Confined Space Hazards: Types, Precautions, and Controls

Over 2 million Americans enter a permit-required confined space on the job every year. And every year, confined space hazards claim the lives of roughly 100 of them.

In this article, we'll discuss confined space hazards and precautions, as well as who must know the hazards of a confined space (in other words, who needs confined space training for construction). We’ll do a deep dive on the top five confined space hazards – toxic atmosphere, oxygen deficiency, oxygen enrichment, flammable or explosive atmosphere, and engulfment – and break down why they’re dangerous, what causes them, and how to protect entrants from them.

Table of Contents

• Understanding Confined Space Hazards? 
• The 5 Most Common Confined Space Hazards 
• Other Hazards in Confined Spaces 
• Who Must Know the Hazards of a Confined Space? 
• Confined Space Hazards and Precautions 
• Identify Confined Space Hazards Better Through Online Training 

Understanding Confined Space Hazards

OSHA’s confined space definition is a space that is large enough that an employee can enter to perform work, has limited means for entry and exit, and isn’t designed for human occupancy. Only some confined spaces require extra precautions, and OSHA calls these permit-required confined spaces (PRCS).

Whether the confined space is a silo, storage bin, tank, cesspool, or large pipe, confined spaces present similar challenges and dangers. They're difficult to get in and out of, even when not fully enclosed. This complicates communication on a good day and hampers escape or rescue efforts when things go badly. Additionally, permit-required confined spaces often come with atmospheric, physical, and configuration hazards.

Atmospheric Hazards

Since they're not designed for continuous human occupation, confined spaces are poorly ventilated, which allows the buildup of bad air. In fact, atmospheric hazards account for four of the five most common hazards of confined spaces and cause 56% of all confined space fatalities.

Examples of atmospheric hazards involve the wrong amount of oxygen, the buildup of toxic substances like carbon monoxide or hydrogen sulfide, and the presence of combustible gases or vapors.

Physical and Configuration Hazards

Most other confined space hazards fall under the umbrella of physical or configuration hazards.

Configuration hazards are physical aspects of the space that can trap, injure, or prevent rescue of an entrant. Examples of configuration hazards in confined spaces include:

• Restricted openings for entry or exit
• Inwardly converging walls
• Downward-sloping, tapering floors
• Dropoffs, pits, ledges, or vertical shafts
• Long, winding, or obstructed travel paths
• Internal obstructions and entanglement points (like pipes, protruding bolts, movable equipment, and permanent structural elements that can snag or trap)
• Low overhead clearance
• Slippery or uneven walking surfaces
• Unstable structural conditions (like weak/corroded floors or damaged walls)

Other physical hazards involve conditions inside or contents of the space, including:

• Engulfment or entrapment
• Extreme temperatures
• Moving equipment
• Electrical energy
• Loud or interfering noise
• Biological threats like mold or pests

The 5 Most Common Confined Space Hazards

While many confined space hazards exist, there are five that cause the majority of deaths, injuries, and illnesses, including four atmospheric hazards plus engulfment.

Familiarity with these confined space hazards and their causes is important for anyone working in or around PRCSs. This knowledge gives you a fighting chance to recognize the potential for these conditions and take necessary precautions.

Confined Space Hazard #1: Toxic Atmosphere

OSHA defines toxic atmosphere hazards in confined spaces based on their ability to cause acute adverse effects (not long-term ones). So, a toxic atmosphere is one that contains a level of toxic substances that can cause death, incapacitation, injury, impairment in the ability to self-rescue, or acute illness.

How do you know what substances and amounts are acutely toxic? OSHA maintains lists of toxic contaminants and their permissible exposure limits (PELs) in the General Industry, Construction, and Marine standards, above which they require mandatory safety precautions. Employers who want to be more cautious than mandatory can reference threshold limit values (TLVs) or recommended exposure limits (RELs) provided by other safety organizations.

