What the title needs to correct first
The “half” in this article’s title is framing meant to make readers remember a field hazard. It is not a universal statistic saying that carbon dioxide causes 50% of all oxygen-deficiency incidents. The OSHA, NIOSH, and HSE primary sources reviewed do not present such an industry-wide ratio. The distribution of causes varies greatly by workplace, process, stored material, season, and ventilation conditions. The title therefore must not be cited as an incident ratio or probability.
There is nevertheless good reason to emphasize CO₂. In confined spaces, CO₂ can displace air and lower oxygen concentration, and it may also be generated in processes that consume oxygen, such as fermentation, decay, and microbial respiration. Moreover, CO₂ can itself cause physiological effects at high concentrations even when oxygen remains near its normal range. A normal reading from one oxygen sensor therefore does not rule out a CO₂ hazard. It is more accurate to read “half” not as a number, but as a warning not to perform only half of an oxygen-deficiency assessment and miss CO₂ sources.
Oxygen deficiency and CO₂ accumulation are not the same hazard
Normal air contains about 20.9% oxygen. OSHA in the United States defines an atmosphere with less than 19.5% oxygen by volume as oxygen-deficient. This value is not a substitute for Korean legal requirements; it is an international public-agency reference point for explaining confined-space hazards. Whether work is actually permitted must also follow Korean occupational-safety law, site procedures, and risk assessment.
Oxygen deficiency means that the partial pressure of breathable oxygen is insufficient. It can rapidly lead to impaired judgment, loss of coordination, unconsciousness, and death, and it is difficult to detect by sensation at low concentrations. NIOSH also warns that many hazardous gases and vapors cannot be seen or smelled, and people cannot tell by their senses whether sufficient oxygen is present.
CO₂ accumulation is a separate chemical-exposure hazard. CO₂ is colorless, odorless, and nonflammable. HSE describes cases in which high concentrations of CO₂ entered confined spaces such as tanks, pits, and cellars, displaced oxygen, and caused fatal asphyxiation. NIOSH recommendations in the United States are 5,000 ppm as an 8-hour time-weighted average, 30,000 ppm for short-term exposure, and 40,000 ppm as the immediately dangerous to life or health concentration. These figures likewise do not replace domestic exposure limits, but they show why CO₂ must not be treated merely as a “ventilation indicator.”
The important point is that the two hazards can overlap or occur separately. A large CO₂ release can lower oxygen while increasing CO₂’s own effects. Conversely, CO₂ may exceed a work limit even while oxygen remains above 19.5%. And if nitrogen displaces air, oxygen may be deficient while CO₂ is not elevated. Both equations—“normal oxygen = safe atmosphere” and “normal CO₂ = normal oxygen”—are therefore false.
The first route by which CO₂ lowers oxygen: air displacement
When gas leaks or is released from CO₂ piping, fire-suppression systems, fermentation tanks, dry ice, welding shielding gas, or process vents, it displaces the existing air. Oxygen does not disappear selectively; the original air mixture is replaced with CO₂, lowering oxygen’s volume fraction throughout the space. The change is faster when a large amount of gas enters a small volume with poor ventilation.
Because CO₂ is denser than air at the same temperature and pressure, it may collect in low-lying areas, pits, sumps, and the bottoms of tanks. HSE notes that relatively heavy welding gases such as CO₂ and argon can accumulate in unexpectedly low places, including vehicle-inspection pits. But it must not be assumed that “heavy gases are always only at the bottom.” Release velocity, heat, fans, worker movement, openings, and structures alter mixing and stratification. That is why a reading at one height must not represent the entire space.
The hazard can return after pre-ventilation, particularly when connected piping remains live. Do not rely merely on a statement that a valve has been closed; where possible, verify physical isolation such as shutoff, disconnection, blinding, and lockout/tagout. Ventilation does not remove the source, so a plan to offset continuously entering CO₂ with a single fan can easily fail.
The second route: biological generation and oxygen consumption
Microbial decomposition and fermentation generate CO₂ in manure-storage tanks, sewers, fermenters, silos, grain stores, and organic-waste spaces. CO₂ can rise while aerobic microbes and stored material consume oxygen, and methane, hydrogen sulfide, and other hazardous gases may also arise under oxygen-poor conditions. HSE guidance on farm slurry facilities explains that bacterial decomposition generates CO₂, ammonia, methane, and hydrogen sulfide, and that oxygen deficiency can occur as well.
