Nitrous oxide scavenging in the 21st Century

09 February, 2011 / indepth
 Janet Pickles  

A frequently asked question – ‘Why should I scavenge when using nitrous oxide sedation?’

The immediate answer is – if you do not, then it is a direct contravention of health and safety on the part of yourself and your staff. Of course, the subject is much more complicated than any simplistic answer and this article will assist with offering a guide to a somewhat misunderstood area of dentistry.

Perhaps a start could be made by looking at some of the standards and white papers that touch on the subject. A white paper commissioned by the Department of Health and published in 2003; Conscious Sedation in the Provision of Dental Care Report of an Expert Group on Sedation for Dentistry mentions scavenging in chapter nine and states: “Scavenging of waste gases must be active and sufficient to fully conform to current COSHH standards. Breathing systems should have a separate inspiratory and expiratory limb to allow proper scavenging. Nasal masks should be close fitting providing a good seal without air entrainment valves.” The definition is fair enough but how does it translate into a good workable system? The paragraph offers three reference sources:

• Anaesthetic and analgesic machines BS4273: 1997 British Standards Institution

• Anaesthetic Agents: Controlling exposure under COSHH. HSAC. HMSO. 1995

• Witcher CE, Zimmerman DC, Yom EM, Piziali RL Control of occupational exposure in the dental operatory. Journal of the American Dental Association (ADA) 1997, 95: 763-776.

Perhaps most interesting is the reference to the ADA. This is a reflection on the fact that the majority of inhalation sedation equipment in use today in the UK is of American origin. Manufacturing of the Quantiflex range; MDM and Mark I (later replaced by Mark II) by Cyprane, began in Keighley in the 1960s under license from Fraser Sweatman. At that time two types of system were available:

• Non-scavenging system (side tubes, exhaling valve)

• Passive system (clear corru-gated 22mm hose, blanking cap)

Manufacturing in the UK ceased in the early 70s, but continues to this day in the USA. Use of the non- scavenging system declined rapidly from the late 90s, but use of the Passive system continued until the then manufacturer, Porter Instruments, ceased production in early 2010. As a result, the standards for scavenging between the UK and USA have always seemed to be intertwined and dependant on the types of equipment available. As standards and attitudes have changed, then what is considered acceptable in scavenging has changed also.

In 1994, NIOSHH (National Institute for Occupational Safety and Health) published a technical report: Control of Nitrous Oxide in Dental Operatories.1 The section on scavenging systems states: “A scavenging system, simply defined, is a means to collect and remove excess gases to prevent them from being vented back into the operating room. Installation of an efficient scavenging system is the most important step in reducing trace gas concentrations. It has been demonstrated that ambient concentrations have been lowered by 90 per cent through the use of an efficient system.” The section is quite comprehensive and details trials using various types of then commercially available systems. This was followed by an article published in General Dentistry, September/October 2002 entitled “Clinical Evaluation of the efficacy of three nitrous oxide scavenging units during dental treatment” Certismo, Walton, Hartzell, Farris. It discussed three breathing systems available at that time; Porter-brown, Matrx ANS and Accutron. The findings concurred with the NIOSHH report in determining that the most efficient system was the Porter-brown – the only ‘double mask’ system.

Now we come to the universally accepted standard of what comprises ‘active’ dental scavenging. The word dental is used quite purposefully, as scavenging for dental application is quite different for that used for general anaes- thesia. This is quite often where the most common misunderstandings occur. The regulations for installation and use of medical gases and anaesthetic gas scavenging are contained within HTM 02-01 published in 2006. However, much of this is concerned with hospital use – operating theatres etc and dental application only receives a very small mention – the addition being made at the 11th hour, so to speak. Dental scavenging is dealt with in Chapter 10 – Anaesthetic Gas Scavenging Disposal Systems and also Appendix L; sections 8, 9 & 10.2

Chapter 10, 10.5 states: “Active Scavenging for dental installation is an entirely different concept. An active system is one in which there is a flow generated through the patient’s nasal mask and this carries away the waste gases exhaled by the patient. This flow is in the order of 45 L/min and is achieved by connection of the mask (via a suitable flow-limiting adaptor) to either; a dental vacuum system or directly to an active scavenging system (BS/EN) wall terminal.”

This obviously reflects the information contained in the NIOSH report dating to 2004 and again reflects the types of breathing systems available in the UK today – again the majority of which are manufactured in the USA. However, this does not alter the fact that 45 L/min has found to be the optimum draw rate to minimise surgery pollution and this has now become a statutory requirement. This requirement is now being taught in all educational courses pertaining to inhalation sedation, an example of which is a publication published in 2009 by Wiley-Blackwell: Clinical Sedation in Dentistry Girdler NM, Hill CM, Wilson KE.

Chapter six – Principles and Practice of Inhalation Sedation, Sub-section Chronic Effects – states: “Active gas scavenging is a statutory requirement during the provision of inhalation sedation with nitrous oxide in the UK. The recognised definition of an active dental scavenging breathing system is an air flow rate of waste gas by the application of low power suction to the expiratory limb of the breathing circuit.

“Passive scavenging – Further ways to reduce trace levels of nitrous oxide include opening a window or door and using floor-level active fan ventilation to the exterior of the building.”

It is interesting to reflect at this point how the definition of ‘passive’ scavenging has altered. It used to refer to the breathing system itself but now refers to additional factors that can be brought into play to reduce ambient surgery pollution, such as low level expelair fans and open doors/windows.

This brings us to a further element of scavenging; how do you actually achieve and monitor an extraction flow rate of 45 L/min? The initial element is obviously the breathing system itself – one capable of withstanding flow rates of 45 L/min and there are several well recognised models available. The second element is the method of achieving the sustained draw and there are basically three ways:

• Use of a BS/EN AGS Wall Terminal fit with an AGS Adapter. This has to be used with a breathing system that has a suitable flow-limiter (often referred to as a vacuum control block).

• Use of chairside suction port. However, the problem here is two-fold. It is most common to employ the high volume suction port which is unpopular and also, the suction has to be capable of offering a sustained 45 L/min during the sedation procedure, something which is not always possible.

• Use of a separate dedicated scavenging pump. The size and decibel level of these units has improved significantly in recent years, increasing their popularity.

When considering any change from current scavenging, it is recommended to seek expert advice on the subject, from a company that specialises in inhalation sedation, as expensive mistakes have been made in the past. The subject should be considered as a whole and no one element taken in isolation
as this can lead to significant error – a very undesirable factor when dealing with nitrous oxide. This article is designed to offer an understanding to the basic requirements. Further detailed information can be found on the R A Medical Services website:

Scottish note: It is heartening to note that Scotland, when considered against other countries comprising the UK, presently ‘leads’ the way in terms of active dental scavenging as approximately 90 per cent of dentists currently employ an active breathing system during inhalation sedation.


1: DHSS (NIOSH) Publication no: 94-129

2: Medical Gases Health Technical Memorandum 02-01: Medical gas pipeline systems. Part A: Design, installation, validation and verification. 2006

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