Department of Chemistry and Department of Biology and Biochemistry: Generic Risk Assessment: The Use of
Liquid Nitrogen.
The assessor should assign values for (a) the level of harm (hazard severity) that could result from using the hazard, and (b) the likelihood of it
occurring (taking into account the frequency and duration of exposure) on a scale of 1 to 5, then multiply them together to give the rating band:
Degree of Harm
(A)
Likelihood of Occurrence
1 – Trivial
(eg discomfort, irritation, slight bruising, self-help recovery)
2 – Minor
(eg small cut, burn or abrasion, basic first aid need)
3 – Moderate (eg strain, sprain, infection, poisoning, incapacitation > 3 days, fire)
4 – Serious (eg fracture, hospitalisation >24 hrs, incapacitation >4 weeks, severe damage
to building)
5 – Fatal
(single or multiple)
(B)
1 – Remote (almost never, not expected at all)
2 – Unlikely (foreseeable, but only occurring rarely)
3 – Possible (could occur, but uncommon)
4 – Likely
(recurrent but not frequent)
5 – Very likely (could occur frequently)
The risk rating (high, medium or low) indicates the level of response required to be taken when designing the action plan.
Risk Assessment Matrix
(B)
(A)
Risk Rating Bands (A x B)
Trivial
Minor
Moderate Serious
Fatal
Remote
1
2
3
4
5
Unlikely
2
4
6
8
10
Possible
3
6
9
12
15
Likely
4
8
12
16
20
Very likely
5
10
15
20
25
LOW RISK
(1 – 8)
MEDIUM RISK
(9 - 12)
HIGH RISK
(15 - 25)
Continue, but
implement
additional reasonably
practicable controls
where possible and
monitor regularly
STOP THE ACTIVITY
Continue, but
review
periodically to
ensure controls
remain effective
Identify new controls.
Activity must not proceed
until risks are reduced to a
low or medium level
Risk Assessment of:
Liquid Nitrogen use at Bath University.
Assessor(s):
Work Authorised by:
Date:
Updated May 2015
Overview of activity / location / equipment /
conditions being assessed:
The handling and use of liquid nitrogen in any experimental/ research work in the Department of
Chemistry or Department of Biology and Biochemistry at the University of Bath.
Hazards
Liquid nitrogen evolves nitrogen gas which is inert and non-toxic but there is a risk of asphyxiation in situations where high concentrations may accumulate
and displace air from the room. The risk of oxygen displacement is increased when nitrogen is stored or used in poorly ventilated areas, or small spaces. Cold
nitrogen gas (from the evaporating liquid) will fill the room from the floor upwards (denser than air).
Short exposures to very cold gas/ vapour leads to discomfort in breathing whilst prolonged inhalation could trigger an asthma attack, and in extreme cases
may damage (burn) lung tissue.
Liquid nitrogen can cause cryogenic burns if the substance itself, or surfaces which are or have been in contact with the substance (e.g. metal transfer
hoses), come into contact with the skin. Local pain may be felt as the skin cools, though intense pain can occur when cold burns thaw and, if the area affected
is large enough, the person may go into shock. Skin may stick to cold surfaces, including clothing saturated with cryogenic liquid, and must not be pulled away
until the affected area has been “thawed” (exposure to tepid water), as tissue can be torn.
Using equipment or tubing made of rubber, synthetic rubber, carbon steel, plastic or glass with liquid nitrogen can lead to failure of the material due to brittle
fracture at low temperature. These materials may implode/ explode from incorrect use/ failure with shrapnel causing cuts/ puncture wounds/ eye injuries.
- Embrittlement of plastic Eppendorf (or similar) tubes can cause the lids to pop off at high speed.
- The glass inner of household vacuum flasks (e.g., Thermos) can shatter/ implode if used to hold liquid nitrogen.
- Lino can become cracked and raised if in contact with cryogens for extended or repeated exposures (this may increase risk of trips).
If a warm hollow tube is inserted into liquid nitrogen, liquid will spout from the tube due to rapid expansion of the liquid inside the tube and gasification.
Ice plugs are formed when moisture in the air condenses at the neck of open dewars, building up to create an icy blockage. If the blockage is complete, this is
potentially explosive as the evaporating nitrogen continues to build pressure in the dewar.
Oxygen in air surrounding pipes and vessels with liquid nitrogen (-196 ˚C) can condense, forming pools of liquid oxygen (-183 ˚C) or liquid air. Liquid oxygen
can also form in cold traps cooled by liquid nitrogen, and appears as a blue liquid. Liquid oxygen can support vigorous combustion of materials not usually
considered flammable, and enriched atmospheres may be explosive.
Dewars may be heavy - a standard 25 L lab dewar will weigh approximately 15 kg empty, and 35-40 kg when full. The larger pressurised dewars are usually
mounted on wheels for easier transport, but these should only be transported on flat surfaces. Control of one of these dewars can easily be lost when
attempting to manoeuvre up or down a slope, and this process will usually require two people. In addition, moving the pressurised dewars on slopes or rough
ground (including bumps) is not recommended by the manufacturers, as the delicate internal structure at the neck may be damaged and the internal vacuum
lost. If this happens, it is usually uneconomical to repair and the dewar will require replacement.
General Controls
•
•
•
•
•
Never store cryogenic substances (or allow them to vaporise) in enclosed areas, including: fridges, freezers, cold rooms, sealed rooms or basements.
Storage of cryogenic liquids must be located away from workplace transport traffic, and site drains.
Dewars should be stored in secure enclosure to prevent access to unauthorised persons. Storage area to be designed fit-for-purpose and maintained in
good order, i.e., kept clear of any combustible materials, with a concrete rather than tarmac floor.
Ensure dewars are completely empty, dry and at ambient temperature before putting into general storage.
Transport of any cryogenic gas in an enclosed vehicle is dangerous, and an open vehicle, trailer, or separate cab from load compartment must be used.
Lab Induction: No staff are allowed to start work until they have been given an induction. The induction includes instruction that all work must be Risk
Assessed. Staff are sent out an induction summary to remind them of the induction details and to send any new staff to the Technical Team.
Hazard
Oxygen/ air
displacement
resulting in
Asphyxiation
Who could be
harmed?
Individual and
others in area
Control measures needed to minimise risk
Risk rating after
controls
implemented
A
B
AxB
5
2
10
A task-specific risk assessment must be filled in, understood and signed before working
with liquid nitrogen.
Large vacuum insulated tanks (dewars) are normally stored outside buildings due to the quantities
of liquid. If outdoor storage is not possible, the storage area must be sufficiently ventilated to ensure
oxygen levels are maintained above 19.5 %, taking into account the normal evaporation of all
dewars within the room and above 18 % taking into account the complete spillage of the largest
dewar.
Always use liquid nitrogen in a well-ventilated area.
•
•
•
•
•
•
Never store in an unventilated cupboard or small room.
Ensure hazard warning signs are displayed (yellow triangle with exclamation symbol and text:
‘Liquid nitrogen’) on dewars and at access points to storage area.
Use only equipment designed for purpose of storing liquid nitrogen.
Never seal an unpressurised/ open dewar (e.g., only use loose-fitting cap).
Vessels must always be attended when filling.
Minimise number of people in the area when filling, and consider personal oxygen monitors.
Do not overfill vessels.
•
•
•
•
Use only proper transfer equipment.
Transfer tubes and dewars must be stainless steel, copper/ copper alloy or aluminium.
Do not use hollow dipsticks - use solid metal or wood.
If fitted (assess according to oxygen displacement calculations for room and dewar sizes),
oxygen monitors should be located approximately 1 metre up from the floor, set to alarm if
oxygen levels reach 19.5 % or lower.
The following points must be considered:
• Ventilation - is it adequate? Some rooms do not have any air supply or extract system and so
there is little or no air change. Can the door be left open to allow gas to dissipate when vessels
are being filled? Can vessels be filled elsewhere?
• Do people spend significant periods working in the room (on unrelated tasks)?
• Is the room fitted with an oxygen deficiency monitor/ alarm?
• Is the door fitted with a viewing panel?
Note: Liquid Nitrogen Containers will be marked with information about where they are to be
stored and filled.
Asphyxiation in
lifts
Lift users
•
•
•
Cold Burns
Liquid nitrogen at
-196 ˚C,
materials liquid
has contacted.
May be as a
result of spillage.
Individual and
others in area
•
•
•
•
•
•
•
•
•
•
•
•
•
Liquid nitrogen containers must not be accompanied in lifts.
If a lift is used then it will be closed to all passengers with signage posted and preferably use of
barrier tape.
The vessel will be manoeuvred into the lift, and the lift called to the destination floor.
5
2
10
Wear dry, thermally insulated gloves, to EN511, designed to handle cold temperatures.
Hands (either bare or gloved) must never be immersed in liquid nitrogen.
Gloves with elasticated cuffs are recommended to prevent ingress of liquid.
Wear eye protection or full face visor (to EN166) when dispensing or handling liquid nitrogen.
To minimise splashing, fill slowly while the vessel cools (increased boil-off to start).
Only use containers suitable for the material being used, i.e., purpose-designed dewar.
Use a transfer tube or funnel to decant liquid into narrow-necked open dewars.
Use a funnel to pour liquid into narrow-necked openings in cryostats and cold traps.
If handling liquids in open dewars, wear closed, non-porous shoes (no sandals, or open-toes).
Wear lab coat, overalls or cryo-apron while dispensing/ filling dewars.
Sleeves and trousers must be worn outside gloves and footwear.
Use tongs to place or retrieve items from cryogenic liquid.
When removing cell-line cages from storage, use a hook to find the handle and raise the cage.
2
3
6
Slips and Trips
Impact
Individual and
others in area
•
•
Individual and
others in area
Manual Handling Individual
Musculoskeletal
injuries may be
sustained from
handling heavy or
awkwardly shaped
vessels.
Inexperienced
users
Individual user
Store dewars away from doors, access routes and fire escapes.
Clear up any frost/ ice or melt-puddles resulting from use of cryogens as soon as possible.
3
2
6
• Wear lab coat and eye protection (to EN 166.F) at all times.
• Use purpose-designed dewars with liquid nitrogen - household vacuum flasks (Thermos, etc)
must not be used.
3
2
6
• Follow the University’s Manual Handling Guidelines.
• Use the associated trolley and tipping mechanism to dispense nitrogen to bench-top (1-2 litre)
dewars.
• Movement of larger containers may require two people, e.g., if there is a requirement to move
between differing levels.
• Individuals must be aware of their own limitations and ask for help if needed.
3
3
9
5
2
10
Never dispose of cryogenic materials down the sink - waste pipes can potentially be cracked
resulting in leaks.
Liquid nitrogen dewars must be visually inspected before use and replaced if damaged.
Storage vessels should be CE standard.
Containers must be used as per manufacturer’s instructions.
Thermos flasks and other household containers must not be used as they are not designed for
liquid nitrogen storage, and they could shatter.
3
2
6
Free-venting dewars must be fitted with their proper loose-fitting cap (in good working order),
which controls vent gas and minimises ingress of moisture.
Dewars will be stored in a well-ventilated place under cover (protected from adverse weather if
outside)
Dewars must not be left wide open to the atmosphere.
3
2
6
Users must be trained how to dispense from these vessels.
Use a phase separator if dispensing into open dewars to reduce splashing. If unavailable, start
the fill slowly until boil-off is reduced.
4
2
8
Provide information, training and supervision: Before using liquid nitrogen, all staff must complete a
risk assessment relating to their specific work and be trained by a competent person.

Implosion or
Individual and
broken equipment others in area
from incorrect use
or failure.




Ice plugs (partial
or complete)
Individual and
others in area



Pressurised
dewars
Individual and
others in area


Inexperienced workers must be supervised by a competent cryogens user.



Oxygen
Enrichment
resulting in fire/
explosion
Transport in
vehicles
Impact from
moving load,
asphyxiation,
burns.
Ensure the vessel is regularly serviced and maintained - tag must be current.
Vessel must have burst disc and pressure release valve.
During transport, avoid slopes and rough ground (including kerbs, gratings and drain covers).
Individual and
others in area
Driver and any
passengers







Ensure pipes, etc. are well insulated and not above tarmac.
Remove any combustible material from storage areas.
Do not suck air into a cold trap for long periods, as liquid air/ oxygen will collect
o Empty traps regularly (after each solvent/ material removed), to minimise adverse reactions
Never transport a dewar in the passenger compartment of a car, or cab of a van/ truck.
Dewars must be fully secured in an upright position during transport (consider collision)
Ensure load compartment is well ventilated.
Remove the dewar from the vehicle as soon as possible after arrival.
5
4
2
1
4
10
First Aid:
Where inhalation has occurred, the victim (who may be unconscious) will be removed to a well ventilated area where possible. Rescuers must not put
themselves at risk - an oxygen-depleted area must not be entered unless over 18% oxygen atmosphere. Breathing apparatus may be required (trained
personnel only). The casualty should be kept warm and rested whilst medical attention is obtained. If breathing has stopped, commence CPR and summon
medical attention immediately.
Where contact has occurred, the temperature of the affected area will be allowed to slowly return to normal. For minor injuries, clothing should be loosened
and the person made comfortable. Clothing must never be pulled away from burned or frozen skin. Flush the affected area tepid water (40 ˚C) for at least 15
minutes and apply a sterile dressing to protect the injury. Seek medical attention.
Never apply any form of direct heat to a cold burn.
For other injuries or shock, apply first aid as far as is practicable and summon medical attention as soon as possible.
All incidents, injuries and near misses must be reported as soon as possible.
Emergency Procedures
Ensure all users know the correct procedures in case of emergency.
In the event of a small spillage (under 1 litre):
• Allow liquid to evaporate
• After return to room temperature, inspect area. Report any infrastructure damage to Estates. If equipment has been damaged, arrange for repair.
In the event of a large spillage (over 1 litre) or accidental release, follow these procedures:
• Evacuate the area and restrict access. Deploy warning signs and cordon off area if necessary.
• Ventilate the area to speed evaporation of any spilt liquid allow resultant gas to disperse.
- Open doors and windows or activate forced ventilation, if safe to do so.
• Try to stop the release if at all possible e.g. turn off valves, but only if it is safe to do so - wear PPE.
• Do not re-enter area unless it is proved safe to do so. The presence of oxygen deficiency monitors will indicate the oxygen levels in the vicinity.
• Prevent liquid nitrogen from entering drains, basements, pits or any confined space where accumulation may be dangerous.
If you find an unconscious person in an area where liquid nitrogen is used, immediately raise the alarm and tell Security and the Emergency Services that
asphyxiation is suspected. Open the door to increase ventilation to the area, and if possible, increase the forced ventilation to the room. Do not attempt to
enter the area unless you know the oxygen level is above 18 %.
Oxygen Enrichment. If present, liquid air/ oxygen will appear as a blue liquid.
• If seen (e.g., under pipes), remove source of liquid nitrogen and allow to warm slowly to room temperature, open to the atmosphere (preferably well
ventilated).
• If seen in a cold trap, keep the pump on (dewar removed) until the blue colour disappears, switch off the pump and slowly open to the atmosphere,
allowing trap to warm to room temperature.
Assessment of Ventilation Requirements for worst case scenario liquid nitrogen spill.
Nitrogen is the main component of air and is present at approximately 78% by volume (oxygen is approximately 21% and argon 1%). Any alterations in the
concentrations of these gases, especially oxygen, have an effect on life. In the case of liquid nitrogen, there is a risk of asphyxiation where ventilation is
inadequate and the nitrogen gas evolved can build up and displace oxygen from the local atmosphere. An atmosphere containing less than 18% oxygen is
potentially hazardous and entry into atmospheres containing less than 20% should be avoided. In these instances, consideration should be given to the
installation of oxygen monitoring equipment.
The process of asphyxiation caused by depleted oxygen levels can happen rapidly and without warning to the victim. The reduction in atmospheric oxygen
results in loss of mental alertness and distortion of judgement and performance. The individual will be unaware of this happening. When going in to help
unconscious colleagues, would-be rescuers often become additional fatalities, overcome by the lack of oxygen very quickly.
Oxygen monitors are usually set to alarm at 19.5 %, and rooms must be evacuated if the level reaches 18%.
The general effects of reduced oxygen content in the atmosphere are given in the table below:
Oxygen content (vol. %)
14-21
11-14
8-11
6-8
0-6
Effects and symptoms
Increased pulse rate and tiredness.
Physical and intellectual performance diminishes without the person being aware.
Possibility of fainting without prior warning.
Fainting within a few minutes – resuscitation possible if carried out immediately.
Fainting almost immediate, death ensues, brain damage even if resuscitated.
Calculation:
Calculate the size of your room in cubic metres (m3) = height x width x length
Need to calculate for worst case scenario, using largest dewar (use volume when full) and gas expansion for nitrogen (1 L of liquid expands to 683 L of gas),
so: potential volume of nitrogen = dewar size in litres x 683 Convert litres to m3 by dividing by 1000.
Calculate remaining air volume = room volume - potential nitrogen volume
Volume of oxygen = 21(remaining air/ room volume)
The volume of oxygen must be above 18 % for safe operation, if below this then ensure operations/storage is in larger volume rooms or use oxygen monitors.
Example calculation:
Room is 3 m wide, 4 m long and 2 m high = 24 m3
Dewar is 25 litres. Potential N2 gas = 25 x 683 = 17075 L
Convert potential N2 to m3 = 17 m3
Calculate remaining air volume = 24 - 17 = 7 m3
Volume of oxygen = 21(7/ 24) = 6.1 %
Remaining oxygen level in the room would be 6.1%, so relocate the vessel to a larger space or consider installation of oxygen monitors.