Sir, in the Opinion article Gone to waste, or something to get your teeth into? (BDJ 2005; 199: 9–12) Geoff Dillow addresses the complicated issue of nitrous oxide scavenging during relative analgesia (or inhalation sedation) in a dental setting. While we applaud any effort to inform and educate staff on this extremely complicated subject, it is felt that certain aspects of the article could also serve to confuse. Much of the information and advice given is correct and pertinent to the subject; however, it is quite clear that the author is not completely up to date on the subject of modern dental breathing systems and methods of exhausting the waste gases through dental vacuum and anaesthetic gas scavenging systems.

A particular area of concern is the specified use of receivers (airbrakes) on AGS systems (or self contained scavenging systems such as the Purair 130). As the recognised definition of an active dental scavenging breathing system is an air flow rate of 45 l/min, using an AGS receiver does not meet strict criteria, induced flow rate on (BS6834: 1997) being only 0.5 l/min at the patient connection. These systems are designed to be passive between the patient and receiving unit inlet and are commonly used with dental passive breathing systems. This low level of induced flow is ample opportunity for breathing system leakage. A further area of concern is with regard to the author's viewpoint of nasal masks and active (45 l/min) scavenging systems currently available on the marketplace. The majority of these are extremely efficient if used correctly and are designed to be exhausted either by connection to a dental vacuum or directly into an AGS wall outlet by means of a special adapter, offering the opportunity of setting and visually checking the 45 l/min flow rate via the use of a vacuum control block — available in-line on most of the modern breathing systems There is a vastly reduced risk of leakage causing ambient surgery pollution from this method if the AGS and vacuum systems are correctly installed, care is taken to ensure a good fit on the nasal mask and the breathing system, including the rebreathing bag, regularly checked.

We strongly suggest that dental surgeries, concerned over scavenging of nitrous oxide seek expert advice on best practice, from a source used to dealing with the specific product — specifically a supplier of said equipment who will be able to advise on exact equipment requirements and therefore minimise ambient surgery pollution and comply with COSHH Regulations.

Note: I write from a very informed viewpoint on the subject, frequently being asked to lecture on the subject for organisations and NHS Trusts. I recently spoke to Mr Dillow who was very interested to hear from me, requested some information on the subject and subsequently thanked me for my advice, stating that he would, in the future, acknowledge me as a reference source, where applicable.

I was quite dismayed to see an article published that could, in parts, only serve to confuse the dental professionals, on what, is admittedly, a very complicated subject. As an individual, backed by my company, we have invested considerable amounts of time and energy over the last two to three years educating on this precise subject and feel, without 'pulling punches' that this type of article will not serve to educate, only confuse. This opinion has already been born out by several telephone calls and emails from senior dental staff requiring reassurance on their choice of nitrous oxide scavenging.

The author of the paper G. Dillow responds: I am always happy to accept correction and criticism where justified and feel that if clarification is necessary this should be brought to everyone's attention. It was pleasing to see that my article was critically examined and obviously stimulated some relevant discussion.

With respect to definitions of scavenging system types, it became clear during conversation with Ms Pickles that 'active' and 'passive' systems could have very different connotations, depending on whether you are a dentist / supplier of dental scavenging systems, or an engineer installing a scavenging system to BS 6834, or to the latest European standard EN 737-2.

To the engineer an active system, as specified in either BS or EN, is one in which a high air flow rate (usually generated by an electrically driven pump) is used to exhaust air through the system's fixed pipework. This air, in turn, entrains waste gases from the patient (or patient ventilator) via a transfer hose and receiver, operating, as Ms Pickles states, at a very low induced flow rate. (Actual system and induced flowrates were stated in my article).

To the same engineer, a passive system, put simply, is nothing more than a pipe through hole in the wall, through which waste gases are driven by patient or ventilator expiratory effort! There is no pump unit involved in such a system.

In the UK, only active systems as defined by the BS or EN Standards above, are considered appropriate for scavenging waste gases from operating areas.

Turning attention now to dental systems, an 'active' system is one in which there is an active flow through the nose mask and this flow carries away the waste gases exhaled by the patient. This flow would, as my article states, be of the order of 45 l/min and obtained by connection of the mask (via a suitable flow-limiting adapter) to either a dental vacuum system or DIRECTLY to an active (BS/EN) scavenging system terminal unit.

A passive system has no such flow through the nose mask and would, as Ms Pickles suggests, lead to considerable spillage of gas, even when connected to the engineer's active (BS/EN) system transfer hose and receiver. This problem was, in fact, stated in my article.

If my article was construed as an encouragement to use (engineer talking) an active system, with its low induced patient flows, in preference to (dentist talking) an active system using a suitable flow controller, then I apologise, for this was not my intention. Rather, the text was intended to highlight possible connections of the nose mask to different types of system and some of the pitfalls (and advantages!) that might occur as a consequence.

Unfortunately, as with many topics, given the unavoidable delay between writing and publishing an article, it was obvious from Ms Pickles' response and my subsequent discussions with her, that practices have moved on somewhat, particularly with respect to the use of 'active' scavenging masks, as understood by most dentists. This is now the 'norm' and, as Ms Pickles states, if used correctly, will offer excellent control over waste gas spillage.

In the light of Ms Pickles' comments, it is my intention to outline the essential features of dental and 'ordinary' active scavenging systems in the forthcoming revision of Health Technical Memorandum 2022, in order to avoid any possible future confusion.