York 1999
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The main characteristics of low flow anaesthesia can be summed up as follows: Increased rebreathing volume, less excess gas, increasing difference between the gas composition within the breathing system and the fresh gas the lower is the fresh gas flow rate, and a significant increase of the time constants. The reservations with respect to low flow anaesthetic techniques are mainly based on these characteristics: Low flow anaesthesia is assumed to increase the risk of accidental delivery of hypoxic gas mixtures to the patient, of occurrence of gas volume deficiency, of inadvertent misdosage of inhalation anaesthetics or inadvertent rebreathing due to unrecognized exhaustion of the carbon dioxide absorbent, and to impair the controlability of the anaesthetic procedure. The essential monitoring devices, to safely avoid any additional risk directly related to the fresh gas flow reduction, are: continuous monitoring of the inspired oxygen concentration, continuous monitoring of the gas volume circulating within the breathing system, the measurement of the anaesthetic concentration applied to the patient, and capnometry or, preferably, capnography.

Obligatorily, following safety devices are demanded by the new common European Technical Standard on Anaesthetic Workstations and Their Modules:

If the workstation is equipped with an anaesthetic gas delivery module for oxygen it must feature an oxygen supply failure alarm module with an auditory larm (EN 740: 51.103). If the gas delivery module delivers other gases than oxygen, air or premixed gases containing more than 21% of oxygen, it must have a protection module for for cut-off of gases other than oxygen (EN 740: 51.110). Whereas the oxygen supply failure alarm module will prevent from hypoxia in high and low flow anaesthesia likewise, the gas flow cut-off device can´t create additional safety in low flow anaesthesia, as if, for instance, pure air is used as the only fresh gas delivered into the breathing system, under low flow conditions, obviously, hypoxic gas mixtures can develop.

To yet more safely protect from hypoxia, the gas delivery module must have means either to prevent delivery of O2 / N2O mixture with an oxygen concentration below 20% in the fresh or the inspired gas, or to give an alarm at an oxygen concentration of 20% in the inspired gas (EN 740: 51.104). Again it must be emphasized, that a device safeguarding not more than an oxygen content of at least 20% in the fresh gas, by no means can prevent from any accidental development of hypoxic gas mixtures within the breathing system. This statement applies likewise to the Ohmeda „Link 25" and the NAD „Oxygen Ratio Controller". Only by safeguarding an adequate oxygen content in the inspired gas, as it is realised for instance by the Penlon anti-hypoxic device, accidental delivery of hypoxic gas mixtures safely can be prevented.

All the other monitors, obligatorily demanded by the new European standard EN 740, will favourably support safe performance of low flow anaesthetic techniques. The oxygen monitor and alarm module for continuous measurement of the inspired oxygen concentration and its adjustable lower alarm limit (EN 740: 51.104), will help to maintain a sufficient inspired oxygen concentration, not only if an oxygen / nitrous oxide, but also if an oxygen / air mixture is used in low flow anaesthesia. The air way pressure monitoring and alarm module (EN 740: 51.106) or the expired volume monitor and alarm module (EN 740: 51.107) are, likewise, devices for helping to early detecting any gas volume deficiency, which inevitably will result whenever the ventilator isn´t filled properly, causing a consecutive decrease of the minute volume or the inspiratory peak and plateau pressure. Early detection of any gas volume deficiency will be ensured, furthermore, by the obligatorily demanded disconnet alarm (EN 740: 51.108), which, however, must be derived from continuous airway pressure or expired minute volume measurement, and by no means, for this purpose, by capnometry. The anaesthetic gas monitor and alarm module (EN 740: 51.105) will help to manage the high differences between fresh gas and anaesthetic gas composition, characterising low flow anaesthesia, and it will facilitate and assist to metering volatile anaesthetics in case of prolonged time constants. Furthermore, this monitoring will help to avoid accidental misdosage in case that the anaesthetist has to switch back from low to high fresh gas flow rates.

Without any doubt, the demand to exclusively use anaesthetic vapour delivery modules (EN 740: 105.1-2) which are concentration calibrated and flow compensated in the working range given by the manufacturer, and which have to feature agent specific filling systems, is a significant obstacle against the deliberate use of draw over vaporizers inside the circle (VIC), which technically are extremely simple devices.

