J. clin. Path. (1961), 14, 11.

A survey of some problems in sterilizationROBERT KNOX

From the Department of Bacteriology, Guy's Hospital Medical School, London

The methods of sterilization in use in many hospitalsin Britain at the present time are inefficient and oftenunsafe and many so-called sterilizers do not sterilize.In recent years there has been increasing awarenessof this unsatisfactory state of affairs. At first a fewindividuals condemned the situation and showedsome glaring examples of breakdown in steriliza-tion (Savage, 1944; Walter, 1948; Howie andTimbury, 1956; Bowie 1955, 1957). More recently,stimulated by the original observations, adminis-trators and hospital authorities are becoming in-creasingly conscious that the whole machinery ofsterilization in hospitals needs a drastic overhaul(Nuffield Provincial Hospitals Trust, 1958; MedicalResearch Council Working Party's Report, 1959).At the same time many new techniques and methodsare becoming available. Rival claims are made fordifferent methods and it is important to keep somesense of perspective and to decide for what purposesdifferent methods of sterilization are most suitable.The aim of this article is to assess the presentsituation, to outline some of the main problems, andto suggest some of the lines along which develop-ment is likely to proceed in the immediate and in themore remote future.

AVAILABLE METHODS OF STERILIZATION

For some articles there are many methods ofsterilization available but others can only besterilized by perhaps one or at the most two differentmethods. The main methods are:

1 a, Dry heat: the conventional hot-air oven,

conveyor belt methods, the conducting aluminiumblock, vacuum ovens; b, Moist heat: steam pressuresterilizers, downward-displacement type and high-vacuum type.

2 Chemical methods: liquid disinfectants, gaseous

disinfectants, especially ethylene oxide.3 Irradiation by cobalt 60 or linear acceleratorThere are, of course, many special methods, e.g.,

the use of ultraviolet light for sterilizing surfaces andfor attempted sterilization of air, and the use offiltration for sterilizing fluids which cannot besubjected to other more drastic treatments. Specialmethods of filtration can also be used for the de-

contamination or attempted sterilization of air. Forsurface disinfection formaldehyde may be satis-factory, provided it is used under proper conditionsof humidity and temperature (Public HealthLaboratory Service, 1958). With some types ofequipment, e.g., bedpans, adequate disinfection maybe achieved by exposure to steam at atmosphericpressure. Finally, for many purposes boiling is aperfectly satisfactory method, but it cannot be toostrongly emphasized that many of these methods areonly suitable in exceptional circumstances and nonecan be universally recommended as guaranteeing asterile product.

GEOGRAPHICAL SCOPE AND LIMITATIONS OF DIFFERENTMETHODS

One of the major problems to decide at the presenttime is to what extent sterilization should continueto be done on a purely local basis and to whatextent it should be organized on a larger scale.Until very recently, sterilization has been a verylocal affair and no large British hospital so far as isknown has had a complete central sterile service.Many new hospitals, however, have already con-structed or are planning central sterile supplydepartments. A valuable account of the departmentalready functioning at Musgrave Park, NorthernIreland, has been given by Allison (1960) who alsodiscussed the development and planning of such aservice. Surgeons and others in the past have beenslow to accept the idea of central sterile supply.They have preferred to have their own instrumentssterilized in the immediate neighbourhood of theirown theatre. Syringes have been boiled up readyfor use in the wards and so on. However, it is nowgenerally agreed that central sterile supply has manyadvantages over purely local sterilization unitsscattered throughout a hospital. With the increasingcomplexity of modern sterilizing plant, it becomesmore and more important to get the maximum useout of each unit. This alone is a powerful argumentin favour of centralizing sterilization arrangementsfor any large hospital.On the other hand there is an increasing feeling

