Oxygen Deficiency
The normal atmosphere is composed approximately of 20.9% oxygen, 78.1% nitrogen, and 1% argon with small amounts of various other gases.
Oxygen Level Chart
- Oxygen deprivation is one form of asphyxiation. While it is desirable to maintain the atmospheric oxygen level at 21% by volume, the body can tolerate deviation from this ideal.
- When the oxygen level falls to 17%, the first sign of hypoxia is a deterioration of night vision which is not noticeable until a normal oxygen concentration is restored. The physiological effects are increased breathing volume and accelerated heartbeat.
- Between 14-16%,the physiological effects are increased breathing volume, accelerated heartbeat, very poor muscular coordination, rapid fatigue, and intermittent respiration.
- Between 6-10%, the effects are nausea, vomiting, inability to perform, and unconsciousness.
- Less than 6%, spasmodic breathing, convulsive movements, and death in minutes.
Oxygen Consumption and Displacement
Reduction of oxygen in a confined space may be the result of either consumption or displacement.
Consumption: The consumption of oxygen takes place during combustion of flammable substances, as in welding, heating, cutting, and brazing. A more subtle consumption of oxygen occurs during bacterial action, as in the fermentation process.
Confined spaces that come into contact with groundwater often have a greater risk of oxygen deficiency. This is common during trenching, excavation, tunnel construction, sewer work, and mining operations. Although groundwater does not use oxygen by itself, it can create conditions that reduce oxygen levels.
Oxygen may also be consumed during chemical reactions as in the formation of rust on the exposed surface of the confined space (iron oxide). The number of people working in a confined space and the amount of their physical activity will also influence the oxygen consumption rate.
- Worker respiration: Workers breathe in oxygen and exhale carbon dioxide. Oxygen levels can decrease in small, poorly ventilated spaces, especially when several workers are present.
- Welding, cutting, brazing, and heating: These hot work activities consume oxygen during combustion.
- Internal combustion engines: Gasoline, diesel, propane, and natural gas engines use oxygen while operating and can also produce carbon monoxide.
- Rust formation (oxidation): Iron and steel surfaces consume oxygen as they rust, especially in damp or flooded areas.
- Bacterial decomposition: Microorganisms consume oxygen as they break down sewage, sludge, manure, food waste, or other organic materials.
- Fermentation: The breakdown of organic materials in tanks, silos, wineries, breweries, and food-processing facilities uses oxygen.
- Composting: Decaying vegetation and organic waste can lower oxygen levels in pits, bins, and storage areas.
- Groundwater and wet soils: Biological activity in groundwater and saturated soils can gradually reduce oxygen in trenches, tunnels, wells, and excavations.
- Oxidation of minerals: In mines and tunnels, exposed rock and certain minerals can consume oxygen through natural chemical reactions.
- Chemical curing processes: Some paints, coatings, adhesives, and resins use oxygen as they cure.
- Fires or smoldering materials: Combustion rapidly consumes oxygen while producing toxic gases.
Displacement: A second factor in oxygen deficiency is displacement by another gas. Examples of gases that are used to displace air, and therefore reduce the oxygen level, are helium, argon, carbon dioxide, and nitrogen.
The use of nitrogen to inert a confined space has claimed more lives than carbon dioxide. The total displacement of oxygen by nitrogen will cause immediate collapse and death.
Carbon dioxide and argon, with specific gravities greater than air, may lie in a tank or manhole for hours or days after opening. Since these gases are colorless and odorless, they pose an immediate hazard to health unless appropriate oxygen measurements and ventilation are adequately carried out.
- Nitrogen: Used to inert tanks, pipelines, and process vessels before maintenance. Nitrogen is colorless, odorless, and can quickly create an oxygen-deficient atmosphere.
- Argon: Commonly used as a shielding gas during welding. Because it is heavier than air, it can collect in low areas.
- Carbon dioxide (CO2): Used in fire suppression systems, beverage production, dry ice applications, and fermentation. It can accumulate in pits, tanks, and manholes.
- Helium: Used for leak testing and specialized industrial processes. It can displace oxygen even though it is lighter than air.
- Natural gas or methane: Leaks from pipelines, landfills, sewers, or mining operations can displace oxygen and create both asphyxiation and fire hazards.
- Propane: Leaks from fuel systems can settle in low areas because propane is heavier than air.
- Hydrogen: Released during battery charging or certain industrial processes. It can displace oxygen and create an explosion hazard.
- Steam: Steam used for cleaning or sterilizing tanks and vessels can temporarily displace air and reduce oxygen levels.
- Refrigerant gases: Leaks from refrigeration systems can displace oxygen in enclosed mechanical rooms or storage areas.
- Fire suppression gases: Clean-agent systems and other suppression gases may reduce oxygen concentrations enough to create an asphyxiation hazard.
Knowledge Check Choose the best answer for the question.
4-3. At what percentage of oxygen do accelerated heartbeat and poor muscular coordination occur?
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