The One-stop Guide to Working with Compressed Gases and Chemicals under Pressure: from Hazard Classification and Leaky Cylinders to Mitigating Risks

What are gases and chemicals under pressure?

Gases under pressure are those that are contained in a receptable at a pressure of 200 kPa (gauge) or more at 20°C. They are categorized according to their physical state when packaged: compressed gas (e.g. oxygen, helium), liquefied gas (e.g. butane, carbon dioxide), refrigerated liquefied gas (also known as a cryogenic gas, e.g. liquid nitrogen), and dissolved gases (e.g. acetylene).

Chemicals under pressure are (typically 50% or more by mass) liquid or solid substances (e.g. pastes or powders), pressurized with a gas at 200 kPa (gauge) or more, also at 20°C, in receptacles other than aerosol dispensers. They are further categorized based on their flammable compound content and heat of combustion.

The ‘gas cylinder’ pictogram represents two GHS physical hazard classes

Gases under pressure

Category Signal Word Hazard Statement Hazard Code

Compressed gas

Entirely gaseous at -50°C and includes all gases with a critical temperature ≤ -50°C. 


Contains gas under pressure; may explode if heated


Liquefied gas

Partially liquid in the cylinder at temperatures above -50°C.


Contains gas under pressure; may explode if heated

Refrigerated liquefied gas

Is partially liquid because of its low temperature.


Contains refrigerated gas; may cause cryogenic burns or injury

Dissolved gas

Dissolved in a liquid phase solvent.


Contains gas under pressure; may explode if heated


Chemicals under pressure

Category Signal Word Hazard Statement Hazard Code

Category 1

Any chemical under pressure that contains ≥ 85% flammable components (by mass) and has a heat of combustion ≥ 20 kJ/g.


Extremely flammable chemical under pressure: may explode if heated


Category 2

Any chemical under pressure that contains > 1 % flammable components (by mass) and has a heat of combustion < 20 kJ/g or that contains < 85% flammable components (by mass) and has a heat of combustion ≥ 20 kJ/g.


Flammable chemical under pressure: may explode if heated

Category 3

Any chemical under pressure that contains ≤ 1 % flammable components (by mass) and has a heat of combustion < 20 kJ/g.


Chemical under pressure: may explode if heated


Nature of the substance

The nature of the substance within the pressurized system is important for evaluating additional hazards associated with these chemicals. Butane for instance is inherently flammable, oxygen as an oxidizer may turn a small fire into an explosion, and methyl acetylene may undergo polymerization or decomposition reactions when exposed to changes in temperature, pressure, or physical damage. Moreover, some gases may also be corrosive or toxic, causing severe health damage. A carbon monoxide leak for instance could be deadly. 

Leaky cylinders

Gas can flow out from leaky pressurized systems to react with nearby flammable, corrosive or oxidizing materials, or it can displace oxygen creating an oxygen deficient atmosphere, which can quickly overtake you. Placing oxygen level sensors in enclosed areas where gas build-up may occur is a good idea since your nose cannot sense an oxygen deficiency.[1]

Make sure to conduct a leak test (e.g. electronic detector, soap solution, etc.) every time a cylinder is changed. Never conduct a leak test with a flame. For minor leaks, close all regulator valves and check all connections. If the leak continues, secure the cylinder next to a fume hood if possible and notify the supplier. For major leaks, initiate building evacuation and your institution’s emergency protocols.

Breached valves and projectiles

Compressed gas cylinders are heavy and awkward to handle: a falling cylinder that strikes the ground can not only cause you bodily injury, but it can also result in a broken (or breached) valve, becoming a projectile with speeds great enough to penetrate concrete walls.[2]

Pressure relief valve not working

A pressure relief valve is designed to relieve excessive cylinder pressure. If this valve fails, over-pressurization may cause forceful rupture and flying fragments.2

Gas pressure regulator installation

Gas pressure regulators (designed for specific gases and pressures involved) are necessary to reduce the cylinder’s pressure, so it can safely discharge substances. Two-stage regulators are generally recommended for lab use2. Never use an adaptor, pipe dope, or Teflon tape to connect a regulator to a cylinder, and only use non-sparking tools if dealing with flammable gases.2 Always check the compatibility of equipment and materials before installation and refer to the manufacturers’ recommendations for first time use.