that many types of sterilization should not be carried11

out on a hospital basis at all but that commercialproduction and distribution would give moreefficient and in the long run more economical results.This new approach is of course closely linked withthe increasing production of disposable items ofequipment, a trend to some extent forced upon usby the increasing cost of labour and the difficulty ofgetting efficient washing and sterilization equipmentinside the hospital area. With certain types ofequipment it looks as though the day of the largecentral sterile supply department in a hospital isalready over before it has begun, and many hospitalauthorities must be wondering whether it is wise tosink a lot of money in the building of what in a fewyears' time may be a white elephant. The answer tomany of these problems cannot be given at thepresent time and will come only as the result ofexperience. The best that can be done is to hopethat experience will not be bought at too great aprice. For example, it seems fair to say that thesupply of disposable equipment sterilized by steamin commercial laboratories and distributed over awide area is not likely to be an economic proposi-tion. One reason is that the installation of largenumbers of autoclaves with the amount of steamrequired to operate them efficiently is very expensiveand the output is rather limited, whereas with othermethods of sterilization, such as irradiation,although the capital cost is high, there is everyprospect of being able to sterilize large quantitiesof material economically, for example, rubbergloves and syringes or needles. Of course it is unwiseto generalize too much. The relative efficiency andcost of different methods must vary enormouslywith the nature of the material to be sterilized andwith the purpose for which it is required.

METHODS OF STERILIZATION

STERILIZATION BY STEAM UNDER PRESSURE This isa field in which considerable advances have beenmade in the last few years, not so much because ofthe discovery of any new principles but because ofan increasing realization that principles alreadywell known should be efficiently applied if steriliza-tion by steam is to yield the best possible results. Ithas been recognized for many years that rapidpenetration of steam into the interior of dressings isessential if reliable sterilization is to be achieved.One of the traditional methods of removing airfrom steam sterilizers has been to make use of asteam ejector to draw out part of the air containedin the cylinder of the sterilizer before steam wasadmitted, and no sterilizer was regarded as efficientunless it had a 'vacuum' apparatus to remove air.It was, however, pointed out many years ago by

Underwood (1934), Savage (1937), and by othersthat this method even at its best gave only a poorvacuum and could not be expected to do more thanremove two-thirds of the air originally present in thesterilizer. Often it was found that it was removingonly about one-third of the air. This meant that agreat deal of air was still left in the sterilizer. Thisinterfered considerably with the penetration of steamand with the attainment of the required sterilizingtemperature and was responsible for many steriliza-tion failures. American opinion then began tostress the desirability of removing air not by thisinefficient vacuum system but by an efficient systemof downward displacement, and a great deal ofAmerican design and practice in hospitals was basedon the assumption that the best way of removingair was by downward displacement (Perkins, 1956).More recently attention has again been drawn

to the need for rapid and efficient removal of air bymeans of the attainment of a really high vacuum,and, especially on the Continent, much progress hasbeen made (Bowie, 1957).A fresh investigation was made by Knox and

Penikett (1958) of the degree of vacuum required inorder that steam might penetrate efficiently intoa standard drum situated in a particular position inan autoclave. The arbitrary figure of 20 mm. orthereabouts was set as the order of vacuum whichmust be attained before admitting steam so as toensure its rapid and reliable penetration. Furtherexperimental data and a full account of this subjectwere given by Penikett (1960).Knox and Penikett showed that in the conditions

they used it was essential to reach this order ofvacuum and emphasized that manufacturers mustnot be content with any machine which was notcapable of rapidly producing it, and subsequentwork has shown that with considerable variations inload and packing and with different sizes of steri-lizers, this is the kind of vacuum which must beobtained in order to ensure uniformly reliablepenetration of steam. The problem which then arosewas how best this kind of vacuum could be obtained.Vacua of this order can be obtained by a number ofdifferent types of pump, for example, rotary oilpumps, water-ring pumps, and steam-jacketedpumps. Each of these has disadvantages andadvantages but the problem has now become one ofmaintenance and engineering practice, and it is aquestion of finding out which method is going togive the best results over the longest period of timewith the least maintenance trouble and the leastcost to hospital authorities.One point that is still unsettled is the exact time

and temperature which should be used for steri-lizing different types of equipment. There has been

