Confined Space Welding: Permits, Atmospheric Testing, and OSHA 1910.146

Q: "What makes a space a confined space?"

"OSHA defines a confined space as a space that is large enough to enter and perform work, has limited means of entry or exit, and is not designed for continuous occupancy. Examples include tanks, vessels, silos, storage bins, hoppers, vaults, boilers, pipes large enough to enter, and pits. If the space additionally has a hazardous atmosphere, engulfment risk, or entrapment risk, it becomes permit-required."

Q: "Can you MIG weld in a confined space?"

"Yes, but GMAW (MIG) in confined spaces requires forced ventilation to remove shielding gas (argon, CO2, or mixtures) that displaces oxygen. Argon is heavier than air and accumulates in low points. CO2 in high concentrations causes asphyxiation. Continuous atmospheric monitoring for oxygen level (19.5-23.5%) is mandatory, and the ventilation must run constantly during welding."

Q: "How often should atmospheric testing be done in a confined space?"

"Initial atmospheric testing must be done before entry. Continuous monitoring is required during welding operations because the arc produces fumes and gases that change the atmosphere rapidly. At minimum, monitor oxygen (19.5-23.5%), lower explosive limit (below 10% LEL), and toxic gases (CO, H2S, and others depending on the space history). Test at multiple levels since gases stratify by density."

Published September 15, 2025 · Updated March 17, 2026 · 7 min read

Welding in a confined space is one of the highest-risk activities in the trade. You’re combining fire, toxic fumes, shielding gas displacement, electrical hazards, and limited escape in a single operation. OSHA 1910.146 (general industry) and 1926.1200 series (construction) set the baseline requirements, but the real safety standard is thorough planning and zero shortcuts.

Confined space incidents kill about 90 workers per year in the U.S., and a significant portion of those involve welding or cutting operations. In many cases, the would-be rescuers also die because they enter without proper equipment.

What Qualifies as a Confined Space

OSHA’s three-part definition:

Large enough to enter and perform work (a person can bodily enter)
Limited entry or exit (not designed for easy access; restricted openings)
Not designed for continuous occupancy (tanks, vessels, crawl spaces)

Common Confined Spaces in Welding

Storage tanks (fuel, water, chemical)
Pressure vessels and boilers
Large-diameter pipe (24 inches and larger)
Ship compartments and double bottoms
Manholes and vaults
Silos and hoppers
Excavations and trenches (deep)
Ductwork large enough to enter

Permit-Required vs. Non-Permit Confined Spaces

A confined space becomes permit-required when it contains one or more of:

Hazardous atmosphere: Toxic gases, oxygen deficiency or enrichment, flammable vapors
Material engulfment hazard: Risk of being buried by stored product
Configuration hazard: Inwardly converging walls, sloped floor leading to a trap
Any other recognized serious hazard

Any confined space where welding will occur is automatically permit-required because welding creates a hazardous atmosphere (fumes, gas displacement, oxygen consumption).

Before Entry: Planning and Preparation

Written Permit

The confined space entry permit must include:

Permit Element	Details
Space identification	Specific location, vessel name/number
Purpose of entry	Description of welding work to be performed
Date and authorized duration	Permits typically expire after one shift
Entrants	Names of all persons who will enter
Attendant(s)	Name of person(s) stationed outside
Entry supervisor	Person authorizing the entry
Hazards identified	Atmospheric, physical, engulfment, electrical
Controls in place	Ventilation, lockout/tagout, air monitoring, PPE
Atmospheric test results	O2, LEL, CO, H2S, and others as applicable
Rescue provisions	Rescue team, equipment, hospital location
Communication method	Voice, radio, visual signals, tug line
Hot work permit	Separate hot work permit also required

Atmospheric Testing

Test the atmosphere before anyone enters. Use a calibrated 4-gas monitor (minimum) testing for:

Oxygen (O2): Normal is 20.9%. Acceptable range for entry: 19.5% to 23.5%.

Below 19.5%: Oxygen-deficient, dangerous. Don’t enter without supplied air
Above 23.5%: Oxygen-enriched, fire/explosion risk greatly increased

Lower Explosive Limit (LEL): Must be below 10% of the LEL for the expected gases. Above 10% LEL, the atmosphere is too close to the explosive range.

Carbon monoxide (CO): PEL is 50 ppm. Welding produces CO, especially with CO2 shielding gas and cellulose electrodes.

Hydrogen sulfide (H2S): PEL is 10 ppm (ceiling 20 ppm). Present in tanks that held petroleum products, wastewater, or biological materials.

Additional testing: Depending on what the space previously contained, test for specific toxic gases (benzene, ammonia, chlorine, etc.).

Test at multiple levels. Gases stratify by density. Heavier-than-air gases (argon, CO2, propane) sink to the bottom. Lighter gases (methane, hydrogen) rise to the top. Test at the entry level, the working level, and the lowest point.