Causes of Toxic Atmosphere Hazards in Confined Spaces

Toxic atmospheres can build up in a confined space due to:

• Residual or stored products. Many confined spaces are tanks, pits, or pipelines that hold, or used to hold, the toxic substance. Even residue can cause illness without proper ventilation.
• Leaks or releases. Even if the confined space hasn’t held a toxic substance, it may be connected to equipment or pipelines that do.
• Decomposition of organic material. Sewers, manure pits, wastewater systems, and food-processing tanks can emit hydrogen sulfide, methane, ammonia, or carbon dioxide as organic matter decays.
• Geological sources. Sewers, utility vaults, crawl spaces, and other underground confined spaces may contain hydrogen from nearby petroleum or natural gas operations, hot springs, geothermal regions, or sulfur-bearing groundwater.
• Disturbing caked material. Shoveling, scraping, washing, or otherwise agitating materials like sludge, sediment, dust, or residue can release trapped gases or toxic dust.
• Work by-products. Running gas- or diesel-powered equipment can generate carbon monoxide. Hot work can produce metal fumes, nitrogen oxides, carbon monoxide, and other toxic byproducts. Cleaning, painting, and other activities can allow the buildup of fumes from solvents, degreasers, paints, epoxies, or adhesives.
• Migration from nearby. Fumes from nearby operations, traffic, or exhaust can easily enter the confined space and get trapped there.

In all these cases, the poor ventilation that is a hallmark of confined spaces plays an important role in creating acutely toxic levels.

Confined Space Hazard #2: Oxygen Deficiency

OSHA considers an oxygen-deficient atmosphere one that’s less than 19.5% oxygen by volume. Below that, individuals may start experiencing drowsiness, difficulty breathing, and/or nausea. Unconsciousness results when oxygen falls to 12%, and below 6%, brain cells begin to die. Eventually, people succumb to death by asphyxiation.

Causes of Oxygen Deficiency Hazards in a Confined Space

Oxygen-deficient atmospheres can be created in confined spaces through processes like consumption, oxidation, displacement, chemical reaction, and adsorption.

Practical examples of conditions that might lead to oxygen deficiency in a confined space include:

• Flash fires, flame-producing equipment, and even human occupation can consume the oxygen in a poorly ventilated confined space.
• Confined spaces with rusty surfaces or freshly poured concrete can become deficient due to high oxidation.
• Naturally occurring methane gas can seep into underground confined spaces and displace oxygen.
• Welding equipment using argon (or any other inert shield gas) can leak and displace oxygen in a confined space.
• Other gases can displace oxygen by accident due to faulty pipes, hoses, or valves.
• Oxygen can get displaced by any gas-forming chemical reaction taking place in the confined space, including biologically driven ones like fermentation.
• Inert gas is sometimes purposefully introduced to displace oxygen in confined spaces – either to prevent oxidation or to reduce the risk of a highly flammable atmosphere.
• If activated charcoal filters are utilized in a confined space, they can adsorb a proportion of the atmospheric oxygen, resulting in a deficiency.

Confined Space Hazard #3: Oxygen Enrichment

OSHA defines an oxygen-enriched atmosphere as one that is more than 23.5% by volume.

The danger here is that oxygen is very reactive with other chemicals. It naturally accelerates the combustion process. Normal air is 20.9% oxygen, but since the rest is mostly inert gas that refuses to react with other chemical compounds, there’s little risk of spontaneous combustion.

When oxygen levels rise above 23.5% in a poorly ventilated space, oxygen can saturate your clothing and other combustible materials. At that point, any spark – whether it’s from hot work or simply static electricity – can cause all oxygen-saturated combustibles to burn.

Causes of Oxygen Enrichment Hazards in Confined Spaces

The source of all oxygen enrichment hazards in a confined space basically boils down to introducing pure oxygen by accident or on purpose.

The most common examples include:

• Equipment utilizing pure oxygen has a leak, or
• Someone using pure oxygen to correct an oxygen deficiency hazard.

Ventilation of confined spaces can be an important safety precaution, but it should always use fresh air from outside. Never, ever use pure oxygen for ventilation.

Confined Space Hazard #4: Flammable or Explosive Atmospheres

It’s common sense that flammable gases, vapors, or mists represent hazards in a confined space; we hardly have to explain why.

There’s a fourth source for flammable or explosive atmospheres that can be less intuitive: combustible dust. When tiny particles of combustible material are suspended in the air in a dense enough concentration, then any spark or source of ignition can burn nearby dust particles, and fire can spread quickly through the dust cloud. Once this process has started, it can also cause an explosion by creating a pressure wave.

Examples of combustible dusts include coal dust, wood dust, corn starch, sugar dust, powdered metal or chemicals (especially aluminum), textile fibers, plastic or rubber dust, and dust from dry goods like animal feed, grain, fertilizer, coffee, tea, flour, or spices.