12 Robert Knox

A survey ofsome problems in sterilization

an unfortunate tendency to link together 'high-vacuum' sterilizers with high-temperature, short-time exposure to steam. It often seems to be takenfor granted that the high-vacuum sterilizer shouldalways be operated at a temperature of 134°C. andat about 30 lb. steam pressure. This association isquite fallacious. There is no reason why conven-tional sterilizing times such as 121 'C. at 15 to 20 min.should not be used with the high-vacuum sterilizerjust as with the downward-displacement sterilizer.There may be certain advantages in higher tem-peratures and shorter times. But slight errors intiming become very important if only very shorttimes are used for sterilization and it would seemthat a greater margin of safety is present if periods of15 to 20 min. are used at temperatures of the orderof 121 to 125°C. rather than periods of only aminute or two at temperatures as high as 134°C.This again is a question which can be answered onlyby future experience. At present opinion is divided.For example, we still have not enough informationas to the correct time and temperature conditionsfor the efficient sterilization of rubber gloves bysteam under pressure. Until very recently it wasaccepted that rubber gloves would not stand steamsterilization. Either the gloves were exposed totemperatures which could not possibly be expectedto render them truly sterile or if they were exposed tosterilizing temperatures they were damaged beyondpossibility of further use. It is now realized that agreat deal of the damage to rubber gloves underthese conditions was due to the long period ofdrying which was essential when the downward-displacement method was being used and that thedamage was due to oxidation. If high-vacuummethods can be used and if drying conditions canrapidly be established by drawing a vacuum afterthe sterilization process is over, there seems to be noreason why rubber gloves should not be adequatelysterilized either at high temperatures, such as134°C. for a very short period of time, say three tofive minutes, or at temperatures of 1210 for 15 to20 min. Which of these is the better is still uncertain.

STERILIZATION BY DRY HEAT This subject is coveredin another article and will not be referred to morethan briefly here. Dry heat is used mainly forsterilizing syringes and certain instruments, especi-ally those of which the cutting edge may be damagedby other methods. For syringes, one great ad-vantage of dry heat is that they can be sterilizedalready assembled and can therefore be relied uponto be sterile inside the container in which they havebeen assembled so that the risk of contaminationbefore an injection is given is reduced to the lowestpossible level. In the conventional hot-air oven

sterilization is carried out at a temperature of160°C. for one hour. Darmady and Brock (1954)have shown that conventional hot-air ovens mayvary enormously in their performances and thatthere is considerable risk of either underheating oroverheating, both of which are undesirable. Inovens fitted with efficient fans the temperature ismuch more uniformly regulated and for manypurposes they are entirely satisfactory. Two recentdevelopments are under trial at the present time invarious forms of conveyor belt sterilizers and thesolid aluminium block which heats the syringesplaced in recesses inside the block by direct con-duction. The conveyor belt seems to be the mostuseful apparatus for the large syringe service. Themachine can be fed continuously with supplies ofsyringes, all of which can be guaranteed, providedthat the temperature control is satisfactorilyadjusted, to receive the same heat treatment as theyare carried past the heating units. The aluminiumblock may perhaps have a considerable future forthe small user. It is, of course, not so quick as asmall autoclave which can indeed be used forsterilizing syringes but the autoclave has the strongdisadvantage that if it is to be reliable then syringesmust not be assembled, whereas the aluminiumblock has the great advantage of sterilizing assembledsyringes which need no further handling until theinjection is actually given. A syringe service usingheat as the means of sterilization has to face com-petition from at least two other agents, namely,ethylene oxide and irradiation. More will be saidabout these in later sections. Here, it may be saidthat syringes sterilized by heat under proper con-ditions represent probably the most absolutestandard of sterility which can be achieved. At thepresent time the same cannot be said of ethyleneoxide except under very special conditions and evenirradiation is not an infallible means of producingsterility. At the present time at least one com-mercial firm is running an efficient syringe servicebased on sterilization by heat from a series of gasovens and making use of the conveyor belt principle.

