Oxygen: Cylinders, Concentrations, Grades, and Labels


Larry "Harris" Taylor, Ph.D.  


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The use of oxygen, particularly in recreational gas mixes, is often a topic for debate, posturing, and rhetoric. This article summarizes the various terms and usage associated with compressed gas cylinders containing oxygen for human respiration and medical procedures.




Oxygen is a chemical element that exists in the atmosphere as a stable molecule containing two atoms (O2). The element has an atomic weight of 16; the molecular weight is 32.   


Oxygen is one of the most abundant chemical elements on earth. Like most gases, oxygen is colorless, odorless, and tasteless. It is chemically reactive and readily combines with a variety of materials. Although by itself oxygen is not combustible, the ease with which it reacts with other materials can lead to fires and explosions if improperly handled. Mixtures above 21-23.45 % O2 are considered oxygen-enriched atmospheres by the Compressed Gas Association (CGA) and the National Fire Protection Association (NFPA). The risk of fire/explosion increases with increasing oxygen partial pressure and/or concentration. Handling oxygen-enriched atmospheres requires specialized equipment and training.


Oxygen is essential for life. The body uses chemical reactions based on oxygen to generate heat and chemical energy. This process, called metabolism, keeps us alive. The oxygen in the breathing gas must be maintained within certain limits. Too little oxygen (hypoxia) can be fatal. Too much oxygen (hyperoxia) can lead to CNS (Central Nervous System) oxygen toxicity with the possibility of seizures at depth or a more long term development, whole body or pulmonary oxygen toxicity. These toxicity concerns for the oxygen provider are discussed here.


Almost all the oxygen (as well as nitrogen and argon) manufactured in the US is prepared from the fractional distillation of liquid air. In this process, air is chilled to a liquid state and then gradually warmed so that oxygen, nitrogen and argon  (which have different boiling points) can be separately collected and packaged. Additionally, a small amount of oxygen is prepared from the electrolysis of water or from solid-state chemical reactions.


USP Entry for Oxygen


The United States Pharmacopoeia (USP) is a listing (compendium) of drugs licensed for use in the United States with the analytical standards necessary to establish purity suitable for human use applied to each of the listed drugs. Drugs sold for human consumption in the US must meet or exceed the purity standards listed in the USP. Drugs meeting these requirements must include a designation on their label that demonstrates compliance with standards listed within the USP; typically, this is shown as the initials USP immediately following the name of the drug. Many nations have their own Pharmacopoeia to list and specify requirements for drugs legally sold within their borders.


To be labeled Oxygen, USP, the gas cylinder must contain a documented minimum of 99.0 percent oxygen by volume and be odor free. The label must also specify whether the oxygen gas was produced by the fractional distillation of air (also called air-liquefaction process) or other means. If produced from liquid air, then the cylinder contents may be labeled USP without an analysis for either carbon dioxide (CO2) or carbon monoxide (CO).


It is illegal in the US to sell non-USP drugs (including oxygen) for human consumption.


Oxygen Grades


The US Department of Transportation (DOT) classifies oxygen as a non-flammable compressed gas. The Compressed Gas Association (CGA) has designated various grades of oxygen based on analysis of the contents of an oxygen cylinder.  The table below, unless otherwise specified, lists possible contaminant components in parts per million (ppm) and the analytical standards for each of the various defined grades. A blank entry means that there is no specification or test required for the listed component in the appropriate grade column. 


Grade A is the minimum requirement for USP oxygen. Grade E is commonly called aviator's grade. 


Grade of Oxygen Gas










Oxygen  Min. % (mole)








Water  (v/v)








Dew Point (oF)








































Carbon Dioxide








Carbon Monoxide








Total Hydrocarbons (as methane)








Ethane & Other Hydrocarbons








Nitrous Oxide
























In addition, there are a variety of terms used in the lay community that have been used to describe various grades of oxygen. These sometimes are specific to a local community or state. One of he most intense discussions commonly encountered in discussing oxygen use for diving centers on the term "welding gas." This is primarily only of historical interest.


