CO₂ is a confined-space hazard that cannot be detected by smell
Carbon dioxide (CO₂) is a colorless, odorless, nonflammable gas. It arises through many routes, including human respiration, fermentation and decay, dry-ice sublimation, carbonated-beverage equipment, fire-suppression releases, combustion, underground strata, and process gases. It is usually diluted in open areas, but can accumulate rapidly where ventilation is restricted, such as tanks, pits, manholes, cold stores, silos, fermenters, and wastewater facilities. The absence of odor or eye irritation does not mean it is safe; by the time a worker begins to feel symptoms, judgment and the ability to escape may already be impaired.
CO₂ hazards must be understood in two ways. High CO₂ itself can cause hypercapnia by affecting respiratory regulation and acid-base balance; CO₂ entering a space can also displace oxygen and create an oxygen-deficiency hazard. CO₂ can be harmful even when oxygen is near its normal range, and a low CO₂ reading does not rule out other gases or oxygen deficiency.
Four figures to distinguish first
For gas concentration by volume, 10,000 ppm equals 1%. Accordingly, 5,000 ppm is 0.5%, 15,000 ppm is 1.5%, 30,000 ppm is 3%, 40,000 ppm is 4%, and 100,000 ppm is 10%. If you do not confirm whether an instrument display is in ppm or vol%, you can seriously misinterpret an alarm value.
The U.S. Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for CO₂ is 5,000 ppm as an 8-hour time-weighted average. The current National Institute for Occupational Safety and Health (NIOSH) Pocket Guide also gives a recommended exposure limit (REL) of 5,000 ppm as a time-weighted average and a 15-minute short-term exposure limit (STEL) of 30,000 ppm. The time-weighted average is a value for managing the average over a work shift, while the STEL supplements it for brief high-concentration exposure. Neither guarantees that every person will have no symptoms.
NIOSH sets the IDLH for CO₂ at 40,000 ppm, or 4%. IDLH is a concentration that is immediately dangerous to life or health or may impair escape ability if equipment fails. It is not a permitted limit for routine work or a value at which one may enter briefly. The 5,000 ppm TWA, 30,000 ppm STEL, and 40,000 ppm IDLH have different purposes and time frames, and must not be read as a continuous “symptom-stage chart.”
Under Korea’s Occupational Safety and Health Standards Rules, “acceptable air” for confined-space work has oxygen of at least 18% and below 23.5%, carbon dioxide below 1.5%, carbon monoxide below 30 ppm, and hydrogen sulfide below 10 ppm. Carbon dioxide at 1.5%, or 15,000 ppm, is an element in judging acceptable air in Korean confined spaces and has a different legal purpose from the U.S. TWA or IDLH. Korean workplaces must apply Korean laws and site procedures first.
Changes that may be expected by concentration range
From ambient levels to below 5,000 ppm
Outdoor CO₂ is generally in the hundreds of ppm, and can be higher indoors because of human respiration and ventilation conditions. Indoor CO₂ in the thousands of ppm can indicate inadequate ventilation, but general indoor-air management values and confined-space entry-permit criteria are not the same concept. In a confined space, one low CO₂ measurement cannot establish safety. Oxygen, combustible gases, hydrogen sulfide, carbon monoxide, and process-specific target substances must also be checked.
At least 5,000 ppm and below 15,000 ppm
This range can exceed an 8-hour occupational limit. It cannot be said that clear symptoms will necessarily occur immediately, but it must not be normalized on the feeling that it is “still tolerable.” Response varies with exposure time, work intensity, temperature, pre-existing cardiopulmonary disease, medication, pregnancy, altitude, and individual sensitivity. Harder work increases breathing volume and therefore CO₂ inhalation.
At least 15,000 ppm and below 30,000 ppm
At 1.5% or higher, the atmosphere is outside the Korean acceptable-air range for confined spaces. NIOSH material explains that high CO₂ exposure may bring changes such as increased breathing, headache, dizziness, fatigue, and increased heart rate. Symptoms do not begin at the same concentration for everyone. The absence of symptoms does not permit ignoring a measurement, and a headache alone cannot establish CO₂ as the cause.
Oxygen can still measure as normal in this range. NIOSH IDLH supporting documentation describes older findings in which some submarine personnel continuously exposed to 30,000 ppm under conditions where oxygen was maintained normally showed only mild effects. This does not mean 3% is safe; it illustrates that limited observations cannot be generalized to ordinary worksites, strenuous work, or mixed-gas environments.