Causes of Flammable or Explosive Atmosphere Hazards in Confined Spaces

The causes of flammable or explosive atmospheric hazards in a confined space include:

• Decomposition of organic matter. Sewers, manure pits, silos that contained rotten grain, and confined spaces near landfills often have a natural buildup of methane as a byproduct of decomposition.
• Residue. If a storage tank or vessel previously held a flammable or explosive substance, the leftover residue can become fumes. Fuel pipelines and tanks are a common example of this.
• Leaks or releases. If the confined space has containers or equipment with flammable or explosive materials, leaky valves or pipes can cause a flammable or explosive atmosphere.
• Work byproducts. As with a toxic atmosphere, work can introduce a hazard that wasn’t already there. Operating an acetylene torch can lead to hazardous levels of the gas in the air. Using flammable paints or solvents can also result in fume buildups. Some work processes might produce or introduce combustible dust to the space, like grinding, mixing, or material-handling.
• Disturbed combustible dust. In many confined spaces, the combustible dust comes with the package – think about storage bins, grain elevators, and silos. Sometimes these dusts hang in the air before entry, but human activity can also stir up dust in sufficient concentrations to be hazardous, whether you’re cleaning with compressed air or simply walking around on the material.

Confined Space Hazard #5: Flowing Liquids or Free-Flowing Solids

OSHA considers a confined space to be hazardous if it contains (or may fill with) any flowing liquid or finely divided, flowable solid. Any solid that’s finely divided enough, like dirt or grain, can behave in a manner that presents similar risks to a flowing liquid.

The hazards presented by flowing liquids or finely divided, flowable solids include:

• Engulfment. Entrants can become inundated by these materials (if they pour in during work) or enveloped by them (if they sink into materials that are already present). Engulfment can lead to two further problems:
  • Entrapment. The entrant is “effectively captured” or trapped inside a confined space where they’re subject to other hazards (like a bad atmosphere). makes it difficult to extract them.
  • Crushing. Particularly with flowable solids, the weight of the engulfing material can press into any available space and crush the entrant. This can lead to asphyxiation; as the entrant exhales, the material squeezes them into a smaller and smaller space, preventing the entrant from inhaling as deeply again. Crushing also complicates rescue because the entrant can’t simply be pulled out without risking injury.
• Aspiration. With or without engulfment, these materials pose a threat to entrants because they’re small enough to be inhaled, plugging or filling the respiratory system and leading to suffocation. Powders, in particular, can present an asphyxiation hazard in the absence of engulfment. As the entrant disturbs the top layer of powder, it goes airborne, and the fine particles can then clog the airways.

Causes Flowing Liquid or Solid Hazards in a Confined Space

How does an entrant encounter a flowing liquid or flowable solid hazard in a confined space? There are several causes:

• Stored products. Many commonly entered confined spaces – like grain silos, ducts, or cesspools – are designed for containing or storing liquids or flowable solids.
  • Poor hazard awareness. Although people often recognize the danger of entering a confined space with liquids, most people underestimate the ease with which someone can become submerged in finely divided solids. Entrants may expect something like grain to function as solid ground when it's more like quicksand.
  • Material collapse. Even when precautions are taken, certain materials can form bridges or crusts over hollow spaces. While the crust supports the weight of the stored material, it may collapse when someone adds their own weight.
  • Work activities. Any work involving vibration or impact can destabilize material and trigger an avalanche or sudden flow.
  • Gravity and tapered configurations. Sloped, funnel-shaped, or cone-bottomed spaces direct materials toward narrow discharge points. If caught in a material flow, entrants can fall into narrowing spaces and become trapped or compressed by the walls.
• Introduction of flowable materials. Sometimes entrants start with an empty space, only to be inundated with liquids or flowable solids from another source.
  • Entry from connected systems. Faulty valves, gates, or pumps can cause unexpected flow of materials, as can inadequate isolation of connected systems like chutes or conveyors.
  • Flooding. Rain, groundwater, tidal water, or runoff can fill manholes, vaults, tunnels, pits, and other confined spaces quickly.

Other Hazards in Confined Spaces

In addition to the five most common hazards above, there are many others that occur less frequently. Other confined space hazards examples include:

Who Must Know the Hazards of a Confined Space?

The confined space standard requires any worker whose duties affect confined-space entry to complete role-appropriate training in the procedures, confined space hazards, and control measures relevant to their responsibilities.