Many moons ago, gas supply vendors typically maintained two separate storage facilities for oxygen: " welding or industrial or technical" (a non-USP, or illegal to sell for human consumption label) variety and gas intended for human use in respirators or medical procedures (a USP label). Since most gas supply vendors have decided that it is simply not cost-effective to store separate grades of medical gases, almost all oxygen sold in the US is a USP grade (meets USP requirements for human consumption). However, there are distinct differences in how the cylinders are filled. (see below). So, the debate as to whether to use a non-USP welding gas or a USP medical grade oxygen for diving is mostly irrelevant since most vendors are now filling all cylinders with USP gas.


Typically, when filling an incoming oxygen cylinder, the fill station operator opens the cylinder valve and "sniffs" the valve opening. If acetylene is smelled (an indication of improper shut down procedures in a welding operation), the cylinder is set aside to be cleaned at some later time (to remove the odor from acetylene and thus prevent a potential explosion from an acetylene-oxygen mix within the cylinder). If no acetylene is smelled, then a whip is connected to the cylinder and the "welding" cylinder is filled with USP oxygen.


If the vendor fill station operator sees that a cylinder is labeled as a medical or emergency oxygen cylinder, or intended for human respiration, then the cylinder is evacuated (see below) and filled with USP oxygen.


So, the difference between a modern US "welding" oxygen or "medical" or "respiratory" oxygen cylinder is not the quality of the oxygen gas used to fill the cylinder, but the manner in which the cylinder is filled (and the legal consequences of inappropriate fill procedures) and the FDA labeling requirements.


Today in the US:


Most welding or Industrial grade oxygen cylinders are filled with USP oxygen without an evacuation-between-fills step.


Respiratory grade oxygen cylinders are filled with USP oxygen with an evacuation-between-fills step.


Medical grade oxygen cylinders are filled with USP oxygen with an evacuation-between-fills step


(Some states differentiate between a respiratory and medical grade. Typically, the cylinder contents are the exactly the same, but the label is different. Historically, this was done to avoid prescription requirements for emergency/rescue organizations using pure oxygen respirators.)


Aviation grade oxygen cylinders are filled with USP oxygen that has undergone additional drying steps with an evacuation-between-fills step. The low quantity of water is a precaution against oxygen line freezing that might occur with higher water content at the chilled temperatures found at altitude. (Typically, "regulator freeze-up" is a result of chilled water vapor within the gas mix condensing and then freezing ... a chunk of ice may block gas flow or interfere with mechanical operation of valve/regulator mechanisms. So, lowering water vapor content of a gas mixture lessens the likelihood of a "freeze-up.")


Grade 4.5 oxygen is USP oxygen that has a purity greater than 99.995 % oxygen


Grade 5 (or "five nines") oxygen is USP oxygen that is certified 99.999 % pure. It is sometimes called research grade.  This is the highest purity of oxygen manufactured in the US and is typically found only in top-end chemical research facilities. Its non-research use is associated with specialty welding of titanium and titanium alloys for the nuclear or aerospace industry.


For divers, the best oxygen available for deco or oxygen-enriched air breathing mixes is aviation grade oxygen.


For emergency-only (DAN) cylinders, the only legally acceptable gas is medical grade oxygen supplied by an FDA licensed facility. (see legal stuff,  below).


The Cylinders


The use of oxygen, particularly in recreational gas mixes, is often a topic for debate, posturing, and rhetoric. This table  summarizes the various types of  compressed gas cylinders containing oxygen for human respiration and medical procedures.





Oxygen Capacity
Service Pressure
Cylinder Length
Cylinder O.D.
Cylinder Weight
IN. - CM
IN. - MM
122 - 3455
2216 - 153
35.75 - 90.8
8.0 - 203
38.6 - 17.55
61.4 - 1738
2216 - 153
23.0 - 58.4
7.25 - 184
21.7 - 9.86
24 - 680
2015 - 139
25.63 - 65.1
4.38 - 111
7.9 - 3.58
Jumbo D
22.6 - 640
2015 - 139
16.3 - 41.4
5.25 - 133
8.1 - 3.68
15 - 425
2015 - 139
16.51 - 41.9
4.38 - 111
5.3 - 2.41
9 - 255
2015 - 139
11.88 - 27.6
4.38 - 111
3.7 - 1.69
7 - 198
2015 - 139
9.18 -23.3
4.38 - 111
3.3 - 1.48
6 - 165
2015 - 139
7.68 - 19.5
4.38 - 111
2.9 - 1.29
6 - 165
2216 - 153
11.59 - 29.4
3.2 - 81
2.2 - 1.0
4 - 113
2216 - 153
8.4 - 21.3
3.2 - 81
1.6 - .74
1.4 - 40
2216 - 153
5.37 - 13.6
2.5 - 63.5
.74 - .34



The Label


The label from one of my E (~650 liter) cylinders is shown below (large enough to read the print).