This route is difficult because the generation rate is not constant. Rising temperature, agitation, pumping, adding contents, collapse of a sediment layer, and changing liquid level can suddenly release trapped gas. Even if a pre-work measurement was safe, the atmosphere can become hazardous after work begins. The fact that a tank has been open for a long time, or that there was no problem the previous day, is not evidence of safety today.
Human respiration also consumes oxygen and releases CO₂ in a small confined space. In an actual risk assessment, however, headcount, work intensity, duration of stay, space volume, air supply, and other sources must all be considered together. Avoid simplistic judgments such as “several people entered, so it is immediately dangerous” or “there is only one person, so it is safe.”
Other oxygen-deficiency causes that must be found besides CO₂
Ending an oxygen-deficiency investigation with CO₂ alone makes the opposite error warned against by the title. HSE confined-space guidance identifies several causes, including the following.
Air may be displaced when inert gases such as nitrogen, argon, or helium are used for purging or as welding shielding gas. Nitrogen in particular has no odor or color, making leaks difficult to detect by the senses.
Oxidation and corrosion of steel tanks, scrap, cutting chips, and broad steel surfaces consume oxygen. HSE describes a fatal case in which rust depleted oxygen in a crane compartment that had filled with water.
Welding, cutting, combustion, engine operation, and some chemical reactions consume oxygen while also creating new hazards such as carbon monoxide, nitrogen oxides, and fumes.
Grain, timber, coal, vegetables, and wood pellets can reduce oxygen through oxidation or biological reactions. Some stored materials also generate CO or CO₂.
Reactions in soil or geological strata, reactions of groundwater with calcareous materials, and gas entering from nearby landfills or pipelines can alter the atmosphere in a space.
Long-term enclosure, inadequate ventilation, and worker respiration compound the effects of other oxygen-consuming processes.
When identifying causes, ask not only “what was originally in the space?” but also “what will be brought in during the work?” Cleaning agents, coatings, welding gases, pump exhaust, dry ice, and emissions from external equipment can create a new atmosphere after work starts. Adjacent tanks, connected piping, drains, and geological strata must also be included in the assessment boundary.
Why one oxygen sensor is not enough
An oxygen sensor reports oxygen concentration, not its cause. If oxygen is low, additional measurement and process checks are needed to determine whether CO₂, nitrogen, argon, combustion, or corrosion is responsible. Even if oxygen is normal, CO₂, hydrogen sulfide, carbon monoxide, or solvent vapor may be at harmful concentrations. A commonly used four-gas monitor usually measures oxygen, combustible gas, hydrogen sulfide, and carbon monoxide, and many models do not include a CO₂ sensor. Never assume that a display with four numbers is also measuring CO₂.
Sensor technology and measurement range must also be checked. If expected concentrations exceed the instrument’s upper limit, it may simply show a maximum reading, an error, or a delayed response. Where CO₂ generation is possible, select a dedicated sensor or multi-gas monitor suited to the relevant concentration range and environment. Check calibration validity, pre-use functional checks, battery, sampling-hose leaks, filter condition, and the manufacturer-specified response time. Record units in the log as well so that the measurer does not confuse %, ppm, and %LEL.
Measurement examines the volume of the space, not only the entrance
Before confined-space entry, OSHA requires testing with a calibrated direct-reading instrument in this order: oxygen, combustible gases and vapors, and potential toxic contaminants. One reason to check oxygen first is that many combustible-gas instruments depend on oxygen and may give unreliable readings in an oxygen-deficient atmosphere. If CO₂ is anticipated, list it explicitly among toxic and asphyxiating agents and measure it as well.
Whenever possible, begin measurement by remote sampling before anyone enters. Include the entrance, work location, floor or tank bottom, upper area, obstructed compartments, and areas around piping. For descending entry where stratification is possible, OSHA Appendix B directs testing in the direction of travel and to each side over distances of about 1.22m, moving at a pace appropriate to the instrument response time. Record the measurement location, height, time, and ventilation state with the numbers to make the results reproducible.