The obligatory demand for a carbon dioxide monitor and alarm module (EN 740: 51.109) is supported entusiastically by the author: The information provided by this device, although it has thoroughly to be analysed, gives extremely valuable informations on both, not only on the patients condition but also on the technical performance of the anaesthesia machine. Low flow anaesthesia is characterised, indeed, by a pronounced increase of the load of the absorbent with carbon dioxide. Capnometry and capnography will ensure instant recognition in case of unexpected exhaustion of the absorbent and, thus, prevent from inadvertant rebreathing.

Last, but not least, the demands on the gas tightness of the breathing systems, gas loss via leaks must be less than 150 ml/min at an presure of 3 kPa, ensures technical preconditions to safely perform all different techniques of low flow anaesthesia.

Concluding, the implications of the new common European technical standard for anaesthetic work stations on the clinical practice with special respect to low flow anaesthesia can be summarized as follows: All monitoring devices and technical features are demanded obligatorily which are needed, and which are technical preconditions for safe performance of low flow anaesthesia. Thus, the perfomance of low flow anaesthetic techniques in routine clinical practice is facilitated significantly, even if lowest fresh gas flow rates are used.

Although the history of standards in the Western world goes back for hundreds of years, primarily related to the need for uniformity in weights and measures, it was the enormous expansion of manufacturing, transport and trade consequent upon the industrial revolution that is responsible for the modern approach to standardization.

Sir Joseph Whitworth published a specification for screw threads in 1839 which was adopted throughout the British sphere of influence and indeed in Europe, (it was not until 1938 that Hitler banned the use of Whitworth sizes in Germany). However in 1864 the USA adopted a different screw thread as a National Standard, an early example of the problems that came to underline the need for International Standards and one that was not solved until the "Declaration of Accord" between Britain, Canada and the USA in 1945.

In 1901, provoked by the multiplicity of sizes and lack of conformity in manufactured steel products, representatives of the Institutions of Civil Engineers, Mechanical Engineers, Electrical Engineers, Naval Architects and of the Iron and Steel Institute set up the Engineering Standards Committee and their first Standard was published in February 1903 (a very short gestation period by modern standards!)

The Committee became the British Engineering Standards Association in 1918 and in 1929 the Association was granted a Royal Charter in which its objectives were defined:-
"that standards should fulfil a recognized need, be arrived at by general consent and preserve community of interest between producer and consumer"
Two years later, with the grant of a supplementary Charter, the Association became the British Standards Institution.

The first World War lead to the forerunners of national standards bodies in France, Germany and the USA and in 1926 the International Federation of the National Standardizing Associations (ISA) was formed but it received limited support. However, the second World War accelerated the demand for international standardization and this lead to the formation of the International Organization for Standardization (ISO) in 1946 (The International Electrotechnical Commission, IEC, had been set up many years previously in 1906 following an Anglo-American initiative).

The European Committee for Standardization (CEN) was founded in 1961 and its electrical counterpart, The European Committee for Electrotechnical Standardization (CENELEC) in 1973. They both include representatives from the national standards bodies of all the EC and EFTA countries.

Unsurprisingly, BSI is not able to retain the minutes of all of its 2888 technical and subcommittees and so the date on which the two original committees concerned with anaesthesia;
SGC/15 - Components for Anaesthetic Apparatus
and
SGC/22 - Respiratory Resuscitation Equipment
were formed is not easy to determine but it was probably in the early 1950s - by 1965 SGC/15 had held 57 meetings and that probably represents some 10 to 15 years of work. This committee published its first British Standard, BS 2927 Oro-pharyngeal airways, in 1957.
Largely as the result of a further UK/US initiative, ISO/TC121 Anaesthetic and respiratory equipment held its first meeting in London in 1967 and from its inception BSI has held the secretariat and the chairman has been from the UK. There are now 56 publications relating to anaesthesia in the draft business plan for ISO/TC121.

CEN/TC215 Respiratory and anaesthetic equipment was inaugurated in 1989, also with BSI holding the secretariat and with a British chairman. From the start this committee and its four working groups - analogous to ISO subcommittees - had an extensive programme, based largely on existing ISO Standards and Drafts but with the obligation to address the then forthcoming Essential Requirements of the EU Directives - and the intention of reaching publication more quickly than was being achieved at that time within ISO. Virtually all anaesthetic equipment falls within the purview of the second of the three Directives relating to medical devices, which was published in 1993 and came into force on 1st January 1995.
This marked a significant point in the history of Standards for although in some European countries National Standards had force of law for the most part they represented good practice and their adoption was voluntary, although strongly recommended; this was about to change.

