Hazardous Atmospheres and Water in Excavations

Most trenches have normal air and a dry floor. The ones that don't can kill quickly, and the danger isn't obvious from the top. This lesson covers the two "invisible" hazards in 1926.651: hazardous atmospheres in paragraph (g) and water accumulation in paragraph (h).

Where bad air comes from

Excavations are low spots, and many gases are heavier than air or collect in low areas. Watch for:

• Landfills and fill areas: decomposing material produces methane (flammable) and hydrogen sulfide (toxic), and can displace oxygen.
• Sewers and manholes: hydrogen sulfide, methane and low oxygen.
• Fuel storage and pipelines: gasoline or diesel vapors from leaking tanks and lines, or natural gas from a damaged service.
• Engine exhaust: gas-powered pumps, compactors, saws and generators at or in the trench produce carbon monoxide, which can build up in a confined trench.
• Chemical or industrial sites: solvent vapors and other contaminants in the soil.
• Some soils and rock: oxygen can be consumed by oxidation of organic material in some soils.

When OSHA requires testing

Under 1926.651(g)(1)(i), where oxygen deficiency (below 19.5 percent oxygen) or a hazardous atmosphere exists or could reasonably be expected to exist, such as in landfill areas or near stored hazardous substances, the atmosphere must be tested before employees enter excavations greater than 4 feet deep.

When controls such as ventilation are used to reduce contaminants, testing must be repeated as often as necessary to make sure the air stays safe (1926.651(g)(1)(iv)). In practice that means continuous monitoring with a calibrated, bump-tested multi-gas meter while people are in the excavation.

Controls

1926.651(g)(1)(ii) requires adequate precautions, such as ventilation or respiratory protection, to prevent exposure to oxygen-deficient and other hazardous atmospheres. Common controls:

• Keep gas-powered equipment out of the trench and place exhaust downwind, or use electric or hydraulic equipment.
• Ventilate with a blower and ducting that delivers fresh air to the work area.
• Test from outside first, lowering the probe to the bottom, since heavier gases settle low.
• Stop work and get out when the meter alarms.

Rescue equipment

Where hazardous atmospheric conditions exist or may reasonably be expected to develop, emergency rescue equipment such as breathing apparatus, a safety harness and line, or a basket stretcher must be readily available, and it must be attended when in use (1926.651(g)(2)(i)). Workers entering bell-bottom pier holes or similar deep, confined footing excavations must wear a harness with a lifeline that is individually attended (1926.651(g)(2)(ii)).

Water: the hazard that changes everything

Water makes soil heavier, weaker and more likely to move. It can:

• Turn Type A or B soil into Type C, since submerged soil or soil with freely seeping water is Type C.
• Add hydrostatic pressure behind walls, shores and shields.
• Wash soil out from behind or below a shield or support system.
• Cause boiling or bottom heave in the trench floor.
• Create a drowning and electrical hazard.

What OSHA requires for water

1926.651(h) sets these rules:

• Employees may not work in excavations with accumulated or accumulating water unless adequate precautions are taken. Precautions vary but can include special support or shield systems, water removal, or safety harnesses and lifelines.
• If water is controlled by water removal equipment (pumps, well points), the equipment and its operation must be monitored by a competent person.
• If the excavation interrupts natural drainage of surface water, use diversion ditches, dikes or other means to keep surface water out.
• Excavations subject to runoff from heavy rains need an inspection by a competent person, and the daily inspection rule in 1926.651(k) requires inspection after every rainstorm.

Key takeaways

• Test before entry into excavations deeper than 4 feet where a hazardous atmosphere could reasonably exist.
• Minimum oxygen is 19.5%. Flammable gas must stay below 20% of the LFL.
• Keep engine exhaust away from the trench, ventilate, and retest as often as needed.
• Rescue equipment must be readily available and attended where atmospheres may be hazardous.
• Water downgrades soil and undermines protective systems. Pump only under competent person monitoring, and inspect after every rainstorm.

Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.