Hazardous Atmospheres in Trenches
A trench is a low spot where heavy gases collect and fresh air doesn’t circulate. Engine exhaust, sewer gas, leaking fuel and natural gas can make the air at the bottom deadly while the air at the top seems fine.
- Construction
- Utility contractors
- Plumbers
Photo: competent person lowering a gas detector probe into a trench before workers enter
Where bad air comes from
| Source | Hazard |
|---|---|
| Engines, pumps, compactors, saws | Carbon monoxide (CO) |
| Sewers, septic systems, swamps | Hydrogen sulfide (H2S), methane, low oxygen |
| Landfills and fill with buried waste | Methane, low oxygen |
| Gas lines and leaking tanks | Natural gas, propane or gasoline vapor (fire and explosion) |
| Contaminated soil | Solvent and fuel vapors |
| Decaying organic material, some soils | Low oxygen, carbon dioxide |
Propane, gasoline vapor, hydrogen sulfide and carbon dioxide are heavier than air and sink to the bottom.
The rule
- In excavations deeper than 4 feet where low oxygen (less than 19.5%) or a hazardous atmosphere exists or could reasonably be expected, the air must be tested before workers enter.
- Ventilation or other controls must keep the atmosphere safe, and testing must be repeated as often as needed.
- Where hazardous atmospheres exist or could develop, emergency rescue equipment (such as breathing apparatus, a harness and line, or a basket stretcher) must be readily available and attended.
Safe practices
- Test with a calibrated four-gas meter (oxygen, flammable gas, CO, H2S), bump-tested before use. See Gas Detectors.
- Test at the bottom of the trench, where heavy gases collect, and at the work location.
- Keep engines and generators away from the edge, with exhaust pointed away.
- Use blowers to ventilate when needed, with the intake in clean air.
- Leave immediately if a meter alarms or anyone feels dizzy, short of breath or gets a headache.
Never go in after a collapsed coworker without testing and protection. Many trench and pit deaths include would-be rescuers. Call 911. See Trench Emergencies.
Confined spaces
Some excavations, pits and vaults also meet the definition of a confined space and fall under OSHA’s confined space rules, including permits, attendants and rescue planning. See Confined Spaces.
Standards & references
- 29 CFR 1926.651(g): Hazardous atmospheres
- See also: Hydrogen Sulfide · Carbon Monoxide
Last updated: