to surface irritation. Examples of secondary irritants include benzene, carbon tetrachloride, ethyl chloride, trichloroethane, trichloroethylene, and chloropropene.

Irritant gases vary widely among all areas of industrial activity. They can be found in plastics plants, chemical plants, the petroleum industry, tanneries, refrigeration industries, paint manufacturing, and mining operations.

Prolonged exposure at irritant or corrosive concentrations in a confined space may produce little or no evidence of irritation. This may result in a general weakening of the defense reflexes from changes in sensitivity. The danger in this situation is that the worker is usually not aware of any increase in his/her exposure to toxic substances.

Examples of Corrosives:

  • Bleach
  • Ammonia
  • Acids

7. Which category of irritants produces systemic toxic effects if exposure occurs in a confined space?

a. Primary irritant
b. Secondary irritant
c. Acute irritant
d. Chronic irritant

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Safety Data Sheet (SDS)

To find out information on the hazardous substances used in a confined space, read the product label and/or the SDS. Labels provide general product information, and the SDS gives useful information on proper use and handling, special precautions, and first aid treatment. When a chemical product is purchased, the manufacturer or supplier of the product provides an SDS. The SDS must be readily available to any employee who wishes to learn about a product that he or she comes into contact with. If you have any questions, contact your safety Department, the manufacturer or supplier of the product, the NIOSH Pocket Guide to Chemical Hazards, or a consultant. Here is a sample SDS.

Monitoring and Testing for Hazardous Atmospheres

Atmospheric testing is required for two distinct purposes:

  1. Evaluation of the hazards of the permit space, and
  2. Verification that acceptable entry conditions for entry into that space exist.

Continual monitoring in confined spaces is necessary because there are unseen and odorless contaminants (or oxygen-deficient atmospheres) that can kill or incapacitate workers. Monitoring is the only way to detect whether a hazardous atmosphere has developed during entry. If this is the case, employees will be alerted to the change so they can leave the space immediately.

Of those contaminants that have odor, some can be detected by our senses only at low concentration. Hydrogen sulfide, for example, will deaden the sense of smell at high concentrations. Because of this, employees might assume that a confined space is safe when it is not. There is no substitute for testing the air in a confined space prior to entry. A worker can also be exposed to a contaminant through skin contact while working in a confined space.

Atmospheric monitoring is necessary whenever:

  • A safe atmosphere cannot be ensured.
  • An existing hazardous atmosphere cannot be removed.
  • The confined space cannot be physically isolated from the penetration of hazardous materials.
  • There is reason to suspect the development of a hazardous atmosphere during work activity.

8. Which of the following is listed as a reason for atmospheric testing?

a. An existing hazardous atmosphere cannot be removed
b. A safe atmosphere can be ensured
c. The confined space can be isolated
d. There is no reason to assume hazardous atmospheres exist

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The "OFT" order of Testing

Always test for atmospheric hazards in the following order:

  1. Oxygen is tested first because most combustible gas and toxic atmosphere meters are oxygen-dependent and will not provide reliable readings when used in oxygen-deficient atmospheres. In addition, both oxygen-deficient and oxygen-enriched atmospheres are extremely hazardous to workers' health and safety. Oxygen levels should be between 19.5% - 23.5%.
  2. Flammable or explosive gases and vapors are tested next because the threat of fire and explosion is both more immediate and more life-threatening, in most cases, than exposure to toxic gases and vapors. Flammability limits should be less than 10% of the Lower Flammability Limit (LFL)
  3. Toxic atmospheres are tested last. Many modern direct-reading instruments provide simultaneous readings of multiple gases. Readings should be less than recognized ACGIH exposure limits or other published exposure levels (e.g. OSHA PELs, NIOSH RELs).

Atmosphere Sampling Procedure

Atmospheric Sampling Procedure
(Click to enlarge)

The atmosphere of a confined space should be analyzed using equipment of sufficient sensitivity and specificity to identify and evaluate any hazardous atmospheres that may exist or arise, so that appropriate permit entry procedures can be developed and acceptable entry conditions stipulated for that space.

