08 Jul Hazardous Atmospheres in Confined Spaces
Hazardous Atmospheres in Confined Spaces
Confined spaces can contain hazardous atmospheres capable of causing serious injury, unconsciousness, fire, explosion, or death within minutes. Because many atmospheric hazards cannot be detected by sight or smell, employees must never assume that a confined space is safe to enter.
Examples of confined spaces include tanks, vessels, pits, vaults, manholes, sewers, silos, bins, and similar spaces that are large enough to enter, have limited or restricted means of entry or exit, and are not designed for continuous employee occupancy.
Atmospheric Hazards
A confined space may contain or develop a hazardous atmosphere due to:
- Oxygen deficiency – less than 19.5% oxygen.
- Oxygen enrichment – greater than 23.5% oxygen, which significantly increases fire hazards.
- Flammable gases, vapors, or mists capable of creating a fire or explosion.
- Toxic gases, vapors, fumes, or airborne contaminants at hazardous concentrations.
- Displacement of oxygen by gases such as nitrogen, argon, helium, or carbon dioxide.
- Work performed inside the space, including welding, cutting, painting, cleaning, or the use of chemicals and combustion-powered equipment.
Atmospheric conditions can change while work is underway. Testing before entry does not guarantee that the atmosphere will remain safe.
Atmospheric Testing
Before entering a permit-required confined space, the atmosphere must be evaluated using properly calibrated direct-reading instruments.
Unless another order is necessary for the specific hazard, test for:
- Oxygen concentration.
- Flammable gases and vapors.
- Potential toxic contaminants.
Employees must not enter until required testing has been completed and acceptable entry conditions have been established.
Atmospheric monitoring shall continue as necessary throughout the entry. Continuous monitoring should be used when conditions could change or when required by the confined-space entry procedure.
Employees must immediately exit the space if monitoring indicates a prohibited atmospheric condition or if an alarm activates.
Carbon Monoxide
Carbon monoxide (CO) is a colorless, odorless, and toxic gas commonly produced by incomplete combustion. Gasoline-, diesel-, propane-, and other fuel-powered engines, heaters, generators, and equipment can produce dangerous concentrations.
Carbon monoxide interferes with the body’s ability to transport oxygen and can cause headache, dizziness, confusion, unconsciousness, and death.
Never operate combustion-powered equipment in or near a confined space where exhaust could enter unless appropriate ventilation, atmospheric monitoring, and other controls have been established.
Hydrogen Sulfide
Hydrogen sulfide (H₂S) is a colorless, highly toxic, and flammable gas that may occur in sewers, wastewater systems, pits, tanks, oil and gas operations, and locations containing decomposing organic material.
At lower concentrations, H₂S may smell like rotten eggs. Never rely on odor as a warning. Exposure can rapidly impair the sense of smell, making the gas appear to have disappeared even though dangerous concentrations remain.
Hydrogen sulfide is generally heavier than air and may accumulate in low areas of confined spaces.
Carbon Dioxide
Carbon dioxide (CO₂) is a colorless, odorless gas that can accumulate from fermentation, decomposition, combustion, process operations, or fire-suppression systems.
High concentrations can displace oxygen and can also have direct harmful physiological effects. A space containing carbon dioxide must be properly evaluated and monitored before entry.
Ammonia
Ammonia is a colorless gas with a strong, irritating odor. Exposure can cause severe irritation or burns to the eyes, skin, nose, throat, and respiratory system.
Although ammonia generally provides noticeable warning at lower concentrations, odor must never be used as a substitute for atmospheric testing. A suspected ammonia leak must be investigated and controlled before employees enter the affected space.
Refrigerants
Refrigerants can create serious confined-space hazards when released from refrigeration or cooling systems. Depending on the refrigerant, hazards may include toxicity, flammability, chemical decomposition products, or displacement of oxygen.
Many refrigerant vapors can accumulate in low areas. Employees must know the specific refrigerant involved, review its Safety Data Sheet (SDS), and use appropriate detection, ventilation, PPE, and emergency procedures.
Welding and Cutting Gases
Welding and cutting operations can quickly change the atmosphere inside a confined space. Hazards may result from:
- Oxygen enrichment or depletion.
- Shielding gases such as argon or helium displacing breathable air.
- Fuel gases such as acetylene accumulating and creating a fire or explosion hazard.
- Welding fumes and gases reaching hazardous concentrations.
Compressed-gas cylinders should generally remain outside the confined space. Torches and hoses must be removed from confined spaces when not in use or when employees leave the space, as required by applicable procedures.
Adequate ventilation and atmospheric monitoring must be maintained during hot work.
Ventilation and Respiratory Protection
When hazardous atmospheric conditions can be controlled through ventilation, clean air shall be supplied as necessary to maintain acceptable entry conditions. Ventilation equipment must be positioned so that contaminated exhaust is not drawn back into the space.
Employees shall not use respirators for confined-space entry unless they are authorized, medically evaluated, fit-tested when applicable, trained, and working under the company’s Respiratory Protection Program.
An atmosphere that is immediately dangerous to life or health (IDLH) requires specialized respiratory protection, rescue capability, and entry procedures.
Never Rely on Your Senses
A hazardous atmosphere may:
- Have no visible indication.
- Have no detectable odor.
- Overcome a worker before there is time to escape.
- Become hazardous after entry.
- Accumulate differently at the top, middle, or bottom of a space.
Never enter a confined space based solely on how the atmosphere looks, feels, or smells.
Remember
Before entering a confined space, identify the hazards, test the atmosphere, establish acceptable entry conditions, provide required ventilation and controls, and follow the company’s confined-space entry procedures. If atmospheric conditions become unsafe or an alarm activates, exit immediately.