At least 30,000 ppm and below 40,000 ppm
30,000 ppm is the NIOSH 15-minute STEL. It does not recommend working for 15 minutes at that concentration; it is a recommended ceiling that the short-term average must not exceed. NIOSH health-hazard evaluations summarize that respiratory rate may increase at about 2–4%. Near 3%, breathlessness, headache, and reduced concentration may occur, but individual variation and exposure duration are large, so estimating concentration from symptoms is dangerous.
At least 40,000 ppm and below 60,000 ppm
40,000 ppm is the NIOSH IDLH. NIOSH revised the IDLH to 40,000 ppm based on human data including observed signs of intoxication following 30 minutes of exposure at 50,000 ppm. Health-hazard evaluation material summarizes that elevated blood pressure and dizziness may occur at about 4–6%. When breathing, judgment, and motor function decline, self-rescue through a ladder or narrow opening becomes difficult.
Entering an IDLH atmosphere without a plan to rescue a collapsed colleague creates a chain asphyxiation incident. The outside attendant must not follow inside, but must use non-entry rescue means and initiate the procedure for calling a rescue team. Entry rescuers must have training, respiratory protection, communications, retrieval, and standby personnel on the assumption of the same or greater hazard.
At least 60,000 ppm and below 100,000 ppm
NIOSH health-hazard evaluation material summarizes that serious effects, including tremor and problems with visual and auditory function, are possible at about 6–10%. NIOSH IDLH documentation cites human data in which exposure to 70,000–100,000 ppm for several minutes could cause loss of consciousness. A worker’s appearance of talking or standing briefly cannot be treated as evidence of safety.
It is also hazardous to assume that high-concentration CO₂ will sit evenly only at the bottom. CO₂ is heavier than air, but its actual distribution varies with temperature differences, entry velocity, agitation, ventilation flow, structural shape, and movement. Before entry, remotely measure upper, middle, and lower levels and the actual travel route; during work, continuously monitor the breathing zone.
At least 100,000 ppm
CO₂ above 10% is an extremely high-risk range that can lead within minutes to drowsiness, loss of consciousness, convulsions, coma, and death. NIOSH IDLH material reports that exposure to 100,000 ppm for only a few minutes can cause unconsciousness. This is not a work condition that can be endured through symptom observation or on-site ventilation; source isolation, unmanned measurement, specialist rescue, and emergency response take priority.
As concentrations rise, direct CO₂ toxicity and oxygen displacement become more likely to occur together, but oxygen must not be estimated by simple calculation. Fermentation and decay create CO₂ while consuming oxygen, and nitrogen purging or other gases may also be present. Measure each with calibrated oxygen and CO₂ sensors.
Why oxygen displacement must be measured separately
Normal air contains about 20.9% oxygen. OSHA treats oxygen below 19.5% as oxygen-deficient. Korean rules define oxygen below 18% as oxygen deficiency and define acceptable air as at least 18% and below 23.5%. Because jurisdictions and purposes differ, a more conservative internal criterion may apply on site. The key point is that both CO₂ and oxygen criteria must be met separately.
At the same CO₂ concentration, oxygen values differ according to the original air composition, other inert gases, oxidation, and microbial activity. CO₂ below 1.5% does not permit assuming oxygen is at least 18%. Even if oxygen appears sufficient, a short-term exposure hazard from CO₂ itself may remain. An oxygen monitor cannot stand in for CO₂ toxicity, nor can a CO₂ monitor stand in for oxygen deficiency.
Three confined-space scenarios
Fermenter inspection
Suppose that, before work, CO₂ near the opening is 8,000 ppm and oxygen is 20.7%. If residue is fermenting and agitation or wash water is introduced, CO₂ generation may increase and the floor concentration may be higher than at the opening. Isolate the equipment, deal with residues, provide forced ventilation, and then remeasure upper, middle, and lower levels. Do not enter without continuous measurement, an outside attendant, communication, and a rescue plan.
Dry-ice cold store
Suppose a monitor displays CO₂ at 18,000 ppm and oxygen at 19.8%. Because this is outside the CO₂ condition for Korean acceptable air, a worker must not enter merely because they do not feel uncomfortable. Dry ice continues to sublimate, and concentration can rise again when the door is closed. Restrict entry, exhaust safely with mechanical ventilation, and then remeasure, including the work area and lower levels.