IRRADIATION In recent years increasing attentionhas been paid to irradiation as a means of sterilizingarticles of various kinds. This is being dealt withmore fully in another article in this series, but hereit may be useful to emphasize some main advantagesof the method.

In Britain the method for which most informationis available is the use of cobalt 60 as a source. Linearaccelerators are also being developed for thispurpose. The use of cobalt 60 is only suitable wherethe process can be carried out on a large enoughscale to justify the capital cost and outlay involved.

13

Robert Knox

It must therefore be essentially an industrial process.If, however, this initial cost is accepted, the processhas many advantages which would seem to make italmost unique. Articles of very diverse size, shape,and of different materials can be sterilized withcomplete reliability and very large quantities ofmaterial can be handled, giving probably a muchgreater capacity than that obtainable by any othermethod with comparable running costs. Materialssuch as syringes or rubber gloves can be assem-bled completely packaged and sterilized readyfor use. With properly designed plant and flow ofmaterials it can be guaranteed that all parts of theload and all items in it receive a sterilizing dose,and those problems of penetration and uniformityof treatment, which are so important in sterilizationby steam and ethylene oxide, do not arise.

GENERAL DISCUSSION

Some of the outstanding problems in sterilizationmay perhaps best be discussed by posing a series ofquestions.

1 WHEN IS STERILIZATION NECESSARY? It is neces-sary here to steer a middle course between twoextreme views. The first is that much of the recentinterest in sterilization is misplaced and that simplemeasures such as boiling or the use of chemicaldisinfectants are perfectly satisfactory for mostpurposes. Those who hold this view believe thatmodern developments in sterilization are un-necessarily expensive and do not give an adequatereturn for the tremendous capital outlay involved.They are supported also by those who believe thatcentral sterilizing services are bad for the morale ofnurses and doctors and that it is better that theyshould be taught how not to sterilize properlyrather than to rely on properly sterilized equipment.At the other extreme are those who demand thatalmost every article of equipment used in hospitalsshould b-e sterile. This certainly goes too far in theopposite direction and there is no doubt that thereare many articles for which all that is needed is areasonable standard of hygiene or cleanliness andnot sterilization at all. Bedpans, Ryle's tubes, andeven much of the water which is used in certaintypes of operative work might come under thisheading. It might in fact be far safer to use good tapwater for many of these purposes rather than so-called sterile water from an inefficient tank sterilizerwhich may be heavily contaminated with Proteus orPseudomonas.

it would certainly make the task of hospitaladministrators a good deal easier if a decision could

be reached as to what articles it is really necessary tosterilize. Once this decision is made then it seemsclear that the best possible methods must be usedfor ensuring that they are properly sterilized andmaintained in a sterile state. It is quite true thatold-fashioned methods may suffice for many pur-poses and those who wish to retain them can oftenargue that no harm appears to have resulted fromtheir use. One of the difficulties in all preventivemedicine is that it is often impossible to trace anydisasters that may occur if there is a breakdown innormal routine. On the other hand, there is certainlyenough evidence to indicate that from time to timefatalities do occur as a result of breakdowns insterilization. There is no refuting the argument thatas far as surgical equipment is concerned it is ourduty to see that those articles which really must besterile are sterile when handed to the surgeon.

