Liquid nitrogen

1. Introduction

This policy is intended to remind users of the dangers associated with the use of liquid nitrogen, including the risks of asphyxiation, cold burns, and explosions caused by trapped, expanding gas.  The term dewar is used throughout to mean a vacuum insulated vessel operating at less than 0.5 bar gauge.

2. Risk assessment

A written risk assessment must be prepared wherever liquid nitrogen is used or stored, describing any control measures needed to minimise its dangers.  The assessment must consider all relevant risks, including the risk of asphyxiation (see sections 4(a) and 5, and Appendix 1).  Where appropriate, emergency procedures must be included (see Appendix 3), as should the names of those authorised to carry out certain safety-related or higher risk activities (e.g. inspection or maintenance work on dewars, or filling dewars or liquid nitrogen refrigerators from bulk supply tanks).

Generic risk assessments (e.g. departmental risk assessments for liquid nitrogen) can usually adequately cover the risks of cold burns or explosion, but they are unlikely to consider the asphyxiation risk for individual areas in sufficient detail.

3. Properties

Liquid nitrogen is a colourless, odourless liquid of boiling point -196°C, density 0.8 kg/litre, and very low viscosity. As the liquid changes to gas at ambient temperature and pressure, the expansion ratio (the gas factor) is approximately 700.  The resulting cold gas is heavier than air, so it accumulates at low level.

4. Hazards

The hazards of liquid nitrogen are largely related to the large volume of gas produced on evaporation and to the liquid’s low temperature. Its very low viscosity means that it rapidly and completely covers surfaces on which it is spilt and it easily penetrates cracks and voids. This means that any spillage on clothing will penetrate much more readily than, say, water. Large spillages on other surfaces may affect areas beneath the surface, by damaging materials or even by causing oxygen depletion in areas remote from the spill.

(a)   Asphyxiation

On boiling, liquid nitrogen produces approximately 700 times its volume of gas. The resulting displacement of oxygen from the atmosphere may be sufficient to cause asphyxiation.

There is no preliminary warning of oxygen deficiency caused by the addition of nitrogen. This is a significant hazard, which has been responsible for a number of deaths in research institutions over the past few years.

In these incidents, asphyxiation is usually sudden. The victims inhale air with little or no oxygen content, causing immediate collapse into a layer of dense, cold, nitrogen-enriched air. Unconsciousness followed rapidly by death is inevitable without immediate rescue and resuscitation.  Rescue attempts often result in the rescuers being overcome as well.

Smaller leaks or spills, or normal boil-off from liquid nitrogen containers in confined spaces (e.g. poorly ventilated small rooms, or cold rooms) may give rise to lesser reductions in oxygen content, but they may still carry a risk of asphyxiation.

The risk of asphyxiation must be assessed wherever liquid nitrogen is used or stored, taking into account the volume present in relation to the room volume, the likelihood of leakage or spillage, the normal evaporative losses that occur with liquid nitrogen use and any ventilation arrangements. 

Appendix 1 shows how to calculate the oxygen depletion arising from normal evaporative losses and from spills.  As an approximation, if the volume of nitrogen gas (m3) produced from the complete loss of the contents of the largest container in the room is > 0.15 x room volume (m3), then this corresponds to an oxygen content of around 18 vol % (air normally contains 21 vol % oxygen) and further action must be taken to control the risk of asphyxiation.

The physiological efforts of reduced oxygen are shown in the following table. Note that exposure to an atmosphere containing less than 18 vol % oxygen poses a significant risk.

  Asphyxia - Effect of O2 Concentration
O2 (vol %)
 Effects and Symptoms
18-21 - No discernible symptoms can be detected by the individual
11-18 - Reduction of physical and intellectual performance without the sufferer being aware
8-11 - Possibility of fainting within a few minutes without prior warning.  Risk of death below 11 vol%
6-8 - Fainting occurs after a short time.  Resuscitation possible if carried out immediately.
0-6

- Fainting almost immediate.

- Brain damage may occur, even if rescued.


(b)   Cold burns and frostbite

Skin contact with liquid nitrogen or cold nitrogen gas may cause severe cold burns, comparable with those caused by boiling water.  Unprotected skin may freeze onto surfaces cooled by the liquid, causing severe damage on removal. Prolonged skin exposure to cold may result in frostbite, while prolonged inhalation of cold vapour or gas may cause serious lung damage.

The eyes are particularly susceptible – even small splashes of liquid nitrogen, or short exposures to cold vapour or gas, may cause instant freezing of eye tissues and permanent damage. 

These injuries can be avoided by ensuring that users always wear appropriate personal protective equipment (PPE) as described in Appendix 2.  First aid treatment for cold injuries is described in Appendix 3.

(c)   Explosions due to trapped, expanding gas

If liquid nitrogen is trapped inside a container that is sealed, then expansion on warming above -196°C may cause an explosion, giving rise to danger from contamination by the vessel’s contents as well as injury from fragments of the vessel itself.