The label designates the following:


  Oxygen, Compressed USP

    This cylinder contains compressed oxygen gas that meets the purity standards for human use as defined by the United States Pharmacopoeia (USP).


  UN 1072

    The DOT chemical compound identification number is 1072.  This is a reference to the Material Safety Data Sheet (MSDS)  which lists physical properties, special handling precautions, chemical reactivity, health hazards, waste disposal and transportation requirements for the cylinder contents. (US OSHA regulations mandate that all chemicals sold in the US be accompanied by a MSDS to facilitate safety when handling purchased chemical materials.)


  Yellow Diamond

    This is a reference symbol established by the National Fire Protection Agency (NFPA) to quickly identify the nature of potential hazards associated with the chemical contents of a container. The yellow color implies the contents may contribute to substantial releases of energy. This symbol contains the name of the chemical, Oxygen, and identifies it as a combustion hazard (the fire symbol) with a relative danger of 2 (of 4 possible). The relative danger of 2 suggests a  potential of violent combustion, an increased fire threat typically without immediate detonation.



    This blank allows the vendor to specify volume of contents contained within the cylinder at the time of filling.



    This is this the legal consumer product safety warning. Notice that this label allows filling of emergency-only cylinders without prescription,


  Produced by Air Liquefaction

    Identifies the method of manufacturer that exempts contents from required-for-USP-label carbon dioxide or carbon monoxide analysis.



    Specific hazards associated with the chemical nature of oxygen. The Chemical Abstracts Service (CAS, a chemical organization that abstracts all published chemical journals, as well as assigning unique numbers to all chemicals known as they are discovered) identification number for oxygen (CAS 778244-7) is also provided as a reference to the chemical and physical properties of the cylinder contents. The caution to use in accordance to the MSDS refers to a reference document (the Oxygen MSDS furnished by the manufacturer) that lists known concerns about the safety issues associated with handling the contents of the cylinder.


  Do Not Remove

    FDA requires that drugs always be properly labeled such that the contents are readily identifiable.


  The Vendor

    The label must contain the source of the drug (in this case, Ann Arbor Welding Supply) since this FDA licensed facility will have records required by FDA Good Manufacturing Practices on the handling of the gas contained in the cylinder.


The Legal Stuff


Before mentioning the FDA regulations for compressed medical gases, it should be pointed out that any business in the United States that has employees must meet both Federal and State compliance with respect to Occupational Health and Safety Regulations ( OSHA). Although originally intended for chemical laboratories, an additional program called "chemical hygiene" is observed by many business operations. "Chemical Hygiene" can be interpreted as having safety data (typically as MSDS (Manufacturing Safety Data Sheets)) for all chemicals (including compressed gases) on the premises. In addition, the employer must document that all employees have been briefed on specific hazards and procedures associated with every chemical present at the business location. To document OSHA compliance, various agencies will supply checklists to assist employers in meeting these guidelines. Failure to meet OSHA guidelines can be quite expensive since OSHA has the power to issue daily fines and close a business until compliance is documented. Wise business owners will seek legal advice to understand steps necessary for compliance with local and Federal OSHA regulations.


