RISK ASSESSMENT: USE OF FLAMMABLE,
EXPLOSIVE, TOXIC & REACTIVE GASES
Hazards

Leakage or escape of flammable gases can produce a serious explosive hazard
in a laboratory. Acetylene, hydrogen, ammonia, hydrogen sulphide, propane and
carbon monoxide are especially dangerous.
Hydrogen:
o Hydrogen is much lighter than air (therefore will collect in roof spaces), is
non-toxic and has no colour or smell. It is highly flammable, with
flammability range of 4% to 75% in air by volume
o Hydrogen flames from leaks can be almost invisible and thus difficult to
detect
o It takes very little energy to ignite an air/hydrogen mixture- an electrostatic
spark is often enough

Apart from explosive hazard, some gases can be reactive e.g. oxygen, and highly
toxic e.g. carbon monoxide.
Oxygen:
o Oxygen behaves differently to air, compressed air, nitrogen and other inert
gases- it is very reactive
o Pure oxygen, at high pressure, can react violently with common materials
such as oil and grease; other materials may catch fire spontaneously
o The main danger to people from an oxygen enriched atmosphere (used to
describe situations where the oxygen level is greater than in air) is that hair
and clothing can easily catch fire
o Oxygen cannot be easily detected by the human senses- it is colourless,
odourless and tasteless
o Even a small increase in the oxygen level in the air to 24% can create a
dangerous situation because it becomes easier to start a fire, which will
then burn hotter and more fiercely than normal air

"Inert" gases such as nitrogen, carbon dioxide and argon can cause asphyxiation
if released in quantity.
Risk
Escape of any gas that is flammable, explosive, toxic or reactive is very serious and
poses extreme danger to all the occupants of a laboratory as well as those beyond.
Precautions


Avoid use of flammable / toxic / explosive / reactive gases where possible
Use gases only in a fume-hood or in a well-ventilated laboratory. Detection
systems are advisable in a confined spaces or poorly ventilated rooms where

















risks from gas leaks would be high.
Hydrogen: Use of a hydrogen generator should always be considered first where
possible. If a generator is not appropriate, the cylinder must be fitted with a gas
detector linked to an automatic shut-off valve. If not, this must be fully justified in
the risk assessment for the work activity involving the use of hydrogen.
Hydrogen: never crack (snift) hydrogen cylinder valves as the escaping gas may
escape
Consideration should be given to using automatic gas cylinder shut-offs for use in
an emergency for all flammable / toxic and explosive gases.
Naked flames or other sources of ignition must be rigorously excluded from the
vicinity.
Gas cylinders, control valves and pressure regulators and gauges should all be
used carefully and according to the manufacturers' recommendations. Broken or
damaged equipment should not be used but must be replaced. Only equipment
that is appropriate i.e. specially designed for use with toxic, explosive or corrosive
gases may be used.
Pressure rating of the gas regulator must be more than the full cylinder supply
pressure.
The smallest cylinder size that is practicable should be used e.g. a lecture bottle
that can be sited in a fume hood.
Flashback arrestors should be fitted to all fuel gas (e.g. hydrogen) supply lines
and to oxygen supply lines when they are used in conjunction with fuel gases.
There should be a regular check for leaks especially in joints. However, leakdetecting fluids on oxygen lines must not be used unless they are compatible.
Soap or liquids that may contain grease must not be used. A FLAME MUST
NEVER BE USED WHEN TESTING FOR LEAKS. Hydrogen: due to its small
molecular size, particular care should be taken with fittings and seals- hydrogen
is able to seep through many porous materials which could be satisfactory for
use with other gases
Consideration should be given to using a gas sensor to detect leakage especially
in poorly ventilated or confined spaces.
Receiving containers must be capable of accepting the gas at the required
operating pressure.
Prior to introducing a flammable gas into a reaction vessel, the equipment must
be purged of oxygen by evacuation or by flushing with inert gas at least three
times.
Exhaust lines must be properly vented e.g. to a fume hood.
Cylinders of "fuel" gases must not be sited on the same rank as oxygen.
As far as possible, cylinders of flammable gases e.g. hydrogen must not be sited
on emergency escape routes.
Ensure that cylinder valves are closed and piped supplies isolated whenever
work is stopped.
Oxygen cylinders:
o always open the valve slowly. Rapid opening, particularly of cylinder
valves, can result in momentarily high oxygen velocities. Any particles
will be pushed through the system very quickly, causing frictional heat.
Alternatively, if the system has a dead end such as where a pressure
regulator is connected to an oxygen cylinder, heat can be generated
through compression of the oxygen. Both cases can result in a fire.
o keep oxygen equipment clean: contamination by particulate matter,
dust, oil greases is a potential fire hazard
o use clean hands or gloves when attaching the pressure regulator,
making connections
o always ensure that the pressure adjusting screw of the pressure

regulator is fully unwound, so that the regulator outlet valve is closed
before opening the oxygen cylinder valve, particularly when opening the
cylinder valve for the first time after changing a cylinder
Depending on the gas being used a separate specific assessment may have to
be made.
Training Requirements
Training by an experienced person is essential. All individuals must have received
training on the safe use of gas regulators and follow the guidelines outlined in the
departmental ‘working safely with gas cylinders’
Risk Remaining
The handling of flammable, toxic and reactive gases will always have some degree of
risk and constant vigilance is required in their use. However, the application of controls /
precautions outlined above should reduce the risk to a low and acceptable level.
Emergency Procedures
Leak



If the leak is small, attempt to close off the cylinder valve but do not endanger
yourself. Eliminate all sources of ignition, ventilate and evacuate the laboratory.
BEWARE of approaching a possible hydrogen leak since the gas burns with an
almost invisible flame - carry a rolled up newspaper in front of you to check for a
flame.
If the leak is large, evacuate the laboratory and sound the fire alarm. See
Department of Biology ‘Working Safely with Gas Cylinders’
Following large scale leakage of an asphyxiating gas e.g. nitrogen, argon never
re-enter a laboratory without permission. Lack of oxygen may not be apparent but
the effect will still be deadly. Be aware that asphyxiating gases may be heavier
than air and accumulate at floor or lower levels.
Fire

Evacuate and sound the alarms.
References
B.O.C. publication "Safe under Pressure" plus any relevant Safety Data Sheets.