These roles and their responsibilities include:

• Authorized Entrants. Responsible for entering permitted spaces, recognizing hazards, following procedures, and evacuating when ordered.
• Attendants. Responsible for monitoring entrants, maintaining communication, summoning rescue, and preventing unauthorized entry.
• Entry Supervisors. Responsible for verifying permit requirements, authorizing entry, coordinating operations, and terminating entry safely.
• Rescue Personnel. Responsible for responding to emergencies, retrieving entrants, administering aid, and conducting rescues safely.
• Employers. Responsible for providing training, equipment, procedures, oversight, and safe confined-space work conditions.
• Contractors. Must coordinate with host employers, train their own workers, follow permits, and communicate hazards effectively. Additional responsibilities vary based on the role for which they’ve been contracted.

Each of these roles requires overlapping but role-specific knowledge, so they shouldn’t all receive identical training. As an online safety training provider, we offer a wide range of confined space training for specific roles.

One important thing we want to mention is that construction trenches often present similar hazards to confined spaces – difficulty entering or exiting, complicated communication and rescue, the possibility of atmospheric hazards, and the chance of engulfment by dirt or liquids. However, trenches present their own problems and require their own solutions, so OSHA requires excavation safety training . Confined space training is not an effective substitute for excavation safety training, but some excavation work may also require confined space training.

Confined Space Hazards and Precautions

You can prevent confined space injuries and fatalities with the right precautions and control measures. Here are a few of the big ones.

Make Training a Priority

We discussed above how role-specific confined space training is critical to confined space safety, but even staff who will not be playing a direct role in confined space work must receive training on the hazards of entering a confined space. A high number of confined space accidents and deaths occur among would-be rescuers who didn’t understand the dangers or proper procedures.

Inspect Confined Spaces Before and During Entry

OSHA's confined space regulation requires a risk assessment by a competent person before any permit-required confined space entry. This assessment determines if the space is free of any harmful materials and substances, as well as identifying potential sources that may arise during entry and any hazards the work itself may cause. Employees should be aware of potential hazards before entering confined spaces.

It is critical to continue monitoring the space after entrance since atmospheric conditions can change swiftly. A monitor with an alert capable of notifying employees approaching the confined space is ideal. Attendants who remain outside the confined space and monitor the entrants are also important to the swift identification of hazards that crop up and an appropriate response.

Identify and Remove any Potential Hazards

Controls should be put into place for any known or potential hazards identified during the pre-entry inspection. Following the hierarchy of hazard controls , you’ll start with any possible eliminations or substitutions, including:

• Whether the work can be accomplished without human entry altogether
• Eliminating entry and exit limitations and/or configuration hazards
• Eliminating hazardous contents before entry, including liquids or flowable solids
• Permanently eliminating stored or incoming energy when practical
• Substituting for less hazardous chemicals, tools, or methods whenever feasible

Next, put engineering controls in place to eliminate or reduce the hazard. Common engineering controls against confined space hazards include measures like:

• Mechanical ventilation
• Purging contaminants or inerting them
• Isolating all piping and process systems that could introduce materials into the space
• Using lockout/tagout procedures on all sources of energy
• Install physical barriers to prevent falls, contact with moving parts, materials release, or accidental entry by unauthorized people
• Install engulfment-prevention equipment
• Install equipment that improves access and egress
• Provide adequate task and egress lighting
• Employ fall-protection and retrieval systems

Administrative controls (like policies and procedures) should be used to restrict access to the confined space and ensure authorized entrants only enter when safe conditions are verified. Common administrative controls include atmospheric testing, communication protocols, rescue plans, scheduling based on exposure limits, and similar precautions.

Personal Protective Equipment (PPE) is the last layer of defense once all other control measures have been exhausted.

Create a Rescue Plan

Even though we mentioned rescue plans under hazard control, it’s worth emphasizing as a critical confined space hazard precaution. OSHA requires employers to plan for appropriate rescue and emergency services that ensure an injured or trapped entrant can be retrieved as soon as possible.

Rescue plan procedures should be clearly defined and documented, and employees should be thoroughly trained in their roles. Insufficient planning can lead to unsuccessful rescue or recovery.

Identify Confined Space Hazards Better Through Online Training

Confined spaces can have many hazards that present an immediate threat to health and life, but role-specific confined space training can provide everyone with the knowledge they need to bring everyone home safely at the end of the day.

As an OSHA-authorized training provider with decades of experience, we’re an expert in effective and easily digestible safety training. Our online courses are self-paced and available on any device with internet access, so all team members can complete the training on their schedule and under conditions that work best for them.

Check out our confined space training for General Industry to prepare for permit-required confined space work. Need construction training instead? We have a large catalog of construction safety courses to meet your needs.