A single pre-work measurement shows only the starting condition. If conditions can change through fermentation, agitation, welding, or pipe leakage, monitor continuously or at intervals suited to the hazard during the work. If an alarm sounds, ventilation stops, or the process, headcount, weather, or liquid level changes, exit immediately and reassess the cause. Fixing a monitor at one point outside the entrance can miss the actual exposure at workers’ breathing height.
Ventilation is a control system, not a device for lowering numbers
First consider whether confined-space entry itself can be avoided. If the work can be completed with external cleaning, a remote camera, long-handled tools, or automatic sampling, there is no need to create entry risk. If entry is unavoidable, isolate CO₂ and inert-gas piping and liquids or solids that could enter, remove residues, and supply fresh outside air.
Keep the air-intake position away from exhaust outlets, vehicles, and generators so it does not draw contaminated air back in. Position ducts so air reaches dead zones and the work location. If CO₂ may collect in low areas, verify by measurement that real air exchange reaches the lower area. Do not conclude that ventilation is adequate merely because a door or manhole has been opened. Measure again after ventilation, during work, and before resuming work.
Do not raise readings in an oxygen-deficient space by blowing in pure oxygen. HSE warns that “improving” air with oxygen creates a serious fire hazard. What is needed is not oxygen enrichment, but appropriate fresh-air ventilation and source control. If ventilation cannot maintain safe conditions, ordinary dust or gas masks are not the solution. NIOSH explains that oxygen-deficient or immediately dangerous to life or health atmospheres require atmosphere-supplying respiratory protection suited to the work conditions. Selection and use must follow professional procedures that include training, fit, emergency air, and a rescue plan.
An entry permit is not one line of measurement values
An entry permit must connect space identification, work scope, expected sources, isolation status, allowable conditions, measuring equipment and results, ventilation method, attendant, communication, exit criteria, and rescue method. Simply writing oxygen 20.9% cannot control the potential for CO₂ leakage, hydrogen-sulfide release, or rapid change caused by agitation.
A practical check can be condensed into the following questions.
Are there pipes, stored materials, microbial processes, dry ice, or combustion sources through which CO₂ could enter or be generated?
Are there causes that could lower oxygen without CO₂, such as nitrogen or argon, corrosion, welding, or oxidation of stored materials?
Are oxygen and CO₂ measured separately, along with the expected combustible and toxic gases?
Have upper, middle, and lower levels and obstructed compartments been tested adequately for the instrument response time?
Is it monitored whether safe conditions remain throughout the work after isolation and ventilation?
When an alarm, ventilation failure, or process change occurs, is there immediate exit and controlled re-entry?
Are an attendant, communications, non-entry rescue, and professional rescue support ready before work begins?
Do not let a rescuer become the second casualty
Oxygen deficiency and high CO₂ concentrations can impair judgment and physical ability within only a few breaths. If someone follows an unconscious colleague in without respiratory protection, the rescuer is exposed to the same atmosphere. NIOSH emphasizes that people without confined-space rescue training, appropriate equipment, and personal protective equipment must not enter to conduct emergency rescue.
A pre-work rescue plan must include an outside attendant, continuous communication, means to call for rescue, available non-entry retrieval equipment, and the rescue team’s arrival and entry conditions. Having rescue respiratory protection alone does not complete preparation. Verify donning training, inspection, fit, air supply, entry route, and the feasibility of lifting a casualty for the actual space. Do not leave site workers to decide a rescue method on the spot.
Conclusion to remember in the field
CO₂ can displace oxygen in confined spaces and can rise along with oxygen consumption in fermentation, decomposition, and respiration. But no universal statistic has been verified showing that CO₂ causes half of all oxygen deficiency. The “half” in the title warns against an incomplete assessment that leaves out CO₂.
A safe judgment looks beyond one number to causes and the possibility of change. Measure oxygen, CO₂, combustible gases, and anticipated toxic gases separately with appropriate sensors, and check multiple heights and work locations. Isolate piping and energy, ventilate with fresh outside air, and monitor conditions during the work. An entry permit becomes meaningful only when exit criteria and a non-entry rescue plan are also ready. What is needed is not a snapshot in which the oxygen reading is normal, but evidence that a safe atmosphere is maintained throughout the work.
Sources
All links were checked on September 9, 2026.