This is most likely to happen if sample storage vials have been immersed in liquid nitrogen.  The shrinkage and embrittlement of materials at this temperature renders any sealing system ineffective and the relatively low surface tension of liquid nitrogen also makes it likely to seep into the vial.  Similar explosions have been reported with glass vessels - the low temperature caused microscopic cracks or holes to open, which then re-sealed on warming.  Great care must be taken to avoid injury from such explosions and Memo M12/01 dealt with this subject in detail. 

Vessels may also become sealed due to ice plug formation (e.g. in the necks of dewars where the wrong type of stopper has been used, or on the pressure relief devices of dewars stored in damp conditions).  Pressure rise may cause the plug to be ejected, or the vessel may rupture.

If glass domestic vacuum flasks are used for liquid nitrogen, its low viscosity may allow it to penetrate the seal between the glass inner and the outer casing, causing an explosion as it warms and expands.  Glass domestic vacuum flasks must not be used for liquid nitrogen.

(d)   Condensation of liquid oxygen

The boiling point of oxygen is -183°C; therefore liquid oxygen may condense in open containers of liquid nitrogen or in open vessels cooled by liquid nitrogen (e.g. cold traps).  Liquid oxygen will accumulate if the liquid nitrogen is constantly replenished, so this type of open cooling system should be avoided where possible.  The unsuspected presence of liquid oxygen may give rise to explosions caused by increased pressure if the vessels are subsequently sealed and allowed to warm up.  If oxidisable material is present, then liquid oxygen may react explosively with it.

(e)   Effects on materials

Many materials become brittle when cooled by liquid nitrogen and may be irreparably damaged.  Other materials (e.g. glass dewars) may fail due to temperature stresses.  Glass dewars should be enclosed in a metal can or wrapped in tape to give protection against flying glass fragments in the event of such failure.

Use only articles or materials designed for use with liquid nitrogen.  Glass domestic vacuum flasks must not be used as they may fail due to thermal shock on filling. 

5. Ventilation and oxygen monitoring

In order to control the risk of asphyxiation, the following conditions should be met for rooms where liquid nitrogen is stored or used.

They should be sufficiently well ventilated, or sufficiently large, to ensure that the oxygen concentration does not fall below 19.5 vol % due to the routine conditions of use, i.e. due to:

  • the normal evaporation losses from all liquid nitrogen containers in the room
  • the losses caused by filling the largest container from a warm condition.

In addition, the loss of the contents of the largest container immediately after filling from a warm condition should not cause the oxygen concentration to fall below 18 vol %.

Appendix 1 shows how to assess the likelihood of oxygen depletion and the effect of ventilation, and this must be done as part of the risk assessment.

In most rooms, natural ventilation will generally provide around one air change per hour.  For basement rooms, cold rooms, or where there are well-sealed windows, less than half an air change per hour will be achieved. Because they are tightly sealed, cold rooms are particularly unsuitable as storage areas for liquid nitrogen and they must not be used for this purpose.  (In any case, there is no benefit to be gained from keeping liquid nitrogen in a cold room – the small temperature reduction relative to the laboratory has an insignificant effect on the evaporation rate of a liquid that is at - 196° C).

Cold nitrogen gas accumulates at low level, so basement rooms, rooms with ventilation openings only at high level, or rooms with floor ducts or pits may pose particular danger in the event of a spill. Where natural ventilation openings are provided, they should be at both high and low level and ideally have a total area of around 1% of the floor area.  Where mechanical ventilation is provided, then air should be extracted from low level and supplied at high level. 

Where ventilation is insufficient to control the build-up of nitrogen gas, or where leaks or spills would reduce the oxygen content to below 18 vol %, then fixed oxygen monitoring equipment must be used. Care should be taken in siting the oxygen sensors in order to avoid persistent false alarms caused by nuisance triggering (e.g. by direct exposure to gas issuing from containers as they are being filled).  Where false alarms persist, then the sensors must be re-sited in order to prevent any consequent complacency in the response to alarms.  In a well-publicised incident in the UK, a worker was killed by asphyxiation following an incident that occurred while filling dewars.  He had no warning of his fate - the alarms had been turned off because they gave continual false readings while dewars were being filled. 

This equipment normally has two alarm levels:

  • the upper level should be set at 19.5 vol % O2 (if this alarm is triggered, then there should be urgent investigation and corrective action)
  • the lower level should be set at 18 vol % O2 (if this alarm is triggered, then the area should be evacuated immediately).

Alarms must be visible and/or audible both inside and outside of the area monitored, in order to give adequate warning of oxygen depletion.

In some circumstances, personal oxygen monitors may usefully supplement fixed ones.  All oxygen monitoring equipment must be installed, operated, serviced, and calibrated in line with the manufacturer’s instructions.  (Users should be aware that the working life of the common electrochemical cell oxygen detectors is only about one year).

6. General precautions for small-scale use

(a)   Liquid nitrogen containers

Generally speaking, in quantities up to about 50 litres, liquid nitrogen is stored and distributed in simple open-topped vessels, designed to operate at atmospheric pressure (“tulips” or dewar flasks).  They are of lightweight construction and should be handled with care to avoid damage to the insulation.  The smaller flasks may be easily knocked over.

