Pollution of operating theatre atmosphere

‘ The Council of the Association of Anaesthetists of Great Britain and Ireland set up a working party in November 1974 to review the available evidence and make recommendations concerning the pollution of the operating theatre environment and other areas by anaesthetic gases and vapours.

The working party has now completed its report and the council is issuing the following statement of advice to its members, and to others who may be concerned.

The working party has been advised by representatives from several divisions of the Department of Health & Social Security and from the Medical Research Council who have assisted with the preparation of this statement.

1 Council’s working party is satisfied that significant quantities of anaesthetic vapours are inhaled by anaesthetists even in well ventilated theatres, and that measurable, though lesser, quantities are inhaled by other persons in the operating department. The quantities inhaled vary considerably from location to location.

2 Although numerous effects on the mortality and morbidity patterns of theatre personnel have been suggested and are being investigated, one which seems reasonably probable is a statistically significant increase in the spontaneous abortion rate of females working in operating theatres. There are many factors which influence spontaneous abortion, and the observed rate depends on the criteria employed. It is generally accepted that under normal conditions the spontaneous abortion rate lies between 9 and 15 per 100 live births. Various surveys indicate that the rate for women employed in operating theatres lies between 17 and 27 per 100 live births. It should be stressed that there is no direct evidence that this observation, even if validated, is causally related to the inhalation of anaesthetic agents.

3 However, it is a sound principle of hygiene to take steps to reduce atmospheric pollution as far as possible. Apart from mitigating any effects on their own performance, health and subjective well-being, anaesthetists have a responsibility to consider the welfare of others working in operating theatres.

4 Attempts to reduce pollution need not wait for the development of adequate monitoring programmes nor for an agreed standard on the maximum limits which should be permitted. The aim should be to reduce atmospheric contamination to the lowest practicable levels.

5 Action should be taken as soon as possible to provide facilities for venting residual anaesthetic gases away from the theatre environment. The DHSS will be issuing guidance on appropriate engineering facilities and procedures for their installation.

6 Anaesthetists are advised to take the appropriate steps to institute measures to lower the atmospheric concentration of anaesthetic agents in operating departments. This should be done in consultation with the engineering and other relevant interests.

There are two methods of removing exhaust gases – passive exhaustion and active scavenging.

i Passive exhaustion to the exterior.

It is necessary either to replace the normal expiratory valve with a special valve or to fit a suitable gas-tight hood to existing valves. The expired gases can then be channelled to a suitable exterior discharge point. The patient will be exposed to an additional expiratory resistance but existing prototype systems suggest that 25 mm diameter smoothbore metal piping is satisfactory in most circumstances. This has a resistance of 0.01 cmH2O per metre at a flow of 30 l/min (corrugated anaesthetic hose has a resistance approximately eight times as great as this). We recommend that the resistance of a passive system should not exceed 0.5 cmH2O at a flow rate of 30 l/min. This resistance would be produced by 50 m of unbent pipe.

The resistance of piping with bends should be measured to ensure that it is acceptable. External discharge points may be subject to significant fluctuations in atmospheric pressure and the siting may be critical. It may be advantageous to use a short T-piece as a terminal. The exhaust duct of the operating theatre ventilation system may be more accessible than a suitable exterior exhaust point and in certain circumstances may be used. The ventilation system must be nonrecirculating at the relevant point and not subject to pressure fluctuations. Not all types of theatre ventilation systems are suitable and the district engineer should be consulted.

ii Active scavenging.

An appropriate device must be fitted which prevents either negative or positive pressure being applied to the respiratory circuit. It is also convenient to include a reservoir. This allows mean extraction flow rates to be reduced from about 100 1/min. to about 25 l/min. An open T-piece of adequate internal volume (greater than the tidal volume) can be used for this purpose, but care must be taken to see that the intake cannot be obstructed, and cannot entrain contaminated or dirty air. A 1 litre canister can also be used. If the canister is open to the atmosphere, the inlet and suction points must be arranged so that expired gases are removed preferentially before room air is entrained.

An enclosed reservoir must allow for free overflow in the event of suction failure, and free ingress of room air to prevent negative pressure being applied to the expiratory valve. The working party recommends that systems should ensure that negative pressure greater than 1 cm H20 cannot be applied to the airway. The source of suction can be either a diaphragm pump, a fan, or a venturi. The use of piped medical vacuum systems is considered in paragraph 9.

7 To avoid introducing any additional hazard from possible misconnection, all expiratory exhaust ports on ventilators or anaesthetic apparatus should be fitted either with non-standard (i.e. not 15 mm or 22 mm conical connections as specified in BS3849) fittings, or a 30 mm male cone. This latter is recommended in the draft International Standard for spirometer outlets and is likely to be the British Standard. Likewise any fixed installation points to which an exhaust tube is to be connected should be fitted with a 30 mm female socket or a matching non-standard fitting. The working party hopes that manufacturers will adopt the 30 mm fitting. This cannot so easily be defeated by amateur adaption and will enable devices to be fitted to spirometer outlets of ventilators.