Ventilation Setup

Forced ventilation is mandatory for confined space welding. Natural ventilation is never sufficient because:

Welding consumes oxygen
Shielding gas displaces oxygen (argon and CO2 are heavier than air and pool at the bottom)
Welding fumes accumulate rapidly in enclosed volumes
Carbon monoxide builds up from the welding process

Ventilation requirements:

Supply fresh air at a rate that maintains O2 above 19.5%
For GMAW welding with argon: typical requirement is 2,000-3,000 CFM depending on space volume
Position supply air to push fumes and heavy gases out the exit point
Don’t just blow air in; create airflow through the space (supply air in, extraction out)
Run ventilation continuously before, during, and after welding
Position the exhaust to capture fumes near the welding point

Lockout/Tagout (LOTO)

Before entering any confined space for welding:

Isolate all energy sources: Electrical, mechanical, hydraulic, pneumatic
Blank or disconnect all piping: Prevent introduction of hazardous materials through pipes
Lock and tag all isolation points: Each entrant should apply their own lock
Verify isolation: Test or attempt to start equipment to confirm it’s de-energized

During Welding Operations

The Attendant

An attendant must be stationed at the entry point at all times while workers are inside. The attendant:

Monitors the atmosphere from outside (many spaces allow remote monitoring)
Maintains communication with entrants (voice, radio, or visual signals)
Tracks who is inside and how long they’ve been working
Does NOT enter the space under any circumstances, even to attempt rescue
Calls for rescue if an emergency occurs
Orders evacuation if conditions deteriorate (atmosphere alarm, ventilation failure, fire)

The attendant must have the training and authority to pull the plug on the entire operation if conditions change.

Continuous Atmospheric Monitoring

During welding, the atmosphere changes rapidly. Continuous monitoring (not periodic) is required:

The 4-gas monitor should be worn by the entrant at breathing zone height
Set alarms at action levels (typically 19.5% O2 low alarm, 10% LEL, 35 ppm CO, 10 ppm H2S)
If any alarm sounds, stop welding and evaluate. If the condition doesn’t improve with ventilation, evacuate

Electrical Safety in Confined Spaces

Welding inside a metal enclosure amplifies electrical shock risk:

Use a Voltage Reducing Device (VRD) on the welding machine
Keep welding cables in good condition; no damaged insulation
Place the electrode holder on a dry insulated surface when not in use (not on the grounded vessel)
Wear dry leather gloves at all times
Stand on dry insulating material if the floor is conductive
Never change electrodes while in contact with the grounded structure

Fume Extraction

Position a local exhaust ventilation (LEV) source capture hood within 12-18 inches of the arc. This captures fumes at the source before they disperse throughout the confined space. General ventilation alone may not reduce breathing zone concentrations below PELs, especially for stainless steel or other high-fume base metals.

Rescue Planning

Every confined space entry must have a rescue plan in place before the first person enters. This is not optional.

Rescue Options

Non-entry rescue (preferred): A retrieval system (tripod, davit arm, or anchor point) with a body harness and retrieval line on each entrant. The attendant or rescue team can extract the entrant without entering the space. This is the safest method and should be used whenever the space configuration allows it.

Entry rescue: A trained, equipped rescue team enters the space to retrieve the victim. This requires:

Rescue team members trained and drilled in confined space rescue
Self-contained breathing apparatus (SCBA) or supplied air respirators
Rescue equipment pre-staged at the entry point
Practice entries in similar spaces

Outside rescue services: If using fire department or contract rescue services, they must be notified in advance and must be able to respond in a timeframe appropriate to the hazards.

What Goes Wrong

Most confined space fatalities involve one or more of:

No atmospheric testing before entry
Ventilation failure during work
Rescue attempt by untrained, unequipped bystanders (leading to multiple fatalities)
Failure to continuously monitor the atmosphere
Improperly isolated piping or energy sources

The pattern repeats: a worker collapses inside, a co-worker rushes in without equipment, and both die. Breaking this cycle requires disciplined adherence to the permit system and rescue plan.

Specific Considerations for Welding Processes

SMAW in Confined Spaces

Electrode coating produces significant fume volume per pass
CO generation from cellulose electrodes (E6010) is high
No shielding gas displacement risk, but fumes require aggressive ventilation
Used electrode stubs create a tripping hazard in tight spaces

GMAW/FCAW in Confined Spaces

Argon and argon/CO2 mixtures are heavier than air and accumulate at the lowest point
CO2 in high concentrations (above 3-4%) causes rapid breathing, headache, and eventual unconsciousness
Pure argon displaces oxygen without warning symptoms (no smell, no irritation)
Continuous O2 monitoring at floor level is critical

GTAW in Confined Spaces

Pure argon shielding gas (same displacement risks as GMAW)
Lower fume generation rate than SMAW or FCAW
Back-purge gas (argon inside pipe) adds to oxygen displacement
Tungsten grinding should be done outside the confined space

Confined space welding is manageable with proper planning, equipment, and discipline. The risks are real but they’re also well understood. Follow the permit system, test the atmosphere, ventilate continuously, keep an attendant at the opening, and have a rescue plan that you’ve actually practiced. The paperwork isn’t bureaucracy. It’s the checklist that keeps people alive.

Frequently Asked Questions

What makes a space a confined space?

OSHA defines a confined space as a space that is large enough to enter and perform work, has limited means of entry or exit, and is not designed for continuous occupancy. Examples include tanks, vessels, silos, storage bins, hoppers, vaults, boilers, pipes large enough to enter, and pits. If the space additionally has a hazardous atmosphere, engulfment risk, or entrapment risk, it becomes permit-required.

Can you MIG weld in a confined space?

Yes, but GMAW (MIG) in confined spaces requires forced ventilation to remove shielding gas (argon, CO2, or mixtures) that displaces oxygen. Argon is heavier than air and accumulates in low points. CO2 in high concentrations causes asphyxiation. Continuous atmospheric monitoring for oxygen level (19.5-23.5%) is mandatory, and the ventilation must run constantly during welding.

How often should atmospheric testing be done in a confined space?

Initial atmospheric testing must be done before entry. Continuous monitoring is required during welding operations because the arc produces fumes and gases that change the atmosphere rapidly. At minimum, monitor oxygen (19.5-23.5%), lower explosive limit (below 10% LEL), and toxic gases (CO, H2S, and others depending on the space history). Test at multiple levels since gases stratify by density.