United States
Hong KongExcavation atmospheric testing
Atmospheric testing is required in an excavation more than 4 feet deep where a hazardous atmosphere could reasonably exist.
- Oxygen must be between 19.5 and 23.5 percent.
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A rule that catches people because the trigger is not depth alone.
The trigger
More than 4 feet deep, AND a hazardous atmosphere could reasonably be expected to exist. Both conditions, at 29 CFR 1926.651(g)(1).
A six-foot trench in clean ground away from anything is not automatically a testing scenario. A four-foot excavation next to a landfill or a fuel line is.
The oxygen band
| Reading | Meaning |
|---|---|
| Below 19.5 percent | Oxygen deficient |
| 19.5 to 23.5 percent | Acceptable |
| Above 23.5 percent | Oxygen enriched, a fire risk |
Candidates remember the lower limit and forget the upper one. An enriched atmosphere makes materials burn far more readily, and 23.5 percent is the ceiling.
What creates a hazardous atmosphere
- Adjacent landfill or contaminated ground
- Nearby fuel or gas lines
- Sewers and manholes
- Engine exhaust from equipment working at the edge
- Decomposing organic material
That last one surprises people. Organic decomposition consumes oxygen and produces methane, and it does so quietly.
What follows a bad reading
Ventilation or respiratory protection, and emergency rescue equipment where a hazardous atmosphere exists or may develop.
An attendant must be present where rescue equipment is required, and they stay outside.
Water accumulation
A related rule. Employees may not work in an excavation with accumulated water unless precautions are taken, and those are supervised by a competent person.
Common questions
When is atmospheric testing required in an excavation?
Where it is more than 4 feet deep and a hazardous atmosphere could reasonably be expected.
What are the acceptable oxygen levels?
Between 19.5 and 23.5 percent.
Is too much oxygen a problem?
Yes. Above 23.5 percent the atmosphere is enriched and materials burn far more readily.
What creates a hazardous atmosphere?
Adjacent landfill, fuel lines, sewers, engine exhaust and decomposing organic material.
What is required if a hazard exists?
Ventilation or respiratory protection, plus emergency rescue equipment and an attendant outside the excavation.