The US Federal Drug Administration (FDA) classifies oxygen as a compressed medical gas (CMG) and, as such, it must be handled in accordance with the appropriate federal guidelines. Use of oxygen as a CMG is covered by a minimum of 2 separate federal documents: Title 21 CFR Parts 200-211 (drug labeling act) and the FDA Medical Gases Guidelines ( as a PDF file from the FDA )


Basically, most "civilized" nations require that drugs intended for human use be prepared under guidelines that have been called "Good Manufacturing Practice (GMP)." The GMP is a set of protocols supervised in the US by the FDA that require all steps, from initial manufacture to dispensing of the drug to the consumer,  document the purity of the material being distributed. So, each phase of drug distribution must maintain records of drug analysis and handling consistent with the standards defined for the specific drug in the USP and GMP guidelines (analogous to a legal "chain of evidence"). Compressed medical gas cylinders can be legally filled only at a FDA inspected and licensed facility.


For medical gases, including oxygen, the primary GMP protocol that affects divers is the transfilling (moving gas between storage and use cylinders) requirement. The FDA Guidelines for this step state:


Requirements for drug containers:

Section 211.94(c) requires that drug product containers be clean.

Section 211.94(d) requires that standards or specifications, methods of testing, and, where indicated, methods of cleaning be written and followed for drug product containers.



One factor to consider regarding the above requirements is the possible presence of foreign gas residues in CMG cylinders before filling. An acceptable method of assuring that cylinders do not contain foreign gas residues is to pull a vacuum on each cylinder equal to 25 or more inches of mercury prior to filling with the CMG. (Cryogenic vessels are seldom completely emptied and need not be evacuated before filling.)


Most medical cylinders and emergency response respirators using oxygen are used repeatedly; often the same cylinder may be used in a variety of locations with differing practices of handling cylinders. The requirement of always evacuating a cylinder, regardless of cylinder pressure, is a step intended to minimize contamination that might occur from "back-filling" of a cylinder improperly connected to a gas cascade system, or left open to the atmosphere for extended periods of time. It also prevents the accumulation of possible contaminants from repeated fills from a compromised source. The primary distinction of oxygen cylinders intended for medical (emergency applications are also medical procedures) procedures and respirators and all other uses is this evacuation step between fills.


The Compressed Gas Association publication P-2.5, Transfilling of High Pressure Gaseous Oxygen To Be Used For Respiration, states that the transfilling station should consist of "a supply cylinder unit, a  receiving cylinder unit, a cylinder evacuation unit, a gas transfer control unit, and detailed written procedures."


So, the cost of filling an oxygen cylinder intended for medical or emergency-only or for human respirators service is primarily not a function of the gas cost, (300 - 700 liter consumer cylinders are typically filled for the same price), but the price for the required documented analysis associated at each step of distribution and the equipment necessary at each step to evacuate a 100% oxygen cylinder to a significant vacuum prior to refilling. There is also the assurance that the cylinder contents have been validated at each handling step.


While many divers might consider this evacuation-between-fills step unnecessary, it is considered by the FDA to be an essential requirement for medical grade gasses. While rare, there have been injuries and fatalities associated with failure to evacuate oxygen cylinders between fills. As such, the evacuation step is considered by the FDA to be absolutely essential in filling cylinders intended for medical (emergency) use.


The FDA does prosecute the inappropriate filling of oxygen cylinders by unlicensed facilities.


From the Medical Gas Guidelines Appendix:


QUESTION: What are the penalties for violating the Act?


ANSWER: If found guilty of violating a provision of the Act, an officer or employee of the firm may be imprisoned for up to one year and/or fined up to $100,000 ($250,000 if death occurs) for each violation. If the prohibited act was committed with intent to defraud or mislead or if an officer or employee is convicted a second time under the Act, the offense is punishable by a fine of $250,000 to $500,000 and imprisonment of up to three years for each violation. The firm can also be fined the same amounts. (Note: The limits of the amount of fine that can be imposed were raised by the Comprehensive Crime Control Act of 1984, for all criminal violations for which the basic Federal law imposes a prison sentence of six months or more.)


One interesting feature of the law as written is that as long as the oxygen gas is USP, it may be used for mixing all sorts of concentrations of oxygen containing breathing mixes. However, the transfilling of 100% oxygen into cylinders intended for medical or emergency-only cylinders requires that the FDA license the transfilling station and that all cylinders filled with compressed medical gas be filled according to FDA specified GMP procedures.