Evaluation and interpretation of this data, and development of the entry procedure, should be done by, or reviewed by, a technically qualified professional (e.g., OSHA consultation service, or certified industrial hygienist, registered safety engineer, certified safety professional, certified marine chemist, etc.) based on evaluation of all serious hazards.

The atmosphere of a permit space should be tested for residues of all contaminants identified by evaluation testing to determine that residual concentrations at the time of testing and entry are within the range of acceptable entry conditions.

When monitoring for entries involving a descent into atmospheres that may be stratified, the atmospheric envelope should be tested about every 4 feet (1.22 m) in the direction of vertical travel and to each side. If a sampling probe is used, the entrant's rate of progress should be slowed to accommodate the sampling speed and detector response.

Results of testing (i.e., actual concentration, etc.) should be recorded on the permit in the space provided adjacent to the stipulated acceptable entry condition.

If entrants leave the confined space for any reason, they should once again test the atmosphere before re-entry into the confined space because it can change rapidly.

Test results that show the composition of an atmosphere to which employees are actually exposed (even if the employees are using respirators) must be available so that they can be reviewed by members of the entry team or representatives.

9. Oxygen levels in a confined space should be maintained at _____.

a. more than 30%
b. less than 10%
c. 25%-30%
d. 19.5%-23.5%

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Direct-Reading Testing Instruments

Electronic gas detectors and color-indicator gas detector tubes are the most common types of instruments used for determining oxygen content, lower explosive limit, and toxic atmospheres.

Purchasing Monitoring Equipment

The typical confined space gas monitors will offer up to four independent sensors for the detection of oxygen, combustible gas, carbon monoxide, and hydrogen sulfide. Before purchasing confined space testing equipment, evaluate the instrument's:

  • Accuracy.
  • testing
  • Environmental operating range:
    • Remote sampling capability.
    • Operating temperature.
    • Relative humidity.
  • Intrinsic safety for explosive atmospheres.
  • Specificity for contaminant of interest.
  • Warm-up time.
  • Response time.
  • Ruggedness.
  • Ease of use and maintenance.
  • Vendor support.
  • Sensor and battery life.
  • Data-logging capabilities.

"Immediately Dangerous to Life or Health" (IDLH) Definition

This term refers to any condition in a permit space that would:

  1. Cause irreversible adverse health effects; or
  2. Interfere with self-rescue; or
  3. Cause immediate or delayed threat to life or health.

Permissible Exposure Limits (PELs)

Permissible exposure limits, or PELs, are occupational exposure standards that refer to the maximum concentration of airborne chemicals to which nearly all healthy persons can be exposed day after day without adverse health effects. Workers exposure to concentration of materials in excess of the PEL can result in detrimental health effects, including illness and/or death.

10. Gas monitors typically measure levels of all of the following toxic gases EXCEPT _____.

a. combustible gas
b. carbon dioxide
c. oxygen
d. hydrogen sulfide

Check your Work

Read the material in each section to find the correct answer to each quiz question. After answering all the questions, click on the "Check Quiz Answers" button to grade your quiz and see your score. You will receive a message if you forgot to answer one of the questions. After clicking the button, the questions you missed will be listed below. You can correct any missed questions and check your answers again.

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Real-Life Accident

A 49-year-old worker suffocated inside of a chemical tank at the Port of New Orleans. The Occupational Safety and Health Administration (OSHA) said the Illinois company, Dedicated Tank Cleaning Services, sent Armond Stack and two others into a tank in October 2015 without first testing the air quality of providing them lifelines. As a result, Stack was killed and the others were hospitalized.


OSHA is proposing $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 Stack’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.



This exercise is optional. To complete the exercise you must speak to trainer John and answer his questions. To do so, you will need to select a response at the bottom of the screen. Do your best to answer his questions. Good luck!


Confined Spaces

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