From 13th June 1998 (the DOW) all conflicting National Standards were withdrawn and the appropriate European Standards (ENs) implemented without change (except where this would be in conflict with existing national laws) throughout the 18 member countries of CEN. A "Competent Authority" (CA) was designated in each of these countries with responsibility to ensure that all medical devices intended to be placed on the market conformed with the Essential Requirements. In turn, the CA appointed several Notified Bodies to carry out the conformity assessment procedures necessary before a manufacturer could affix a CE mark to a particular product which would then have unrestricted circulation throughout the EC.

The future
There is now in existence a Global Harmonization Task Force (GHTF) which has agreed the Essential Principles for safety and performance and ISO/TC210 (Quality management and corresponding general aspects for medical devices) has this year issued a Technical Report as a Committee Draft (CD) for comment entitled:
"Medical devices - Guide to the selection of standards in support of recognized essential principles for safety and performance of medical devices"
The aim of this draft is to identify significant standards and guides of use to manufacturers, standardization bodies and regulatory authorities for the assessment of product conformity. It is conceivable that this could result in the granting of an international equivalent of the CE mark.
There is thus a clear trend, already in place in Europe, towards legally enforceable requirements backed by risk assessment and analysis and intended to protect the consumer, the user, third parties, the environment, etc.
The process of harmonization between CEN and ISO standards for anaesthetic equipment started in June this year with the first meeting of an ISO/IEC joint working group, with CEN participation, charged with harmonizing ISO 8835 parts 1, 2 & 3 with EN 740 under the Vienna Agreement.

Is there a downside to what seems to be the inevitable march of progress?
All current standards acknowledge the need not to deter innovation but General Requirement 6 of the European Essential Requirements states that:-
"Any undesirable side-effect must constitute an acceptable risk when weighed against
the performance intended."
so one has to be concerned that the manufacturer of a truly innovative medical device might well hesitate before placing the device on the market.

Secondly, the need to protect the patient against hazardous output of energy or substances from the equipment leads to the specification of appropriate monitoring, alarm and protection devices - which is entirely consistent with the aims of device standard. However, there is also a tendency to specify how the equipment is to be used, which is clearly the responsibility of the operator (the anaesthetist) under professional guidelines and not of the manufacturer (nor of the Standards drafting committee).
This leads to consideration of the role of the clinician in medical device standards drafting. Because of their historical background, international technical committees often contain a preponderance of members with engineering expertise and given the present trend to ever larger multi-national companies it would not be surprising if there were to be a potential conflict of interest as an increasing number of the established members of a particular subcommittee or working group find themselves working for the same parent company. So the clinician is likely to remain of crucial importance in realizing a realistic balance in a highly technical field.
And herein there are problems.
Apart from difficulty in recruiting anaesthetists with an interest in this less-than-glamorous pastime, the expense of their attendance at meetings within the UK has to be borne by their employer. Assisted travel to overseas meeting comes from a government fund (£2.7M this year) administered by BSI and normally granted to only one person - who may or may not be a clinician - for each committee, subcommittee or working group. The allowance is based on a maximum of 70% of the published economy airfare.
I believe that it is essential for specialist groups such as ALFA with a particular interest in anaesthetic equipment to be involved in the process of standardization and to bring pressure to bear on the major anaesthetic institutions to maintain an appropriate level of support.

Finally, because of the multiplicity of technical committees at all levels of the standardization process in so many different fields there is a real risk of parallel developments taking place in separate committees without this necessarily becoming apparent, even at the stage of publication; for example:- CEN/TC79, Respiratory protective devices, published two European Standards EN 137 and EN 250 in 1993 relating to self-contained open-circuit compressed air breathing apparatus and diving apparatus respectively in which some of the requirements and test methods are of potential relevance to anaesthetic breathing systems. It would obviously be of value to know of experience gained with these standards.

At the beginning of this year BSI declared its commitment to electronic document distribution and this suggests a possible answer to the practical difficulties of obtaining appropriate participation in standardization, given the potentially widespread availability of Drafts for Public Comment on the internet. Let us hope that this will prove to be a realistic approach.

