Oxygen-Deficient & Oxygen-Enriched Air
Normal air is about 20.9% oxygen. In tanks, pits, trenches, vaults and closed rooms, that number can drop low enough to knock you out in a breath or two, or rise high enough to make your clothes burn like paper. You can’t see, smell or feel either change. Only a meter can tell you.
- Construction
- General Industry
- Confined Spaces
The limits
| Oxygen level | What it means |
|---|---|
| Above 23.5% | Oxygen-enriched (OSHA): serious fire hazard |
| 20.9% | Normal air |
| Below 19.5% | Oxygen-deficient (OSHA): not safe to enter without supplied air |
| About 12 to 16% | Faster breathing, poor judgment and coordination |
| About 10 to 14% | Faulty judgment, extreme fatigue, blue lips |
| Below about 10% | Nausea, collapse, unconsciousness |
| Below about 6% | Collapse within seconds; death in minutes |
Any drop below 20.9% means something is displacing or using up the oxygen. Find out what before anyone goes in.
How oxygen gets used up or pushed out
- Displacement: nitrogen or argon purging, welding shielding gas, carbon dioxide from dry ice or fermentation, and leaking cryogenic liquids. See Inert Gas Asphyxiation and Cryogenic Liquids.
- Consumption: rusting steel tanks, rotting material and bacteria in sewers and manure pits, burning heaters and engines, and curing coatings.
- Low spots: gases heavier than air settle in pits, trenches and tank bottoms. See Trench Atmospheres.
Oxygen-enriched air
- Common causes: leaking oxygen torch hoses and cylinders, oxygen left on in a closed space, and liquid oxygen.
- In enriched air, clothing, hair, grease and dust catch fire easily and burn very fast.
- Never use oxygen to ventilate a space, blow off dust or “freshen” the air. Use fresh air from a blower. See Oxygen Hazards.
- Take torches and hoses out of confined spaces at breaks and at the end of the shift.
Testing the air
- Use a calibrated, bump-tested multi-gas meter. See Gas Detectors.
- Test in this order: oxygen first, then flammable gases, then toxic gases. Low oxygen can make flammable readings wrong.
- Test at the top, middle and bottom of the space.
- Ventilate, retest, and keep monitoring while people are inside. Conditions can change.
Rescuers die too
NIOSH has found that a large share of confined space deaths are would-be rescuers who rushed in to help a coworker. If someone collapses in a space, don’t go in. Call for help, start the rescue plan, and use retrieval lines from outside. Only trained rescuers with supplied air should enter. See Confined Spaces.
Before entering any tank, pit or vault
- Meter bump-tested today
- Oxygen between 19.5% and 23.5%
- Flammable and toxic gases below limits
- Ventilation running with fresh air, not oxygen
- Attendant outside and rescue plan ready
Standards & references
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