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Oxygen (O). Oxygen is a diatomic element (exists naturally as O2) that is an odorless, tasteless and colorless gas making up 21.5% of the air we breathe. It is not flammable or toxic, but does support combustion. Oxygen can be liquefied, becoming a slightly bluish liquid with a boiling point of –183 degrees Celsius. It has a vapor density (gas) of 1.429 and it may explode (liquid) on contact with heat or oxidizable materials. Therapeutic overdoses can cause convulsions.
Flammable atmospheres in confined spaces usually result from methane, also known as natural gas, hydrogen sulfide and carbon monoxide. Other flammable vapors can also be present as a result of spills or leaks from other locations. Most gases and vapors are heavier than air and may enter confined spaces underground when released.
Even if flammable vapors are present, there must be a proper mixture of flammable vapor and oxygen in the presence of an ignition source for ignition to occur. This mixture of oxygen and flammable vapor is known as the flammable range. Flammable range is measured on a scale from 0% to 100%. On this scale, each flammable vapor has a range in which there is a proper mixture of oxygen and vapor for combustion to occur.
Flammable ranges are composed of a lower explosive limit (LEL) and an upper explosive limit (UEL). Monitoring instruments are available that measure for a percentage of the LEL. When 10% of the LEL is reached, response personnel should not enter the confined space until fresh air is pumped in. The danger of introducing fresh air into a confined space where a flammable vapor is present is that you may well bring the mixture into its flammable range, if it was not already. Therefore, after the induction of air and during entry, the space must be continually monitored to detect changes in the atmosphere that may present a danger to personnel.
Methane (CH4). Methane is a colorless, odorless and tasteless gas that is lighter than air, with a vapor density of 0.554. It is an asphyxiant gas that is flammable with a flash point of –306 degrees Fahrenheit and an autoignition temperature of 1,000F. Its flammable range is 5-15% in air.
Gasoline fumes. Gasoline is a mixture of liquid hydrocarbons. Different manufacturers produce different formulations affecting physical characteristics. As a liquid, gasoline is very flammable and volatile (readily forms vapor at ambient temperatures) with vapors that are heavier than air and can enter below ground and other confined spaces. The boiling and flash points are well below zero.
Propane (C3H8). Propane is a colorless liquefied gas that is highly flammable, a potential asphyxiant, and heavier than air so it can pool in below ground confined spaces. It has a boiling point of –42.5C and a flash point of –150F. The ignition temperature for propane is 874F. Propane has a flammable range of 2.4% to 9.5% in air.
Toxic vapors in a confined space can also be dangerous to workers and rescuers. Examples of toxic gases that have caused deaths in workers in confined spaces are carbon monoxide, hydrogen cyanide, and hydrogen sulfide (all also flammable), chlorine and ammonia. Ammonia is particularly dangerous in confined spaces because it is fairly easy to be found within its flammable range in confined areas, so in addition to toxicity, you need to worry about flammability.
Vapors can accumulate in confined spaces naturally (residue) or from chemical or biological process, other leaks and spills, through the use of other chemicals to clean or process tanks, and from welding or other hot work operations within a tank or space. Some gases such as hydrogen sulfide are extremely toxic and can cause paralysis of the olfactory system (loss of sense of smell), loss of reasoning, respiratory failure, unconsciousness and death. When the sense of smell is lost the person in the space thinks the hazardous material is gone when in fact it is not, they just can’t smell it any longer. Here are some examples:
Carbon monoxide (CO). Carbon monoxide is a colorless, odorless gas that is slightly lighter than air at 0.96716 (air equals 1.0). It is highly toxic by inhalation, highly flammable and has a wide flammable range of 12 to 75% in air. Carbon monoxide is a chemical asphyxiant that displaces oxygen in the blood. It has an affinity for blood over 200 times that of oxygen.