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Thank you for the chance of responding to the comments of Drs King and Hepp who make two points. First, that trace anaesthetic concentrations in our labour suite would constitute an occupational hazard and, second, that nitrous oxide and isoflurane are significant atmospheric pollutants when released by medical use. Neither of these statements is supportable. All researchers in the UK should be aware that they work under the statutory requirements of the Health and Safety at Work Act (1974) and the various regulations issued under the Act. The Management of Health and Safety at Work Regulations (1992) require a risk assessment to be performed for all aspects of work. Any research project therefore can only proceed after a risk assessment has been performed and documented. Our study was no exception and a risk assessment was carried out. Part of the assessment involved personal monitoring of midwives for nitrous oxide exposure during their shift in the labour ward which was not at that time equipped with scavenging. The highest 8-h time-weighted average exposure (TWA) measured was 85 parts per million (p.p.m.). The UK approved occupational standard for nitrous oxide is a long-term exposure limit (8-h TWA reference period) of 100 p.p.m. Our study introduced scavenging in order to further lower these already permissible levels. At around the same time we conducted a study which failed to identify any significant haematological impact of nitrous oxide exposure in the labour ward [1]. There are also no studies identifying a health effect of trace levels of anaesthetic gases in areas where scavenging is used [2] and therefore we were unable to identify any risk to maternity unit staff. Had we been able to do so, the study would not have been performed. The environmental impact of the release of isoflurane, and other volatile anaesthetic agents, was the subject of a line of correspondence in the Lancet in April–May 1989. The outcome of this was to point out that, in reference to industrial activity, the production of volatile anaesthetic agents was trivial [3]. At that time it was concluded that the volatile anaesthetic agents posed a minimal threat to the ozone layer [4]. At the fourth meeting of the parties to the Montreal Protocol in 1992 it was agreed to phase out chloroflurocarbons by 1990 and hydrochlorofluorocarbons (HCFCs) by 2030. Having said that, the production of chemicals graded as lower grade greenhouse gases by the 1997 Kyoto conference, such as HCFCs, perfluorocarbons and sulphur hexafluoride, is on the increase, at least in the United States [5]. For the foreseeable future therefore the modest production of volatile anaesthetic agents is going to remain trivial in comparison with the activity of industry as a whole. The authors state that the medical usage of nitrous oxide is responsible for 10% of all emissions of this gas. I find this statement extremely difficult to believe. The article they quote is in Russian and I regret to say that I have been unable to read it. In 1989, it was estimated that the UK medical use of nitrous oxide was 1 × 109 litres and this was compared to an estimate of production from microbial breakdown of agricultural nitrates of 8 × 109 to 3 × 1010 litres [4]. At that time therefore medical usage was thought to be about 10% of release from agricultural sources. This does not take into consideration other sources of nitrous oxide such as industrial processes and fossil fuel combustion which contributed 37% and 10.8%, respectively, of total UK emissions in 1996 [6]. Intriguingly, nitrous oxide emission from cars is on the increase due to the use of catalytic converters. While there is no reason to assume medical nitrous oxide consumption has risen in the past 10 years, in 1990 the method of estimating nitrous oxide emissions from agriculture was changed. This change multiplied these emissions by a factor of nearly 15 [6]. These considerations imply that the contribution of the release of nitrous oxide from medical sources to the total anthropogenic nitrous oxide emissions in the UK is about 0.35%. Other workers have come to similar conclusions elsewhere [7, 8]. As a result of international agreements, many countries complete an inventory of their production of greenhouse and ozone-depleting chemicals and many of these are available on the Internet. Nowhere is the medical use of volatile anaesthetic agents or nitrous oxide stated as being of measurable concern. Neither the Montreal nor Kyoto Protocols concern themselves with the emissions of anaesthetic gases from the medical industry. The problems of global warming and ozone depletion arise from areas of human activity several orders of magnitude greater and until these can be controlled, no purpose will be achieved by limiting the very useful application of these agents in medical practice.