Posted on: January 5, 2024

The 5 Most Common Confined Space Hazards

The 5 Most Common Confined Space Hazards

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 100 to 150 of them.

Whether it's a silo, grain bin, trench, cesspool, or large pipe, confined spaces have similar challenges and dangers, especially due to limited access and poor ventilation. They're difficult to enter and exit, even when not fully enclosed. That complicates communication on a good day and hampers escape or rescue efforts when things go badly. And since they're not designed for continuous human occupation, confined spaces are poorly ventilated. That can allow the buildup of bad air.

In fact, four of the five most common confined space hazards involve a hazardous atmosphere.

Below, we'll discuss the top 5 confined space hazards that entrants may encounter. We'll discuss how OSHA defines the hazard, how the condition occurs, and why it can be immediately dangerous to life and health.

Confined Space Hazard #1: Toxic Atmosphere

OSHA considers a confined space to have a toxic atmosphere hazard if it contains an acutely toxic level of a substance that can cause death, incapacitation, injury, impairment in the ability to self-rescue, or acute illness due to its health effects.

In other words, only acute adverse symptoms make something a toxic atmosphere hazard. The presence of substances that cause damage on a longer time scale, like asbestos, would not constitute a toxic atmosphere, though separate OSHA protections would still apply.

OSHA maintains a list of toxic contaminants and their permissible exposure limits (PELs) in their General Industry, Construction, and Marine standards. OSHA has mandatory safety precautions when a toxic substance is at or above its PEL. Other organizations maintain separate sets of values called threshold limit values (TLVs) or recommended exposure limits (RELs). Employers may voluntarily enact precautions based on more conservative TLVs or RELs.

If OSHA has not published a PEL, you should refer to exposure recommendations on the substance's safety data sheet (SDS).

How Do Toxic Atmosphere Hazards Occur in Confined Spaces?

Toxic atmospheres can build up in a confined space for a few reasons.

The confined space itself may be the storage tank or vessel for a toxic product (or it was, in the past, and residue remains). It could also be the case that a toxic substance is being utilized in some way inside the confined space.

Underground confined spaces like sewers, utility vaults, and crawl spaces may accumulate hydrogen sulfide.

Operating combustion-powered equipment inside a confined space can result in a buildup of carbon monoxide.

Confined Space Hazard #2: Oxygen Deficiency

OSHA considers an oxygen-deficient atmosphere to be less than 19.5% by volume. Most healthy adults won't notice ill effects at this level, but below that, it's a different story.

Why is Oxygen Deficiency Hazardous?

When oxygen levels fall to 16% by volume, people start feeling the effects of hypoxia. You might have notable difficulty breathing, nausea, and/or drowsiness. At 12%, you run the risk of losing consciousness, and brain cells start to die when oxygen levels fall below 6%.

Eventually, these levels of oxygen deficiency can lead to death through asphyxiation.

How Do Oxygen Deficiency Hazards Occur in Confined Spaces?

There are a handful of ways that oxygen-deficient environments can occur as confined space hazards:  through consumption, oxidation, accidental or purposeful displacement, chemical reaction, and adsorption.

Practical examples of confined space or entry conditions that might lead to oxygen deficiency:

  • 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 like sewers or vaults and displace oxygen.
  • Welding equipment using argon, or another 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

Under normal conditions, oxygen makes up 20.9% of Earth's atmosphere. OSHA considers an oxygen-enriched atmosphere to be more than 23.5% by volume.

Why is Oxygen Enrichment Hazardous?

You may not realize it, but most of the air you breathe is inert gas. Specifically, over 78% of our atmosphere is nitrogen by volume. That makes our normal atmosphere relatively non-reactive to other chemical compounds.

Oxygen, on the other hand, is highly reactive. It accelerates the combustion process, and that's what makes an oxygen-enriched atmosphere so dangerous.

When oxygen levels rise above 23.5% in a confined space, oxygen may saturate your clothing and any other combustible materials. At that point, it's easy for any spark, whether it's from hot work or simple static electricity, can cause oxygen-saturated combustibles to burn quickly.

How Do Oxygen Enrichment Hazards Occur in Confined Spaces?

There are a few different scenarios that can result in an oxygen-enriched atmosphere inside a confined space, but they all boil down to the introduction of pure oxygen into the space, either on accident or on purpose.

One scenario is that equipment utilizing pure oxygen has an unknown leak, typically due to faulty hoses, pipes, or valves. OSHA requires any oxygen-fueled equipment to be removed from a confined space when it's not in use for this reason. For the same reason, you should never bring oxygen cylinders into a confined space.

Another scenario is that someone purposefully introduces pure oxygen to deal with an oxygen deficiency hazard, not knowing how dangerous that is. Oxygen-deficient confined spaces should always be ventilated using fresh air from outside the space. You should never use pure oxygen as ventilation.

