Confined-Space Work Safety Guide — Oxygen Deficiency and Hazardous Gas Monitoring Essentials (Nationwide)

Confined-Space Work Precautions — Key Rules for Safe Entry and Gas Monitoring

What Is a Confined Space?

A confined space is any location where inadequate ventilation creates a risk of oxygen deficiency, toxic gas exposure, fire, or explosion. This includes not only spaces that are actively oxygen-deficient, but also any space not regularly occupied by workers where poor airflow means hazardous or inert gases could be present or accumulate.

• Oxygen deficiency is defined as oxygen concentration below 18%.

At oxygen concentrations below 18%, hypoxia symptoms begin. As levels fall further, conditions can become rapidly fatal. Oxygen and hazardous gas concentrations must be measured before entry using calibrated gas detection equipment.

CO (Carbon Monoxide) — Concentration and Health Effects

H₂S (Hydrogen Sulfide) — Concentration and Health Effects

Other Asphyxiants Commonly Found in Confined Spaces

Four Pillars of Confined-Space Safety

• ① Inventory all confined spaces — identify and document every confined space on site
• ② Mark confined spaces — post clearly visible signage at every identified confined space
• ③ Equip each team — ensure oxygen monitors, multi-gas detectors, supplied-air equipment, and communication devices are available for every team working in confined spaces
• ④ Train and drill — conduct mandatory confined-space safety training for all workers who may enter

For reliable confined-space monitoring, professional-grade instruments with verified accuracy should be used. The WANDI GasTiger 2000 provides industry-leading durability and measurement precision for demanding field conditions.

Correct Measurement Technique

Gas concentrations vary by location inside a confined space. Two essential practices:

• First: sample multiple points accounting for the space's area and depth — do not rely on a single measurement
• Second: for deep vessels, use a rubber hose or PVC sampling tube to reach lower sections without entering

Gas detectors must be calibrated on a regular schedule. As a quick field check, the O₂ reading in normal ambient air should be approximately 20.9% — if it deviates significantly, calibration is needed before use.

When entering deep spaces, supplied-air breathing apparatus or air-line respirators must be worn. Where flammable gas may be present, explosion-proof (Ex-rated) lighting must be used inside the space.

Safety is the priority. Know the rules — work without incidents.

Frequently Asked Questions

Q1. What symptoms appear when oxygen falls below 18%?

Below 18%: dizziness and headache begin. Below 16%: impaired judgment and muscle weakness. Below 10%: loss of consciousness. Below 6%: death within minutes. Korea's OSHA defines below 18% as oxygen deficiency and requires measurement before confined-space entry.

Q2. Which gases must be measured before confined-space entry?

The standard four-gas set — O₂, CO, H₂S, and EX (combustible gas) — is the baseline. Depending on site-specific hazards, CH₄, NH₃, CO₂, and other gases may be added to the measurement protocol.

Q3. Why must multiple sampling points be measured?

Gas density determines where gases accumulate: heavy gases (H₂S, CO₂) settle near the floor; light gases (H₂) rise to the ceiling. A single measurement point can miss hazardous concentrations elsewhere. Upper, middle, and lower zones should all be sampled using a probe tube before any worker enters.

Q4. How often should gas detectors be calibrated?

Formal calibration is typically performed every 6–12 months. Before each use, confirm that the O₂ reading in ambient air is approximately 20.9% — significant deviation indicates the instrument needs calibration before it can be trusted for confined-space entry decisions.

Q5. Why is explosion-proof lighting required in confined spaces?

Flammable gases such as LPG or methane can accumulate in confined spaces. Standard lighting fixtures can produce electrical sparks or localized heat sufficient to ignite a flammable atmosphere. Explosion-proof (Ex-rated) lighting is specifically designed to eliminate ignition sources, making it safe for use in potentially flammable environments.

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