Equipment room with a CO₂ fire-suppression system
Suppose that after an alarm, CO₂ is 45,000 ppm and oxygen is 18.9%. It is fatal to compare only the oxygen value with Korea’s 18% criterion and decide it “passes.” CO₂ has already exceeded the NIOSH IDLH. Do not enter without protection; follow emergency procedures, remote isolation, and specialist-rescue-team callout. Also assume worse conditions in light of sensor over-range or possible concentration stratification.
Entry procedures that must operate before concentration numbers
Safe confined-space work is created by a management system, not a single monitor. The following elements must be connected under Korean rules, the workplace work program, risk assessment, and entry permit.
Identify the space, process, residues, piping, and gas-entry sources, and control entry.
Isolate energy and material inflow through valve shutoff, blinding, lockout, and tagging.
Before entry, measure oxygen, combustible gases, CO₂, and anticipated hazardous gases from outside at upper, middle, and lower levels and along the travel route.
Check the sensor measurement range, response time, calibration, and bump-test status.
Provide forced ventilation with fresh air, but do not ventilate with oxygen.
Continuously measure workers’ breathing zones and evacuate immediately if an alarm or rising trend appears.
Prepare an outside attendant, communications, entry list, non-entry rescue equipment, and trained rescue response in advance.
Move anyone with abnormal symptoms to a safe place and request emergency medical support. Rescuers must not enter without protection.
Individual variation and the limits of interpreting symptoms
Response to CO₂ is not determined by concentration alone. Exposure duration, physical activity, body temperature, altitude, respiratory and cardiovascular condition, anxiety or panic sensitivity, medications, age, and individual physiological differences all matter. In the same space, one person may report headache or breathlessness first, while another may suddenly lose function without symptoms.
Headache, dizziness, rapid breathing, palpitations, fatigue, and confusion can be consistent with CO₂ exposure, but may also occur with CO, hydrogen sulfide, oxygen deficiency, heat stress, dehydration, and other illnesses. Do not diagnose the cause or decide when to re-enter from symptoms alone. Atmospheric measurement, exposure assessment, and medical judgment are needed.
This article is educational material for understanding hazards, not medical diagnostic or treatment advice. It also does not replace a workplace confined-space program, statutory training, entry permits, measurement, ventilation, respiratory-protection selection, monitoring, or rescue procedures. Actual work must follow applicable laws, KOSHA guidance, manufacturer instructions, and workplace approval procedures.
Key points
A 5,000 ppm TWA is an occupational long-term average management value, not an unconditional safety line.
15,000 ppm is the point at which carbon dioxide falls outside acceptable air for Korean confined spaces.
30,000 ppm is the NIOSH 15-minute STEL; it does not mean a permitted stay time.
40,000 ppm is the NIOSH IDLH; unplanned entry or rescue must not be attempted.
At 10% or above, the concentration is extreme and risks loss of consciousness and death within minutes.
Direct CO₂ effects and oxygen displacement must be measured separately and managed together.
Decide with calibrated measurement, ventilation, monitoring, and a rescue plan—not personal symptoms or odor.
Sources
NIOSH Carbon dioxide IDLH documentation — exposure limits, human acute-exposure data, and the basis for the 40,000 ppm IDLH
NIOSH Pocket Guide to Chemical Hazards: Carbon dioxide — CO₂ properties, REL, STEL, symptoms, and measurement information
NIOSH Health Hazard Evaluation Report 2004-0075-2944 — CO₂ health effects at 2–10%, oxygen deficiency, activity level, and field cases
OSHA Occupational Chemical Database: Carbon dioxide — OSHA PEL and measurement methods
OSHA Oxygen-Deficient or Oxygen-Enriched Atmospheres — oxygen displacement by inert gases including CO₂ and oxygen-deficiency hazards
National Law Information Center, Occupational Safety and Health Standards Rules, Article 618 — Korean definitions of confined space, acceptable air, and oxygen deficiency
Korea Occupational Safety and Health Agency, Confined-Space Work Safety for Field Workers — training material on bodily responses to CO₂ concentration ranges and work safety in confined spaces
The final verification date for all web materials is September 9, 2026.