Useful references:-

1. BSI: the story of standards C Douglas Woodward 1972
2. The European Directives: safeguarding the patient and staff Roger Feneley and Susanne Ludgate Health Trends, Vol. 26 No. 4, 1994
3. Historical note The story of the gauge J S Poll Anaesthesia, 1999, 54, 575-581

To follow:

Cylinder colours in various countries and according to ISO and Pr EN standards

The principles of inhalational anaesthesia have remained essentially unchanged since Morton demonstrated the effect of ether in 1846. This long period alone suggests a reappraisal of the technique is needed and this is made more urgent by the current threats to inhalational anaesthesia These are firstly, that it is environmentally damaging and secondly, that there is an alternative technique, total intravenous anaesthesia (TIVA), that does not cause environmental damage. If inhalational anaesthesia is to remain a mainstream anaesthetic technique then these threats must be challenged.

The environmental threat comes from the effects of nitrous oxide and volatile agents on theatre personnel and in the atmosphere. Nitrous oxide is significantly more harmful than the volatile agents as it is both a greenhouse gas and an ozone layer depleter. The simple solution is to stop using it and to use air/oxygen as a carrier gas instead. The reasons for using nitrous oxide - analgesia, less volatile agent required and reducing the possibility of awareness - are no longer valid given the new analgesic drugs, new volatile agents and the ability to monitor vapour concentrations accurately. There are also many other good reasons not to use nitrous oxide as well as its environmental effects, such as its effect on closed body spaces, its potential to cause hypoxia, postoperative nausea and vomiting (PONV), its effect on the bone marrow etc.

The environmental effects of the volatile anaesthetics vary according to the agent. Halothane, enflurane and isoflurane, being chloroflurocarbons are more harmful to the ozone layer than sevoflurane and desflurane which only contain fluorine. On environmental grounds these latter should be the preferred volatile agents to use. They are however expensive and to make their use economically feasible they should be used in as small a quantity as possible. In practice this means using a circle technique with as low a flow as possible.

If nitrous oxide is not being used then low flow circle anaesthesia becomes a much simpler technique as varying flows are no longer needed at the start and finish of the anaesthetic and the same low flow can be used throughout. However, a continuous low flow creates the problem of getting an adequate concentration of volatile into the circle with the vaporiser out of the circle (VOC). The solution is to use an in circle vaporiser (VIC) as vaporisation is then related to the patients minute volume not to the fresh gas flow. This would only work with sevoflurane, as the low BP of desflurane makes it impractical for use in an in circle vaporiser.

As yet there is no in circle vaporiser designed to current standards. The Oxford Miniature Vaporiser is a very satisfactory in circle vaporiser . It is still in production and could be modified to bring it up to current standards. An alternative to using a VIC is injection of the volatile agent directly into the circle using a syringe pump although this does not give the fine control provided by a vaporiser dial.

If nitrous oxide is not being used, then scavenging by absorption with activated charcoal would effectively prevent the discharge of exhaust volatile agents into the atmosphere and so eliminate atmospheric pollution. However, this would then create the environmental problem of the disposal of the exhausted activated charcoal. It is possible that recycling of the absorbed volatile agent could be developed.

Very low flow and closed circle techniques introduce the problem of reactions between volatile agents and soda lime to produce potentially harmful products, such as compound A with sevoflurane. Although in practice this is probably not a clinical problem, it would still be desirable to develop a carbon dioxide absorbent that does not react with volatile agents, as is being done by the group from Belfast.

TIVA is being promoted as an alternative to inhalational anaesthesia on the grounds that it is not environmentally damaging, it has a better recovery profile and it causes less PONV. If the changes described above ie to stop using nitrous oxide, to use the least environmentally damaging volatile agents in the smallest quantities possible and to scavenge with activated charcoal were adopted, then the environmental effects of inhalational anaesthesia would be virtually eliminated such that it would become
environmentally acceptable Sevoflurane has a good recovery profile and removing nitrous oxide from the technique could help to reduce PONV. To find out if TIVA has a real advantage over inhalational anaesthesia a study is required comparing TIVA with sevoflurane in air/oxygen with regard to recovery and PONV. The cost of each technique should also be incuded in such a study as very low flow circle techniques are cost effective.