Accident Litigation


It is pretty much a given in the US that a dive accident will lead to litigation. In such an event, the plaintiff will explore as many as possible avenues to convince  a jury that the defendant was not a responsible, caring person ... someone whose disregard of accepted safety standards was the proximate cause of an injury or death. If oxygen was on the site and used in accident management, then the nature of the cylinder contents becomes a legal matter. Using a FDA licensed facility as a medical gas vendor facilitates defense; use of FDA non-compliant cylinders, irrespective of the actual gas purity, potentially  provides the plaintiff (and jury) with documentation that the defendant was acting outside the law.




The historical basis of prescriptions for oxygen is discussed in the article Rx:Oxygen. Historically, there have been 4 types of oxygen cylinders exempt from prescription requirements. These are cylinders used only for:

 1. emergency use by trained personnel

 2. chemical/medical research or education

 3. extremely limited capacity

 4. private or commercial aviation


However, medical insurance providers, either third party or the federal government, pay for most non-hospital use of oxygen cylinders. This means the oxygen supply vendor is often audited to document amounts of gas distributed and the conditions of distribution. Regardless of the nature of an emergency-only cylinder, it is simply easier and more convenient to provide the oxygen supplier with a prescription. Besides, many commercial vendors will simply not fill an oxygen cylinder for individuals without a prescription.


This is how I handle the situation. 


Each year (Michigan limits oxygen prescriptions to one year), I have a physician write me a prescription for "oxygen to be used in dive accident management or education." This satisfies the vendor's need for documents when audited and allows me to fill my cylinders. I keep a photocopy of my personal prescription for oxygen in my DAN O2 case, in case there is ever a need to refill a used oxygen cylinder while traveling.


DAN Agreement With Praxair


The Diver's Alert Network (DAN)  has made arrangements with a commercial medical gas supplier, Praxair, to assist divers in filling their DAN oxygen cylinders. 


CGA SB-107


In 1992, the Compressed Gas Association issued a special warning about oxygen cylinders to the diving community; special bulletin 107 (SB-107). This was a result of several catastrophic failures during refill operations of oxygen cylinders that had been immersed in ocean water. These failures included several fatalities of fill station operators. It has now been documented that water can enter a cylinder via the regulator when the purge is depressed, regardless of cylinder gas pressure. The combination of seawater with high oxygen concentration can destroy the integrity of a steel cylinder within 30 to 60 days. As such, the CGA has recommended:


 1. if possible, do not submerge cylinders containing high concentrations of oxygen

 2. cascade system working pressure should  never drop below 2 x  the at-depth pressure.

 3. never transfill oxygen (or any other gas)  cylinders without the owner's consent.


So, What to do?


Personally, I get my oxygen-enriched mixtures from my local dive shop (that uses aviation grade oxygen in their gas mixing operations). However, I will only fill my DAN O2 cylinders (and additional D & E cylinders used in teaching DAN O2 classes) from an FDA licensed transfilling station. To me, a reasonable and prudent person would always fill their emergency response cylinders at an FDA licensed facility.


Another guideline I use is that if the cylinder has a scuba regulator compatible valve, then I fill the cylinder at a dive shop. However, any oxygen cylinders with a medical regulator valve (uses the pin-indexing system) are only filled at a FDA licensed facility.



-----, COMPRESSED MEDICAL GASES GUIDELINE, U.S. Public Health Service, Food & Drug Administration, Rockville, MD. 1989, 24 pages. 

From CGA

 G-4: Oxygen, 1987, 16 pages. 

 G-4.3: Commodity Specification for Oxygen, 1988, 12 pages.   

 P.2.5 Transfilling of High Pressure Gaseous Oxygen To Be Used For Respiration, 13 pages, 1992.

 SB-7: Rupture of Oxygen Cylinders in the Diving Industry, 1980, 2 pages.   


 -----, "Medical Gases," Chapter 7 in NFPA Fire Protection Handbook, p. 5-49 - 5-57.    




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 About The Author:

Larry "Harris" Taylor, Ph.D. is a biochemist and Dive Safety Coordinator at the University of Michigan. He has authored more than 200 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information in North America.

  Copyright 2001-2024 by Larry "Harris" Taylor

All rights reserved.

Use of these articles for personal or organizational profit is specifically denied.

These articles may be used for not-for-profit diving education