Refer to the following video for installation instructions.

How to mitigate the risks of working with gas cylinders?

1. Receiving:

  • Examine the cylinder before accepting it. Do not accept it if:
    • the cylinder or valve assembly is corroded or broken.
    • the cylinders are not properly labelled with its contents and associated hazards.
    • the cylinder has not been tested or inspected recently. Depending on your local regulations and the type of gas, cylinders should be inspected every 3-12 years.
  • Record when cylinders are received, and do not keep them longer than recommended by the supplier.

2. Storage:

  • Store cylinders in a well-ventilated area. The setup will be dependent on the size and layout of the room.
  • Store cylinders away from incompatible chemicals such as flammables, oxygen, fuels, and combustibles, as well as heat, cold, direct sunlight, emergency exits, corridors, loading bays, or areas with high traffic.
  • Secure the cylinders with chains, belts, or bench/wall-mounted brackets to prevent falling.
  • Keep the cap on the cylinder when not in use.
  • Do not store gas cylinders in lockers as this makes it more difficult to detect leaks.
3. Handling:
  • Additional training may be required to handle gases if they pose other hazards such as toxicity.
  • When transporting cylinders, use an appropriate trolly or cart with a securing system. Make sure the cylinder is maintained upright, and do not remove the valve cap until the cylinder has been secured at the point of use.
  • Do not lift cylinders by their valve cap or assembly to prevent damage.
  • Do not use adaptors to fit cylinders with incompatible pressure regulators, gauges, and tubing. Never mix regulators and tubing of incompatible substances. For instance, do not use copper fittings with acetylene cylinders.
  • Do not allow grease or oil to contact the valves especially for oxidizing gases.
  • Wear appropriate PPE including goggles, gloves, lab coats, and closed toed shoes.
  • Do not use a compressed gas cylinder unless the cylinder is clearly marked or labeled with the cylinder’s content. Never rely on the color of a cylinder to identify its contents.
4. Awareness of surroundings:
  • Remove all items that could cause flames or sparks such as electrical equipment and burners.
  • Ensure proper (high and low) ventilation in the laboratory, and regulate the temperature and/or pressure if possible. The pressure in a cylinder will increase when environmental temperatures increase.
  • Check regularly for leaks. Gases that are heavier settle to lower areas such as pipes or basements resulting in a gas trail from the cylinder. If the gas trail encounters a spark, the fire produced can “flash-back” to the cylinder.
5. Emergencies and spills:
    • Ensure operational firefighting and first aid equipment are accessible.
    • Store cylinders away from emergency exits, hallways, elevators, staircases, or areas of high traffic.
    • In case of a major leak, immediately unplug electronic equipment and initiate building evacuation and your institution’s emergency protocols.
    6. Minimize the use of compressed gases:
      • Order an appropriate number of cylinders. Do not store excess cylinders in your laboratory.
      • Examine your SOP to determine if there are safer alternatives.
      • If substitutions are not possible, consider installing fixed pipelines for gases. This will mean that cylinders do not have to be moved frequently, and all lab members will be aware of their location, which can decrease injuries and minimize the possibility of improper setup.
      7. Waste:
        • Keep empty cylinders closed. They should not be emptied completely since a positive pressure must be maintained.
        • Contact your supplier to determine if they have a cylinder recycling program.
        • Separate empty and full cylinders, and mark them as “empty” before disposal.



        1. Compressed Gases | Department of Chemistry (
        2. Safe Operating Procedure (
        By Qinling Li


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