A recent study [1] comparing TIVA with sevoflurane in nitrous oxide/oxygen showed comparable recovery between the two groups, an increased incidence of PONV in the sevoflurane group and a cost advantage to the sevoflurane group with a FGF of 1 l/min. Omitting nitrous oxide from the sevoflurane technique could well reduce the incidence of PONV and using a closed circle with a FGF of only basal oxygen could
further improve the cost advantage of a sevoflurane technique.

In summary, the measures proposed to ensure the continuation of inhalational anaesthesia as a mainstream anaesthetic technique are:-

1. Stop using nitrous oxide, use air/oxygen as a carrier gas instead.
2. Use sevoflurane as the preferred volatile agent.
3. Use a closed circle anaesthetic technique.
4. Design a modern in circle vaporiser.
5. Scavenge with activated charcoal and investigate the possibility of recycling the absorbed volatile agent.
6. Develop a carbon dioxide absorbent that does not react with volatile agents.
7. Carry out a study to compare TIVA with sevoflurane in air/oxygen delivered by a closed circle technique to quantify the threat from TIVA.

If no changes are made to the current way of practising inhalational anaesthesia, then it is likely to become a fringe technique in the foreseeable future.

1 Smith I, Thwaites A J. Target-controlled propofol vs. sevoflurane: a double-blind
randomised comparison in day-case anaesthesia. Anaesthesia 1999; 54: 745-752

ANAESTHETIC GASES OCCUPATIONAL EXPOSURE CONSIDERATIONS

Maureen Meldrum.

Health and Safety Executive.

Bootle, Merseyside

This presentation describes the background to the setting off Occupational Exposure Limits  (OELs) in the UK for anaesthetic gases, with particular reference to nitrous oxide, as this was the gas which attracted most debate.

Within the UK, the Control of Substance Hazardous to Health Regulations (COSHH) 1988 provides the legal basis for OELs. Under COSHH there are two types of OEL: Maximum Exposure Limits (MELs) and Occupational Exposure "Standards (OESs). OESs represent airborne concentrations averaged over a specified time period which, according to available scientific knowledge, will not damage the health of workers exposed to those levels by inhalation day after day. Hence, OESs are often referred to as 'health-based' standards. MELs have a different legal status to that of OESs, and are applied to substances with serious health concerns for which it is not always possible to identify a threshold level of exposure below which there would be no residual risk. The establishment of MELs is primarily based on the lowest level of occupational exposure which can be reasonably achieved by industry.

Anaesthetic gases were selected for review by the UK Health and Safety Executive (HSE) in   \/ the early 1990s following reports of spontaneous abortion in operating staff personnel linked to occupational exposure to anaesthetic gases. Prior to this time, no OELs had been in existence in the UK for any of the four key anaesthetic gases in use at that time (enflurane, halothane, isoflurane and nitrous oxide).

HSE compiled a review of the toxicological and occupational exposure data on these gases which was evaluated by WATCH an independent committee of experts in occupational health (the Health and Safety Commission's Working Group on the Assessment of Toxic Chemicals).  WATCH recommended that OESs be set and these recommendations were endorsed by  ACTS, the Health and Safety Commission's Advisory Committee on Toxic Substances.  As  with all OES proposals, there followed a period of public consultation. However, there was some controversy which developed surrounding the proposal for nitrous oxide, based on the interpretation of the toxicological evidence, and also on concerns for possible excessive costs of achieving the standards proposed. Therefore there was a particularly extended period of public consultation.

The WATCH position on nitrous oxide centred on the evidence from studies in animals for effects on the developing fetus. WATCH concluded that the underlying toxicological mechanism was the inhibition of vitamin 812 methionine synthetase, leading to an impairment of folate metabolism and DNA synthesis. Although the rate of onset of methionine synthetase inhibition in the liver was faster in the rat than the human, the eventual extent of inhibition was indeed to be similar in both species. Hence, although there was no entirely convincing evidence in humans for developmental toxicity resulting from occupational exposure to nitrous oxide there was felt to be a plausible biological mechanism by which this could occur. This view was reinforced by the evidence for inhibition of DNA synthesis in dentists occupationally exposed, and from patients given nitrous oxide anaesthesia.

WATCH proposed an OES of 100 ppm (as an 8-hour time weighted average) for nitrous oxide.

