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Compressed Gas: Classification and Requirements

See UC San Diego requirements for safe storage and handling of compressed gases.


NFPA 55: Compressed Gas definition:
A material, or mixture of materials, that (1) is a gas at 68°F (20°C) or less at an absolute pressure of 14.7 psi (101.3 kPa) and (2) has a boiling point of 68°F (20°C) or less at an absolute pressure of 14.7 psi (101.3 kPa) and that is liquefied, nonliquefied, or in solution, except those gases that have no other health or physical hazard properties are not considered to be compressed gases until the pressure in the packaging exceeds an absolute pressure of 40.6 psi (280 kPa) at 68°F (20°C).

Requirement for researchers

UC San Diego researchers working with compressed gases must follow an approved Hazard Control Plan (HCP) in using the HCP application

This HCP must be pre-approved by the principal investigator prior to beginning any work with this material.

Information on this Blink page is supplementary and is not intended to replace the approved HCP.

Hazard classification

Use and storage of compressed gases is strictly regulated according to hazard classification.

Compressed gases are classified as class I, II, III, or IV in order of decreasing hazard.* Classes are based on the lethal concentration to 50% of test animals (rats) of each gas (LC50).

University of California, San Diego Compressed Gas Hazard Classifications
UCSD Compressed Gas Hazard Classifications
Class I LC50 (rat) ≤ 200 ppm
Class II 201 ppm LC50 (rat) ≤ 2000 ppm
Class III 2001 ppm LC50 (rat) ≤ 5000 ppm
Class IV LC50 (rat) > 5001 ppm

* Note: The California Fire Code uses a different classification system. For more information, contact EH&S Fire & Life Safety, (858) 534-3659.

Storage and use

Storage and use requirements apply to all compressed gases.

Additional precautions

Implement additional safety precautions for strong>particularly hazardous substances (classes I, II, and III). Requirements are relaxed for small quantities and short term usage.


  • Follow the Chemical Hygiene Plan and Compressed Gas at all times.
  • Complete your Compressed Gas and/or gas specific Hazard Control Plan (HCP) in the HCP application prior to ordering your material.
  • There are fire code restrictions on the quantities of some gases regarding use and/or storage within research buildings. Examples include:
    • flammable (hydrogen, methane, acetylene)
    • oxidizer (oxygen, nitrous oxide)
    • highly toxic (arsine, phosphine, nitric oxide, nitrogen dioxide)
    • toxic (ammonia, hydrogen sulfide, carbon monoxide)
    • corrosive (hydrogen chloride, chlorine, hydrogen fluoride)
    • Unstable reactive (acetylene)
  • Order only what is needed for a reasonable amount of time.
  • Keep quantities to a minimum for all gases.
  • Reduce concentrations when possible.
  • Read Compressed Gas Cylinders Overview for details on ordering, services, and rental fees to complete your purchase.
  • Order all in-stock and specialty gases through Marketplace.
Important: To ensure safe use and storage of Class I, II, or III gases, contact the EH&S Chemical Hygiene Officerbefore ordering material.

Compressed Gas Safety Training for UC San Diego

Compressed gas cylinders thumbnail image

This 2-video set was presented by Eugene Y. Ngai of Chemically Speaking LLC in May 2013 for UCSD researchers and Environment, Health & Safety staff. Key elements of a CG safety program and pre-planning for emergency response are covered.

Compressed gas cylinders thumbnail image

Safety training handouts presented with this program are below:

All files courtesy of Chemically Speaking LLC

Compressed gas alternatives

Gas cylinders are not always ideal in a lab setting for transportation, storage, safety, or other practical reasons. Consider the alternatives below:

  • Gas generation

    Gas is generated on demand. This may alleviate fire code limitations on the storage and use of these gases:

    • Oxygen
    • Hydrogen
    • Arsine
    • Fluorine
    • Methylsilane
    • Ozone
  • Subatmospheric gas cylinders

    Gas will flow out of the cylinder only if a vacuum is drawn on the cylinder valve outlet. Four types of systems are available that operationally meet this criteria:

    1. Gas adsorbed on a solid
    2. Gas complexed with a liquid
    3. Mechanical
    4. Gas generator


UC San Diego requirements for engineering controls, protective equipment, storage, emergency response, warning systems, and employee training are based on:

Questions? Contact the EH&S Chemical Hygiene Officer.
Note: this page has a friendly link that's easy to remember: