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Health and Safety Services

Liquid Nitrogen - Code of practice for handling

1. Status of this Code. Compliance with this code of practice is mandatory for all staff and students.

2. Risk Assessment. A current COSHH risk asssessment must be in existence for all work with liquid nitrogen. See:

3. Cold Burns. Liquid N2 can cause severe frostbite and cold burns. Safety glasses and appropriate gloves (see 13) must be worn when handling liquid N2. Never put any part of your body in front of a liquid N2 supply. Never touch an uninsulated pipe or vessel. The cold metal may stick fast and tear the flesh when you attempt to withdraw from it.

4. Ventilation. Always handle liquid N2 in well-ventilated areas to prevent excessive concentrations of gas. If such are not available then oxygen monitoring should be fitted and great care exercised to avoid spills. Note: The cloudy vapour that appears when liquid N2 is exposed to the air is condensed atmospheric moisture; not the gas itself.

5. Asphyxiation. Liquid N2 can rapidly be converted into gas at 700 times the liquid volume. It may then kill by asphyxiation. When the oxygen concentration in the air is sufficiently low, a person can become unconscious without sensing any warning symptoms, such as dizziness. See Appendix Two for the physiology of asphyxiation.

6. Pressure. Liquid N2 boils off very quickly. Do not put liquid N2 in closed vessels that cannot withstand the pressure. No thermos flasks. Punch holes in cryovials.

7. Signage. The presence of liquid N2 must be indicated by a suitable sign

8. Oxygen deficiency alarms. All rooms where volumes in excess of 25 litres of liquid N2 are held are equipped with oxygen monitoring. The sensors have a life of only 12-18 months. Systems in continuous use must be checked monthly and the results recorded. Systems not in regular use and only used for short periods should be tested before use each time. One or more members of staff must be appointed to carry out these tests. It is essential that the unwary do not enter dangerous conditions to investigate an alarm. Accordingly, in the case of devices that do not automatically silence when the oxygen level returns to normal, the guage should be outside the room in order that it may be checked before entering the room to investigate an alarm. Instructions on the meaning of alarms and the action to take should be posted outside each room with an alarm.

9. Transport including manual handling.
(a) Only use closed "onion" (25 litre) Dewars and "transport" dewars when moving liquid N2.
(b) Moving around large Dewars is always a a two person operation.  The Manual Handling Regulations (1992) require all persons involved in transporting large Dewars to receive suitable training before commencing any such tasks.  Contact the College Health and Safety Officer X6218.  See also Manual Handling Safety.

10. Use of Lifts. It is extremely unlikely that a well-maintained and properly handled Dewar will spill its contents whilst being transported in a lift and so place an accompanying handler at risk of injury or death. However to eliminate such risk altogether the following practice should be adhered to when transporting Dewars in lifts.
10.1 Always work in pairs.
10.2 Use the Goods lift whenever possible. If the Main lift must be used then it must only be used between ground and basement level. This will avoid the lift being stopped and entered by a member of the public at other floors.
10.3 One person should send and another should be waiting to receive the Dewar at the floor destination. No one should accompany the Dewar.

11. Maintenance of Dewars. All large Dewars (>25litres) must be subject to annual maintenance checks by a contract company. Smaller Dewars must be visually inspected each time they are refilled and any defects must be reported to the appropriate Departmental Superintendent.

12. Decanting of liquid N2. Never overfill Dewars. Spillage damages flooring and could cause injury. Insert pipes and funnels slowly to avoid slashing. Great care should be exercised to ensure that space is left to replace lids/tops on Dewars especially those that insert a considerable distance into the vessel. Spills and splashes can set off oxygen monitors. Moving Dewars away from the sensors on oxygen monitors before topping up and taking care to minimise splashing as suggested will avoid alarms being set off unnecessarily.

13. Local rules for Liquid N2 safety. Local rules exist for handling Liquid N2 in association with specialist equipment. For example within room L. All such local rules must be adhered to.