Larger quantities (up to 250 litres) are generally held in transportable liquid cylinders that may be designed to deliver liquid or gas.  They operate at above atmospheric pressure, so they are fitted with safety devices to allow them to vent excess pressure.  The manufacturer’s recommended intervals for inspection and replacement of the safety devices must be observed.  Care must be taken to ensure that any venting takes place safely (as supplied, many such cylinders have safety devices discharging horizontally at eye level) and venting may need to be directed to a safe place outside of the storage area.  Transportable cylinders should be handled with care.  In particular, trolleys used for moving them, or the trolley bases fitted to some cylinders, must be suitably designed and in good condition to avoid accidents resulting in the cylinder tipping over.

Users should be alert to the signs of insulation failure (the need for frequent topping-up, or excessive condensation on the dewar) as the high boil-off rate increases the risk of oxygen depletion.

(b)   Labelling

Liquid nitrogen containers must be clearly labelled showing basic safety-related information, using a label of the type shown in Appendix 4.

(c)   Filling

Only those who have been suitably trained may fill dewars using a hose from a transportable container or bulk tank.  This is a potentially dangerous operation and appropriate PPE must be used (see Appendix 2).  Care must be taken to secure the hose, to purge the line of excess moisture or dust, and to initiate the fill slowly.  If an excessively high fill rate allows an unsecured hose to whip out of the dewar, then the situation may rapidly get out of control, with a high probability of injury or death from cryogenic burns and asphyxiation.

(d)   Handling

Dewars should be handled with care and not ‘walked’, rolled or dragged along the floor - rough handling may damage them, as may severe impacts.  Manual handling assessments (UPS S7/99) will be needed for larger dewars (say > 20 litres) and these may identify a need for trolleys or tipping trolleys.  Stairs and doorways present an added risk of spillage due to tripping, or colliding with someone.  If a large dewar (say > 20 litres) must be carried on stairs, then two people should carry it and the use of additional body protection (e.g. an apron) is recommended.

(e)   Lifts

Liquid nitrogen must not be transported in occupied lifts, because of the danger of asphyxiation in the event of a leak or spill, especially in the event of a lift breakdown. Where this is considered to be impractical, then the University Safety Office must be consulted in order that a safe system of work can be devised instead.

(f)   Transport

For similar reasons, liquid nitrogen must not be transported in a closed vehicle such as a car or van.

(g)   Sample storage containers

Users should be aware that there is an oxygen-deficient atmosphere inside large storage containers.  Care must be taken to ensure that people retrieving samples cannot lean over the containers in such a way that they might breathe this atmosphere and collapse into or over the container, resulting in asphyxiation.

7. Bulk liquid nitrogen

Where bulk supply tanks are used, the consequences of an accident are potentially much more serious because of the quantity of liquid nitrogen present.  Unless steps are taken to prevent it, the entire contents of the bulk tank may be lost.  When such incidents occur, there is a high risk of loss of life.  Therefore special care must be taken in the design and operation of such systems.  Where new installations are planned, or where existing installations are to be altered, then this may not take place without first consulting the University Safety Office.  All such installations must comply with BCGA Code of Practice CP 36 (copies available from the Safety Office).

Where liquid nitrogen take-off points from a bulk supply tank are provided inside a building, whether for manual operation or for automatic filling of storage tanks, then oxygen monitoring must be provided at those points.  In these cases, the low oxygen (18%) alarm must be linked to an automatic valve that cuts off the supply from the bulk tank in the event of the alarm being set off.  This link must operate in a fail-safe mode and be capable of operating in the event of mains power failure.  Additional mechanical ventilation linked to this alarm should also be considered, with low-level extract and high-level air make-up. 

Only suitably trained and experienced individuals should be allowed to operate the system (e.g. to fill dewars or liquid nitrogen refrigerators) or to carry out installation or maintenance work on the system.  They should be identified in the risk assessment.

8. Information, instruction and training

All users of liquid nitrogen must have received enough information, instruction and training to enable them to understand the dangers associated with liquid nitrogen and how this relates to their own work.

9. Summary of departmental action

Where liquid nitrogen is used or stored, then departments must ensure that:

(a)  a written assessment covers the risks relevant to the circumstances of use or storage

(b)  the risk assessment describes, where necessary, any control measures, any emergency procedures, and who is authorised to carry out various tasks associated with the use of liquid nitrogen

(c)  only suitably constructed dewars or transportable liquid cylinders are used for liquid nitrogen; this equipment must be maintained according to the manufacturer’s instructions and must comply with any requirements imposed by the University Safety Policy on Pressure Systems (UPS S7/00)

(d)  any oxygen monitoring equipment and warning systems are suitably sited and maintained

(e)  the provisions of this policy with respect to bulk liquid nitrogen are observed

(f)  sufficient information, instruction and training is provided to users to enable them to understand the dangers associated with liquid nitrogen and how this relates to their own work.

THIS STATEMENT FORMS PART OF THE UNIVERSITY SAFETY POLICY.  PLEASE AMEND THE INDEX.

June 2003