Confined Space Atmospheric Hazards

Talking Points

1. 1 Test in this order: oxygen first (19.5-23.5%), then LEL (lower explosive limit), then toxic gases
2. 2 Oxygen below 19.5%: do not enter — atmosphere is oxygen-deficient, can cause collapse without warning
3. 3 Oxygen above 23.5%: increased fire risk — do not enter
4. 4 LEL above 5%: explosive atmosphere — evacuate immediately, do not use hot work equipment
5. 5 Hydrogen sulphide (H2S): rotten egg smell at low levels, but causes olfactory fatigue — do NOT rely on smell as your indicator. Deadens at ~50ppm — you stop smelling it while still at dangerous levels
6. 6 Carbon monoxide (CO): produced by engines, heaters, compressors — can accumulate in manholes near traffic or machinery
7. 7 Test at multiple points — top, middle, bottom — gases can layer at different heights
8. 8 Test before entry, during work, and after any disruption (door opened, material disturbed)

Control Measures

• Four-gas detector (O2, LEL, CO, H2S) calibrated to manufacturer schedule
• Trained operator checks calibration before each use — recorded in permit
• Multi-point testing procedure documented on permit
• Mechanical extraction ventilation to prevent gas accumulation
• Standby rescue personnel with breathing apparatus on standby