14. Appropriate Gloves - Spills when wearing gauntlets or other loose fitting gloves can result in liquid N2 running inside the glove and causing severe burns. Always use thermal protective gloves specifically designed for cryogenic use with close fitting ribbed cuffs that will prevent this from happening. They also protect the hands from cold burns from materials that have been in contact with cryogenic liquids. These can be as much of a problem as the liquid itself. The surface of the glove is textured to provide a sure grip on cold slippery surfaces thus helping to prevent other associated accidents.

15. First Aid Measures for Skin/Eye Contact - Immediately flush thoroughly with water for 15 minutes. In case of frostbite spray with water for at least 15 minutes, apply a sterile dressing obtain medical assistance.

16. Size of rooms to accommodate Dewars without oxygen monitoring
Worst case scenario - full contents of the vessel are released to atmosphere over a short period of time.
Resulting % oxygen concentration. %O2 = 100 x Vo / Vr
Where for nitrogen Vo = 0.2095 (Vr-Vg). Vg = maximum gas release which is the liquid volume capacity of the vessel V x gas expansion of 682.7. Vr = Volume of Room.
If the calculation suggests an oxygen content in the atmosphere lower than 18% then fit permanent oxygen monitoring

17. Further reading:
a) BOC (Cryospeed) G4521 - Siting of liquid cylinders or vessels in buildings.
b) BOC Gases - Liquid Nitrogen Safety Data Sheet.
c) BOC - Prevention of oxygen enrichment or deficiency accidents
d) BOC - Care with Cryogenics
e) Queen Mary College, London - Liquid Nitrogen, Code of Practice
f) J.A.G. Collinson, Safety in the Laboratory, The Medical Technologist, February 1990
g) British Cryogenics Council, Cryogenics Safety Manual, Oxford, 1991
h) National Physics Laboratory, Safety Measures in Chemical Laboratories, London, 1964

Appendix One

Physiology of Asphyxiation

Large leaks of nitrogen may produce local oxygen deficient atmospheres that will result in asphyxia if breathed. This is especially true in confined spaces. Atmospheres containing less than 18% oxygen are potentially dangerous and entry into atmospheres containing less than 20% is not recommended.

Asphyxia due to oxygen deficiency (anoxia) is often rapid with no prior warning to the victim. A general indication of what is liable to happen in oxygen deficient atmospheres is given below but it should be appreciated that the reactions of some individuals may be very different from those shown.

Sudden asphyxia. In sudden and acute asphyxia, such as that from inhalation of a gas containing practically no oxygen, unconsciousness is immediate. The person falls as if struck down by a blow on the head and may die in a few minutes, unless immediate remedial action is taken.

Gradual asphyxia. Degrees of asphyxia will occur when the atmosphere contains less than 20.9% oxygen by volume.

Oxygen Content (vol. %)
Effects and symptoms (at atmospheric pressure)
20 - 14
Diminution of physical and intellectual performance without person's knowledge.
14 - 10
Judgement becomes faulty. Severe injuries may cause no pain. Ill temper easily aroused. Rapid fatigue on exertion.
10 - 6
Nausea and vomiting may appear. Loss of ability to move vigorously or at all. Inability to walk, stand or crawl is often first warning and it comes too late. Person may realise they are dying but does not care. Resuscitation possible if carried out immediately.
0 - 6
Fainting almost immediate, painless death ensues, brain damage even if rescued.

If any of the following symptoms appear in situations where asphyxia is possible:
i) Rapid and gasping breath.
ii) Rapid fatigue.
iii) Nausea.
iv) Vomiting
v) Collapse or inability to move.
vi) Unusual behaviour
Immediately move the affected person to the open air. Keep victim warm and rested. Call an ambulance. Apply artificial respiration if necessary

However, attempts to rescue affected persons from confined spaces or where oxygen deficient atmospheres may be present should only be made by those trained in the use of breathing apparatus and confined space entry procedures. This excludes all Birkbeck personnel. The Fire Brigade should be called in all instances where a trapped person requires rescue.

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