During the extended period of public consultation on the OES proposals for the anaesthetic gases, the Committee on Toxicity (COT) was consulted. The COT is the foremost independent advisory committee to UK government departments on toxicology issues. Overall, the COT ⁽¹⁾ concurred with the position reached by WATCH for nitrous oxide. It concluded that the large number of studies in the rat were consistent in showing adverse effects on reproductive outcome. At very high exposure concentrations there are teratogenic effects and at lower concentrations (1000 ppm and above), there are fetotoxic effects (reduced fetal size and weight). The COT agreed that the critical NOAEL from the rat data was 500 ppm.

The COT also reviewed the human evidence i.e. the results of studies which had investigated reproductive outcomes in people who had been occupationally exposed to nitrous oxide for prolonged periods. It concluded that these did not allow the animal evidence to be discounted. Indeed, the COT considered that the human data, despite its limitations, tended to reinforce the concerns which arise because of the effects seen in animals.

The concerns about the potentially excessive costs of control to the proposed OES of 100 ppm as an 8-hour time weighted average for nitrous oxide were shown to be unjustified. HSE published guidance giving practical advice on ventilation control measures noting that adherence to these measures would achieve an appropriate degree of control without incurring undue additional costs ⁽²⁾.

Agreement on the OES proposals for the remaining three anaesthetic gases was more straightforward, and eventually the OES proposals for all four gases were agreed and were implemented in the UK. in 1996. The OES values for these gases are listed in EH40 ⁽³⁾.

References.

1. Department of Health Committees on Toxicity Mutagenicity Carcinogenicity of Chemicals Food, Consumer products and the Environment. 1995 Annual Report. HMSO.

2. Health and Safety Executive. Anaesthetic agents: Controlling Exposure under COSHH. HSE Books. ISBN 0717610438

3. Health and Safety Executive. EH40/99 Occupational Exposure Limits 1999, HSE Books. I

Man and the Atmosphere.

JF Nunn.

The atmosphere of the earth contains oxygen and is not in chemical equilibrium: this is unique in the solar system and is due to life. Any primary atmosphere was lost by the combined effect of solar wind and the intense heat of impacts during formation of the earth (c. 4,600 Ma (= 4.6 x 10⁹ years) ago). After surface cooling, a secondary atmosphere developed as a result of outgassing, supplemented by a secondary veneer from comets and meteorites. Water vapour condensed forming oceans, probably by about 4,000 Ma ago. The secondary atmosphere was mainly carbon dioxide with some nitrogen, as are the present atmospheres of Venus and Mars. Ammonia would have undergone photo-dissociation and hydrogen would be lost from the earth's gravitational field so the secondary atmosphere would be only weakly reducing. Carbon dioxide steadily decreased by reaction with silicates to form carbonates and silica.

Photosynthesising cyanobacteria certainly existed 3,500 Ma ago, and probably earlier. Oxygen released by these organisms reacted with soluble bivalent iron, leached from basalt, to deposit enormous quantities of trivalent iron as the ''banded iron formations". For about 1,500 Ma, this immense sink prevented oxygen achieving more than trace concentrations in the atmosphere. However, from 2,000 Ma ago, iron was deposited in the trivalent "red beds", indicating an appreciable atmospheric concentration of oxygen. Banded iron formations were seldom formed after about 1,800 Ma ago.

Decay of living material released methane into the atmosphere, particularly between 3,500 and 2,500 Ma ago. This provided an appreciable greenhouse effect at a time when carbon dioxide concentrations were rapidly declining, and solar radiation was minimal.

The overt fossil record began about 570 Ma ago (Palaeozoic Era) with the abrupt appearance of all the main invertebrate phyla. It is thought that carbon dioxide concentrations had then fallen to about 0.5 % and oxygen had risen to about 15 %. It is unclear to what extent the increase in oxygen concentration contributed to the apparent explosion of life. There was a huge increase in land based vegetation during the Devonian and Carboniferous Periods (490 -290 Ma ago). This resulted in a reduction in carbon dioxide concentration to about the present atmospheric level, with carbon burial as fossil fuel. Solar radiation was then less than at present, and an extensive glaciation ensued. During the Devonian and Carboniferous, enhanced photosynthesis also caused the atmospheric oxygen concentrations to increase above its present level. Some life forms evolved to utilise a high ambient oxygen partial pressure.