8 Termination points should be clearly marked: DANGER ANAESTHETIC GASES. Since flammable gases may be involved, equipment should also take account of Hospital Technical Memorandum I. The siting of the exterior discharge point should be such that other areas are not contaminated. When overhead pipe runs are used, the possibility of condensation must be considered, and water traps incorporated. Periodic sterilisation of such lines may be necessary.

9 The use of hospital piped medical vacuum systems, though feasible, introduces several special problems and they should only be used with the approval of the hospital engineer. They should not be used with flammable anaesthetic agents. Since anaesthetic agents are soluble in pump oils, routine maintenance of the pumps may need adjustment. The exhaust point for hospital suction may be unsuitable. It must be established that the pumps can handle the additional flow without significant loss of vacuum and a metering orifice or flowmeter should he included in the line. Such systems cannot therefore be regarded as a preferred method.

10 While partial or complete rebreathing techniques associated with low input gas flows will obviously reduce contamination, partially closed systems will still require the use of scavenging devices. Activated charcoal will absorb anaesthetic vapours but is not able to deal with nitrous oxide.

11 There are circumstances in which vapours cannot be reliably scavenged by the above methods, for example, outpatients, dentistry and oral surgery with a Boyle-Davies gag. Continuous removal of large volumes of (contaminated) air as close as possible to the site of loss will minimise the level or general pollution, in proportion to the flow rate. The flow necessary to reliably remove exhaust gases depends on many factors, and it is not possible to specify the optimum scavenging flow rate. Systems unable to handle 250 l/min. are likely to be ineffective in many circumstances.

12 Anaesthetists should consult theatre nursing staff to ensure that anaesthetics are not used for cleaning or for disinfecting surfaces. The importance of avoiding spillage should be included in the in-service training of theatre staff and improved methods of filling vaporisers may be desirable. It is good technique to arrange for vaporisers to be filled at the end of the day, or if necessary, at the end of an operating session, and not at the start of a session.

13 Many of the causes of spontaneous abortion are not well understood. Nevertheless, women with a history of previous spontaneous abortions and those who become pregnant may seek advice as to their environment. The association is unable to give general advice since statistical factors cannot be applied to the individual case. If there is no apparent cause for repeated abortions, counselling should be sought by individuals from their obstetric advisor.

14 Council has advised the DHSS that there is a need for further research into methods of controlling atmospheric contamination, from the point of view of effectiveness, safety and acceptability in various situations. They have also suggested that there is a need for a continuous programme of monitoring of the health of operating department staff. ‘

A footnote to the article appearing in the May 1976 issue of Hospital Engineering states that the text was prepared by Dr M. D. Vickers (Birmingham), secretary of the Association of Anaesthetists of Great Britain and Ireland, at the request of the association’s president and council.

The ongoing story…
Mike Arrowsmith, Health Estate Journal technical editor, provides updating facts and comments.

In fact the debate about exposure to waste anaesthetic gases continued until the Health & Safety Commission published Anaesthetic agents: controlling exposure under COSHH.

This was followed in March 1996 by Advice on the Implementation of the Health & Safety Commission’s Occupational Exposure Standards for Anaesthetic Agents published by the NHS Executive.

To some extent the debate was of little relevance because a British Standard for anaesthetic scavenging systems had already been published in 1987 (scavenging was included in HTM 2022, 1994). A European Standard followed, which has been subsequently revised to take account of the developments in anaesthesiology that have resulted in reduced flows being used in breathing systems, (a revision of BS 6834 is in hand).

Between the publication of the article in Hospital Engineering and that of the British Standard BS 6834 in 1987 there was considerable activity in trying to achieve a standard design of scavenging system. Early scavenging systems were passive, simply being a connection between a breathing system and wall terminal to allow the dispersal of exhaled gases.

There was work on assisted passive systems whereby waste gas from a breathing system was discharged into an exhaust ventilation duct. One such system was pioneered by North West Thames Regional Health Authority, under the aegis of Dr Gill-Rodriguez from St Mary’s, Paddington, and Frank White, assistant regional engineer, and was called the AETU (anaesthetic exhaust terminal unit).

During this period various manufacturers were producing their own active scavenging systems, which all differed in basic design. Because of the unpredictability of passive systems and assisted passive systems, and because of growing awareness of the effect active systems could have on breathing systems – induced flows could be imposed that reduced the available gas for respiration – the Department of Health commissioned a design study with the aim of developing a standard scavenging system. This study by BSRIA eventually led to the publication of the British Standard. This development work was not without its critics, but that is another story…

Whereas the control of exposure to anaesthetic agents in operating rooms is now a simple matter of installing a scavenging system, other areas, such as recovery rooms, obstetrics and dentistry, remain a problem and control of exposure relies on good ventilation.