Compliance · US OSHA 1910.146
Confined-Space Atmospheric Limits
Occupational: confined-space entry
A confined space (a tank, vessel, silo, sewer, pit, vault, or similar enclosure not designed for continuous occupancy) can hold a lethal atmosphere that gives no warning at the entrance. Confined-space safety rules, such as OSHA's permit-required confined space standard and equivalents worldwide, require the air to be tested before anyone enters and monitored continuously while they are inside. Four atmospheric hazards define the core tests: oxygen (too little suffocates, too much raises fire risk), combustible gas or vapor measured as a percentage of the lower explosive limit (LEL), and toxic gases, principally carbon monoxide (CO) and hydrogen sulfide (H₂S).
The standard
What Confined-space Atmospheric Limits Are
These limits exist because confined-space atmospheres are a leading cause of multiple-fatality workplace incidents, often killing would-be rescuers as well as the original entrant.
Limit values
OSHA confined space Limit Values
Every value US OSHA 1910.146 publishes for the 4 parameters below, with its averaging period, as the standard states it.
Who it applies to
Who Must Comply
Employers and contractors whose workers enter confined spaces across water and wastewater, oil and gas, chemical processing, agriculture (silos and manure pits), utilities, marine, and construction all fall under these rules. The obligations include atmospheric testing by a competent person, continuous monitoring during entry, ventilation, and a rescue plan. Because the hazards are invisible and fast-acting, the gas monitor is not optional equipment. It is the primary safeguard, and its correct calibration and alarm settings are a matter of life and death.
Monitoring
How Monitoring Demonstrates Compliance
Confined-space entry uses a multi-gas monitor that measures oxygen, combustible gas (LEL), and the toxic gases simultaneously, with audible and visual alarms set to the entry thresholds. Testing follows a defined order (oxygen first, then combustibles, then toxics) and samples the space at different depths before entry, since gases stratify by density. During occupancy the monitor runs continuously in the breathing zone. Sensors are electrochemical (for O₂, CO, and H₂S) and catalytic or infrared (for LEL), and require regular bump-testing and calibration to remain trustworthy.
Recommended
best match-
Thiamis connects 1 instrument
A gateway that connects the third-party instruments you choose to the Aethair Platform.
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Carbon Monoxide (CO)Hydrogen Sulfide (H₂S)Combustible (LEL)Oxygen (O₂)RAE Systems (Honeywell) portable gas detectors.
Also measures 8 more
Methane (CH₄)Chlorine (Cl₂)Carbon Dioxide (CO₂)Ammonia (NH₃)Nitric Oxide (NO)Nitrogen Dioxide (NO₂)Sulfur Dioxide (SO₂)VOC (Total) -
Who measures each parameter
| Parameter |  Aethair PRO |  Aethair IAQ | External |
|---|---|---|---|
| Oxygen (O₂) | O₂ cartridge | — | 3 instruments via Thiamis |
| Combustible (LEL) | — | — | 1 instrument via Thiamis |
| Carbon Monoxide (CO) | CO (electrochemical), MultiGas+ +1 cartridges | — | 3 instruments via Thiamis |
| Hydrogen Sulfide (H₂S) | H₂S cartridge | — | 4 instruments via Thiamis |
Devices
Devices That Cover OSHA confined space
How much of this standard one Aethair monitor covers, followed by the third-party instruments in the sensor catalog that measure its parameters. Each connects to Environet through a Thiamis gateway, which adds whatever the monitor leaves out.
Aethair monitors
Covers all 4 on its own
Covers part of it (combine on one Thiamis)
Solutions
Solutions Judged Against OSHA confined space
Landfill, Biogas & Odor
Methane, H₂S, and odor complaint response
Occupational Safety & Industrial Hygiene
Worker exposure, confined space, and heat stress
Questions
OSHA confined space FAQ
What are the four gases tested in a confined space?
Oxygen, combustible gas (measured as % of the lower explosive limit), and the toxic gases carbon monoxide and hydrogen sulfide. Oxygen is tested first because combustible and toxic sensors depend on adequate oxygen, then combustibles, then toxics. Additional gases may be added depending on the specific hazard.
Why test the atmosphere before and during entry?
Conditions change: ventilation, work activity, and disturbed sludge or residues can release gas or consume oxygen after entry begins. Pre-entry testing confirms it is safe to enter; continuous monitoring during occupancy catches any deterioration in time to evacuate. Many confined-space deaths occur when only a pre-entry test was done.
Why do confined-space incidents often kill more than one person?
Because the atmosphere is invisible and incapacitates quickly, and untrained rescuers rushing in without their own monitor and breathing protection are overcome by the same hazard. This is why the rules require a rescue plan and continuous monitoring rather than reactive rescue.
How do I monitor a confined space?
Use a multi-gas monitor covering oxygen, LEL, CO, and H₂S with alarms set to the entry limits, bump-tested and calibrated. Use the configurator to build a confined-space gas panel and add any site-specific gases your process requires.
Monitor Against OSHA confined space
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