Confined Space Hazard #4: Flammable or Explosive Atmospheres

OSHA defines flammable and combustible atmospheres as hazardous for obvious reasons.

Specifically, OSHA considers the atmosphere in a confined space to be hazardous when it contains:

For combustible dust, the rule of thumb is that when the dust is thick enough to impair visibility within 5 feet, you should assume there's a high risk of a flash fire or explosion.

It's worth noting that while OSHA uses "LFL," a lot of gas detection equipment uses "LEL" or Lower Explosive Limit. These terms are essentially interchangeable.

How Can Dust Be Flammable or Explosive?

Most people can easily make the connection that flammable gases, vapors, or mists can present an immediate threat to health and life, but combustible dust is sometimes less intuitive.

When combustible dust is 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, and spices.

How Do Flammable or Explosive Atmosphere Hazards Occur in Confined Spaces?

It's not unusual to encounter methane, fuel vapors, grain dust, and other flammable substances in the air inside confined spaces.

Many confined spaces may accumulate a natural buildup of methane because it's a byproduct of the decomposition of organic matter. Sewers, manure pits, silos that have held rotted grain, and confined spaces near landfills can all have flammable or explosive atmospheres, as a result.

Even when empty, vessels that previously contained flammable or explosive substances may contain residual traces or fumes. Fuel pipelines and tanks are common examples of vessels in this category that often must be entered.

If a confined space contains any vessels of flammable or explosive materials, then a leaky valve or pipe may result in a hazardous atmosphere.

Flammable or explosive atmospheres can also build up due to the work being done inside a confined space. For example, operating an acetylene torch can lead to hazardous levels of acetylene in the air. Using flammable paints and solvents can cause hazardous fume buildup. Processes like grinding, mixing, or other materials handling can create or kick up combustible dust in sufficient concentrations, as can disturbing settled dust by cleaning with compressed air.

Settled dust can become airborne when it's disturbed, and certain kinds of confined spaces present a special risk of combustible dust when they've contained the right materials. Examples include storage bins, grain elevators, silos, hoppers, chutes, mixers, blenders, powder-processing equipment, ovens, and furnaces.

Confined Space Hazard #5: Flowing Liquid or Free Flowing Solids

OSHA considers a confined space to be hazardous if it contains (or may fill with) a material that can engulf an entrant. This includes any flowing liquid or finely divided, flowable solid like dirt or grain.

Why Are Flowing Liquids or Free Flowing Solids Hazardous?

The potential hazards of being engulfed by a liquid are pretty self-evident, but what you may not realize is that any solid finely divided enough to be "flowable" can present similar risks.

One hazard is that liquids and free-flowing solids can engulf an entrant. Engulfment is its own risk when the material can exert enough force on the body to cause death by strangulation, constriction, or crushing.

Engulfment can also mean that the entrant is "effectively captured" or trapped inside the confined space where they're subject to other hazards like flooding or a toxic atmosphere. People who are engulfed by flowing solids are particularly hard to rescue due to the weight of the material bearing down on them.

Finally, aspiration is another threat posed by flowing liquid or free flowing solids. Liquid or fine solid material may be inhaled, filling or plugging the respiratory system and leading to asphyxiation and death. Powders may be an asphyxiation risk even without engulfment because the top layer can go airborne and clog the airways.

How Does a Flowing Liquid or Free Flowing Solids Hazard Occur in Confined Spaces?

Many commonly entered confined spaces are designed for containing or storing liquids or flowable solids, like grain silos, ducts, or cesspools.

Although people often recognize the danger of bodies of liquid, it's surprisingly easy for someone to become submerged in finely divided solids. Entrants may expect something like grain to function as solid ground when it's more like quicksand. It's also possible for some materials to form bridges or crusts over a hollow space. The crust can support the weight of the stored material but will collapse when someone walks over it.

That's why OSHA recommends draining any confined space of liquids or finely divided solids before entry, when possible.

In other cases, a confined space may be empty upon entry but fill up very quickly. Trenches are an example of this. Rain can cause a flood, while a collapse can engulf someone in dirt. Even when a storage space or conduit has been drained, faulty valves or doors can give way, releasing flowing liquid or free-flowing solids into the confined space.

Meet Confined Space Training Requirements Online

To ensure that employees who enter confined spaces understand the risks and their remedies in detail, OSHA requires all entrants to complete training in confined space hazards and precautions.

Our 8-hour General Industry Confined Space course is a thorough primer on confined space safety, appropriate for both entrants and their supervisors. It lays out the basics of confined space standards, then covers roles and responsibilities, atmospheric testing, hazardous energy control, and rescue/emergency protocol. We also offer a Confined Spaces 8-Hour Entry Training for Construction course.

We've been an OSHA-authorized training provider for over 20 years, and online courses like ours can be a consistent and cost-effective way to meet OSHA training requirements and ensure comprehension. The courses are online, mobile-friendly, and self-paced so that learners can focus on the training when it's convenient for them.

Enroll today!