During the following Period (the Permian), there is evidence that these atmospheric changes reversed, with a sharp increase in carbon dioxide and decrease in oxygen. Either or both have been blamed for the extinction which terminated the Palaeozoic Era about 250 Ma ago.

The last 2 Ma (the Pleistocene) have been characterised by well documented cycles of glaciations. Fossil remains indicate unequivocal evidence of fairly rapid variations in temperature with England, for example, oscillating between sub-arctic and sub-tropical climates. Ice cores have been drilled in both Antarctica and Greenland, dating back to snow which fell 420,000 years ago. Trapped gas bubbles show that the atmospheric concentrations of both carbon dioxide and methane closely mirrored the temperature levels inferred from the ratio of ¹⁸oxygen to ¹⁶oxygen (a measure of global ice volume), which is in agreement with the fossil record. However, careful analysis shows that the gas changes followed (lie temperature changes by some thousands of years, thus appearing to be the effect rather than the cause of  the temperature change, although the gas changes must have provided some positive greenhouse feed-back. Oceanic sediment samples go back more than 2 Ma, and the findings are similar in principle. During the last 250,000 years, atmospheric carbon dioxide concentrations have generally remained within the limits of 200 - 300 ppmv.

The cause of these temperature changes now appears to be cyclical astronomical forcing, as proposed many years ago by Croll and Milankovitch. The three most important cycles are:

a) changes in the ellipticity of the earth's orbit round the sun (periodicity 96,000 years)

b) changes in the inclination of the earth's axis (periodicity 42,000 years)

c) precession of the equinoxes (periodicity 26,000 years) The first appears to place the earth in a zone of interplanetary dust particles. It is possible to calculate the summation of these influences on Earth's temperature, and the results accord remarkably well with observed temperature inferred from ¹⁸O/¹⁶O ratios. Thus it appears highly unlikely that greenhouse gases were the prime factors controlling glacial cycles.

In the last 250 years, man has had major influences on the atmosphere. The most obvious is the increase in atmospheric C02 concentration as a result of burning fossil fuels. The concentration was 280 ppmv at the beginning of the industrial revolution in 1750, slowly increasing to 310 by 1950, but now increasing by 1 ppmv/year, which is 100 times the usual rate of increase at the end of an ice age. Methane is increasing in parallel. Other greenhouse gases include nitrous oxide and CRCs, both of which have very long atmospheric half-lives. In the long term, these trends in greenhouse gases are unlikely to be of trivial significance. However, the Milankovitch cycles suggest that the next ice age is imminent, and an increase in greenhouse gases might well diminish its impact.

It would be an oversimplification to believe that global temperature is determined solely by Milankovitch cycles and greenhouse gases. Sulphuric acid droplets in the atmosphere reflect solar radiation, and their increase has hitherto offset much of the global warming expected from the CO-i increase. Changes in thermohaline circulation could be immensely important, especially for Europe. The Gulf Stream is the main factor which gives England a temperature so much more favourable than Hudson Bay, which is at a similar latitude. Closure of the Panama Isthmus and the continuing elevation of the Himalayas must have had profound effects. Unpredictable factors include a major asteroid impact or a flood basalt eruption.

The other main impact of man on the atmosphere is destruction of the ozone layer in the stratosphere. Ozone can be destroyed by many free radicals including chlorine and nitric oxide. Highly reactive chlorine cannot normally pass through the troposphere to reach the stratosphere, and so is generally harmless. However, CFCs are highly stable, and pass freely through the troposphere, then undergoing photodissociation to produce chlorine radicals, each of which may destroy 10,000 molecules of ozone. Nitrous oxide will also pass the troposphere to produce nitric oxide radicals with similar effect on ozone. Minimal Antarctic levels of ozone fell from 300 Dobson Units on 1960 to 100 in 1994 and are still falling.

REFERENCES 70 key references can be found in: . Nunn, JF, 1998, Evolution of the Atmosphere. Proc. Geol. Assoc. 109, 1-13.  and also:

Chapter 1 (Nunn) in: Nunn's Applied Respiratory Physiology (5th Edition, 2000) by AB Lumb. Butterworths.

See also:

Muller RA & MacDonald GJ, 1997, Glacial cycles and astronomical forcing. Science. 277, 215-218. Kortenkamp SJ & Dermott SF, 1998, A 100,000-year periodicity in the accretion rate of interplanetary dust. Science, 280, 874-876.

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