On this page
In this lesson
What you'll learn
- State OSHA's five conditions that define a hazardous atmosphere
- Explain how oxygen deficiency and enrichment happen in confined spaces
- Identify common flammable and toxic gases and where they come from
- Recognize engulfment, configuration and physical hazards
A confined space can hurt you in many ways, but the atmosphere is the hazard behind most deaths. You can't see it, it often has no smell or a misleading one, and it can change while you are inside. This lesson covers the air first, then the other hazards that make a space permit-required.
OSHA's definition of a hazardous atmosphere
Under 1910.146(b), a hazardous atmosphere is one that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue, injury or acute illness from one or more of these causes:
| Condition | Threshold |
|---|---|
| Flammable gas, vapor or mist | More than 10% of its lower flammable limit (LFL) |
| Airborne combustible dust | At or above its LFL (roughly when dust obscures vision at 5 feet or less) |
| Oxygen | Below 19.5% or above 23.5% |
| Toxic substances with an OSHA PEL or dose | Above the dose or permissible exposure limit |
| Any other condition | Immediately dangerous to life or health (IDLH) |
Oxygen deficiency
Normal air contains about 20.9% oxygen. Below 19.5%, the atmosphere is oxygen-deficient. Oxygen is lost in confined spaces by:
- Displacement by other gases, such as nitrogen or argon used for purging or inerting, carbon dioxide from fermentation, or methane from decay.
- Consumption by rusting steel, decomposing organic material, bacteria, combustion such as welding and cutting, or fermentation.
- Absorption by some materials, such as fresh activated carbon.
As oxygen drops, judgment and coordination fail before the victim notices, which is why people don't simply climb out.
Oxygen enrichment
Above 23.5%, the atmosphere is oxygen-enriched, and materials such as clothing and hair can ignite and burn violently. Enrichment usually comes from leaking oxygen hoses or cylinders, or from someone trying to "freshen" the air with pure oxygen. Never ventilate with oxygen.
Flammable atmospheres
Fire and explosion need fuel, oxygen and an ignition source. In confined spaces, fuel comes from:
- Methane from sewers, landfills and decomposing material.
- Vapors from fuels, solvents, paints, coatings and cleaning chemicals.
- Gases from welding and cutting operations, or leaking lines.
- Combustible dusts such as grain, flour, coal, sugar and some metals.
OSHA considers more than 10% of the LFL a hazardous atmosphere, which gives a margin of safety well below the point where the mixture can burn.
Toxic atmospheres
Common toxic gases in confined spaces include:
- Hydrogen sulfide (H2S): rotten-egg smell at low levels, but it deadens the sense of smell at higher concentrations. Found in sewers, wastewater, pulp and paper, and oil and gas. See our hydrogen sulfide guide.
- Carbon monoxide (CO): odorless, from engine exhaust, combustion and some processes. Gasoline-powered equipment near a confined space opening is a common source.
- Ammonia, chlorine and other process chemicals left in tanks and lines, or released by cleaning.
- Solvent vapors from coatings, adhesives and degreasing inside the space.
Engulfment
Engulfment is the surrounding and effective capture of a person by a liquid or finely divided (flowable) solid substance that can be aspirated to cause death by filling or plugging the respiratory system, or that can exert enough force on the body to cause death by strangulation, constriction or crushing. Grain bins, sand and gravel hoppers, sawdust silos and sewers with upstream flow are common engulfment hazards. Bridged or crusted material can collapse under a worker without warning.
Internal configuration
Some spaces trap or asphyxiate because of their shape: inwardly converging walls or a floor that slopes downward and tapers to a smaller cross-section, such as hoppers, cones and some tank bottoms. A worker who slips can wedge into the narrowing space.
Other serious safety and health hazards
The fourth permit-space category covers any other recognized serious hazard, such as:
- Moving parts: agitators, mixers, augers, conveyors and fans.
- Hazardous energy: electrical, hydraulic, pneumatic, steam and stored pressure.
- Temperature extremes: heat stress inside hot vessels, or cold.
- Falls from ladders and platforms, and slippery surfaces.
- Noise, poor lighting, falling objects, and biological hazards such as sewage.
These are controlled by isolation, lockout and other measures covered in lesson 6.
Key takeaways
- A hazardous atmosphere: flammables above 10% LFL, combustible dust at its LFL, oxygen below 19.5% or above 23.5%, toxics above their PEL, or any IDLH condition.
- Oxygen is lost by displacement, consumption and absorption. Never ventilate with pure oxygen.
- H2S and CO are the most common toxic gases. The work itself can create hazards.
- Engulfment, converging configurations and physical hazards also make a space permit-required.
Knowledge check
Lesson 3 quiz: 8 questions
Choose one answer for each question, then select Check my answers. Score 80%% or higher to pass. Your answers stay in your browser and are never saved.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.