Monitoring for Oxygen


T.R. Consulting, Inc.

November 2003 Safety Article

Written and compiled by:

Tony Rieck

Copyright 2003 T.R. Consulting, Inc.



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Note: T.R. Consulting, Inc. presents the information contained in this article as an aid in understanding of the subject matter. Referenced standards must be read and thoroughly understood in order to assure compliance with the standard. T.R. Consulting, Inc. attempts to provide accurate information, but makes no warranty with regard to either the completeness or accuracy of the information contained herein.



What Is Oxygen?


Oxygen is a colorless odorless and tasteless gas that makes up approximately 20.9% of the air that we breathe.  There are three commonly referenced forms of oxygen:


- Elemental oxygen is a single oxygen molecule.  Elemental oxygen is very reactive and is a component of hundreds of thousands of organic compounds and will combine with most elements.

- The oxygen we breathe is a pair of oxygen molecules bonded together and is represented chemically as O2

- Triatomic Oxygen (O3) is also called ozone.  Ozone plays an important part in the filtering of UV radiation from the sun in the upper atmosphere, but is considered a toxic air contaminant when emitted into the air at this level of the atmosphere.




Oxygen Deficiency


The body requires oxygen to live, if the oxygen concentration decreases, the body reacts in various ways. Death occurs rapidly when the concentration decreases to 6%.  Oxygen can be reduced in the atmosphere through chemical reduction reactions, combustion, and displacement with atmospheric contaminants or inert gases.



          Physiological Effect of Oxygen Deficiency


% Oxygen (by volume)

At sea Level                      Effects

--------------------              ----------------------

21                                Nothing abnormal


16 - 21                           Increased breathing volume.  Accelerated heartbeat. Impaired attention and thinking. Impaired coordination.


14 - 10                           Very faulty judgment. Very poor muscular coordination. Muscular exertion brings on rapid fatigue that may cause permanent heart damage. Intermittent respiration.


10 - 6                            Nausea. Vomiting. Inability to perform vigorous movement, or loss of all movement.  Unconsciousness, followed by death.


<6                                Spasmodic breathing. Convulsive movements. Death in minutes.




Physiological effects of oxygen deficiency are not apparent until the concentration decreases to 16%. The various regulations and standards dealing with respirator use recommend that percentages ranging from 16-19.5% be considered indicative of an oxygen deficiency. Such numbers take into account individual physiological responses, errors in measurements, and other safety considerations. In hazardous response operations, 19.5% oxygen in air is the figure that decides between air-purifying and atmosphere-supplying respirators.



Oxygen Enriched Atmospheres


An atmosphere is considered to be oxygen enriched when the concentration of oxygen exceeds 23.5%.  Oxygen enriched atmospheres pose a serious risk of fire and explosion because flammable and combustible materials will ignite readily and burn violently.  This is why you will see signs where oxygen is in use, such as at hospitals, reading “No Smoking, Sparks or Open Flames - Oxygen in Use”.  In oxygen bottling plants, special static preventive footwear, clothing and flooring is required to prevent a static spark from causing ignition (for example, clothing or hair could catch fire due to the simple act of removing work clothes at the end of a shift).  Oxygen enriched atmospheres are relatively uncommon unless certain chemicals are encountered that will produce oxygen upon decomposition (i.e. hydrogen peroxide) or oxygen storage systems are present.




Importance of Oxygen Monitoring


Since we require oxygen to live and oxygen (or a lack thereof) cannot be detected with the senses, we need a reliable means of determining the oxygen content of an atmosphere before we enter it.  Additionally, determining the level of flammability associated with an atmosphere is also oxygen dependant as combustible gas indicator readings are unreliable unless appropriate concentrations of oxygen are present (19.5% to 23.5%).


