--- title: "OSHA Confined Space Limits: O₂, LEL, CO, H₂S | Aethair" description: "Confined-space atmospheric limits for oxygen, LEL, CO, and H₂S: acceptable ranges, common alarm setpoints, who must test, and a recommended configuration." url: https://aethair.io/standards/confined_space/ --- 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)**. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure OSHA confined space Monitoring](https://aethair.io/build/?params=o2,lel,co,h2s) At a glance **US OSHA 1910.146** Occupational: confined-space entry 4 parameters · 4 limit values [Oxygen (O₂)](https://aethair.io/parameters/o2/) [Combustible (LEL)](https://aethair.io/parameters/lel/) [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) [Hydrogen Sulfide (H₂S)](https://aethair.io/parameters/h2s/) 1 device measures them all - **Thiamis** RAE Gas Detectors [Official text from US OSHA 1910.146](https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.146) 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. [**Oxygen (O₂)** Gases & VOCs](https://aethair.io/parameters/o2/) **19.5–23.5 %** *Acceptable range* [**Combustible (LEL)** Gases & VOCs](https://aethair.io/parameters/lel/) **10 %LEL** *Action level* [**Carbon Monoxide (CO)** Gases & VOCs](https://aethair.io/parameters/co/) **35 ppm** *Common alarm setpoint* not set by 1910.146, which defers to the [OSHA PEL](https://aethair.io/resources/osha-air-quality-standards-compliance/) (50 ppm, 8-hour TWA); 35 ppm is the [NIOSH REL](https://aethair.io/standards/niosh_rel/) and a common gas-detector alarm setpoint [**Hydrogen Sulfide (H₂S)** Gases & VOCs](https://aethair.io/parameters/h2s/) **10 ppm** *Common alarm setpoint* not set by 1910.146, which defers to the OSHA limit (a 20 ppm ceiling); 10 ppm is a common gas-detector alarm setpoint 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. [Customize this configuration](https://aethair.io/build/?params=o2,lel,co,h2s) ### Who measures each parameter | Parameter | Aethair PRO | Aethair IAQ | External | | - | - | - | - | | [Oxygen (O₂)](https://aethair.io/parameters/o2/) | O₂ cartridge | — | [3 instruments](https://aethair.io/parameters/o2/) via Thiamis | | [Combustible (LEL)](https://aethair.io/parameters/lel/) | — | — | [1 instrument](https://aethair.io/parameters/lel/) via Thiamis | | [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) | CO (electrochemical), MultiGas+ +1 cartridges | — | [3 instruments](https://aethair.io/parameters/co/) via Thiamis | | [Hydrogen Sulfide (H₂S)](https://aethair.io/parameters/h2s/) | H₂S cartridge | — | [4 instruments](https://aethair.io/parameters/h2s/) 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 [**Aethair PRO** Built in + Aethair CO (electrochemical), Aethair O₂, Custom MultiSense (H₂S) 3 of 4: O₂, CO, H₂S](https://aethair.io/products/pro/) Covers all 4 on its own [**RAE** Gas Detectors](https://aethair.io/sensors/rae-gas-detectors/) Covers part of it (combine on one Thiamis) [**TSI GM460** Gas Detectors 3 of 4: CO, H₂S, O₂](https://aethair.io/sensors/tsi-gas-detectors/) [**RKI Instruments GX6000** Gas Detectors 3 of 4: CO, H₂S, O₂](https://aethair.io/sensors/rki-instruments-gas-detectors/) [**AMETEK Brookfield Jerome** Specialty Air Analyzers 1 of 4: H₂S](https://aethair.io/sensors/ametek-brookfield-specialty-air-analyzers/) Solutions ## Solutions Judged Against OSHA confined space ### [Landfill, Biogas & Odor](https://aethair.io/solutions/landfill-biogas/) Methane, H₂S, and odor complaint response ### [Occupational Safety & Industrial Hygiene](https://aethair.io/solutions/occupational-safety/) Worker exposure, confined space, and heat stress [All solutions](https://aethair.io/solutions/) 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.