At rest, a person inhales six to eight liters of air per minute.  At the height of exertion, a person may breathe as much as 75 liters of air per minute.  As one’s activity level increases, so too does one’s body demand more oxygen.  Since work does not typically involve lying on the couch and eating bon bons while watching one’s favorite talk show host (being at rest), it is assumed that some exertion will be taking place.  Also, since individuals can be more sensitive to oxygen depletion and because of the potential ranges of instrumentation error, regulations require oxygen concentrations in the workplace to exceed the level at which physiological effects of oxygen depletion are expected to occur.  Thus, even though physiological effects of oxygen deficiency are not expected to manifest themselves at oxygen concentrations greater than 16%, a work atmosphere must contain no less than 19.5% oxygen ( a little “breathing” room) to allow for worker occupancy without the use of air-supplied respirators.


How Oxygen Monitors Work


A typical oxygen monitor is an electronic box containing a circuit board, display screen, battery, pump or fan unit, and an oxygen sensor cell.  The fan or pump draws a metered volume of air over the oxygen sensor cell.  The oxygen sensor cell is a sealed plastic electrochemical transducer containing an anode and a cathode immersed in an electrolytic solution such as potassium hydroxide (KOH).  The sensing surface of the oxygen sensor cell is a diffusion membrane (typically Teflon)that allows the oxygen to diffuse into the cell.  The resultant reaction causes an electrical charge that is proportional to the amount of oxygen that has diffused into the cell.



Inert Atmospheres


A common method for reducing hazards from flammable vapors and gases is the introduction of an inert gas, such as carbon dioxide or nitrogen, into a container to displace the oxygen.  Since combustible gas indicator readings are unreliable in oxygen deficient atmospheres, the only way to be certain that the space has been properly inerted is to use an oxygen meter.  It is important to remember that inerting is a process that removes oxygen from an atmosphere; the fuel (usually a flammable vapor or gas) is still present.  Once inerted, any task that could allow the reintroduction of air will result in a potentially explosive situation.  Inerted containers should be provided with a pressure relief valve (to prevent over pressurization) and pressure gage, pressurized slightly (no more than 1 psig) with the inert gas, and sealed. 


* Special attention should be paid to inerted containers that are stored indoors as any leakage into the storage room could cause a reduction of oxygen in the room. 


* An inerted container should never be considered safe for cutting.  As soon as the cutting begins, air is again free to mix with the atmosphere in the container.


* A rule of thumb for determining the appropriate level of oxygen to consider a container inert is that the available oxygen should be no more than half of the oxygen required for the ignition of the stored material.  For example, many common petroleum products can be ignited in atmospheres containing as little as 11% oxygen.  Thus, in order to assure a truly inert atmosphere, an oxygen reading of somewhat less than 6% would be required.  Note: this point also demonstrates that fires and explosions can occur in atmospheres containing far less oxygen than one requires for breathing.



Oxygen Monitor Limitations


Like any other monitoring instrument, the oxygen monitor has a standard calibration error and there are components that can age and cause inaccurate readings.


* Calibration error - many manufacturers will instruct the user to calibrate the instrument to 21% in fresh air.  Since the actual concentration of oxygen at sea level is 20.9% and can vary slightly due to altitude, a slight error has already been introduced.  Additionally, some oxygen monitors will self calibrate each time that they are turned on.  If they turn on while being worn in an atmosphere containing somewhat less than normal oxygen, the error will be even greater.


* Oxygen sensor cell degradation - aging of the oxygen sensor cell will cause the oxygen meter to take longer to come to a stable reading after initially being turned on.  As the cell ages more, the instrument will require frequent calibration adjustments and will provide low readings.  Moisture and carbon dioxide can shorten the life of an oxygen sensor cell.  On most instruments, this cell can be easily replaced by removing a cover, loosening two screws, removing the old cell, placing the new cell into the meter, tightening the screws and replacing the cover.


* Altitude of calibration - the meter should always be calibrated at the altitued and under the proximate conditions that it will be used.  A meter calibrated at 5,000 feet elevation may alarm, indicating an oxygen deficient atmosphere, at 9,000 feet.  Certainly, the meter would be less accurate.


* Battery - always check the battery to assure that it has an adequate charge.  Dead battery = no reading + no alarm = no clue.



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