2 HOW SHOULD STERILIZATION BE DONE ? Differenttypes of equipment demand different methods ofsterilization. For surgical dressings, packs, and manyother fairly bulky items of equipment, steam underpressure is the most widely used and can be highlysatisfactory. Here the problem is one of penetrationof steam into materials from which it is importantthat the last trace of air should be removed. Forthis reason the pre-vacuum type of autoclave is verydesirable. Autoclaves working by downward dis-placement of air can be effective and many whichhave not been working properly can be put into goodworking order (Howie and Timbury, 1956). Some ofthese if they have been manufactured to standhigher pressures and if they can satisfactorily passinsurance tests can be adapted for use as pre-vacuum sterilizers. But wherever new sterilizers arebeing installed it is desirable that pre-vacuum auto-claves should be specified. It has already beenemphasized that these can be a good deal smallerthan downward-displacement sterilizers intended tocarry the same overall load (Medical ResearchCouncil Report 1960; Wells and Whitwell, 1960).The capital and running costs may therefore bemuch less than might be expected.For emergency sterilization of instruments for use

in theatre areas, there is no need for a high pre-vacuum sterilizer and a small downward-displace-ment type is useful. In these it is important thatsteam should enter very rapidly and air be removedquickly. Sterilizers of this sort, even though small,are best fitted with at least two discharge channels ofadequate bore.For the sterilization of bottled fluids, if it is not

done on a commercial basis, a central sterile supplydepartment needs a large downward-displacementsterilizer. The process is slow but devices are being

14

A survey ofsome problems in sterilization

developed to give more rapid cooling and shortenthe whole cycle of sterilization.For syringes, the standard of sterilization at

present is by dry heat either in the form of a hot airoven or a conveyor belt or other devices which arebeing developed. Autoclaves may be used forsterilization of syringes but they are not so reliableor effective unless the syringe is dismantled andreassembled after sterilization. This is in itself adisadvantage and for that reason alone dry heat ispreferable. Other methods such as the aluminiumblock and perhaps the pressure cooker haveadvantages for the small-scale user.

In the future, it seems that irradiation may well bethe most satisfactory method of sterilizing syringes,especially if these are produced industrially andpre-packaged. The same applies to rubber gloves.These can of course be sterilized effectively by steamunder pressure but disposable rubber gloves sterilizedby irradiation are likely to be the answer to many ofthe special problems created by rubber goods.Irradiation on an industrial scale may also be usedincreasingly for many other articles.

It seems that there will still remain for some timemany articles which cannot be sterilized by any ofthese methods, and for this type of equipmentethylene oxide is likely to be the most appropriate.The probable scope of ethylene oxide in the futureis discussed by Kelsey (1961).

3 WHERE SHOULD STERILIZATION BE DONE ? Thisraises many problems of organization and adminis-tration. The whole subject is in such a fluid state atthe present moment that what is said today may beuntrue tomorrow. As we move from the old-fashioned boiler in the ward or in the theatre to asystem of centralized sterilization in a hospital, thelogical conclusion is to move still further in thedirection of large-scale production of pre-packagedsterile articles on a commercial baisis at competitiveprices. But the question how far this process shouldgo and what functions central sterile supply de-partments will have depends on future developmentsin the costs of materials, packaging, and transport,the ease with which supplies can be distributed at eco-nomic rates, and the area over which this can be done.

4 HOW OFTEN SHOULD ARTICLES BE STERILIZED ?One of the dangers of sterilization which is notalways realized is the fact that an article oncesterile may gradually become contaminated if it isstored for any length of time, and the time necessaryfor contamination to occur is quite unknown inmany cases. Any competent bacteriologist, howevergreat his faith in the filtering effect of the con-ventional cottonwool plug, is rightly suspicious of

any sterile test-tubes with cottonwool plugs whichhave been lying about in his laboratory for weeksor months, especially if he has no means of beingcertain that the cottonwool plug has not at sometime been removed and replaced. The extent towhich contamination occurs in surgical dressingsand other equipment depends on the wrapping andthe container in which they are stored. This in itselfis an important subject on which more work needsto be done.

5 WHAT IS MEANT BY STERILITY? This question isnot so easy to answer as it seems at first sight. A fewyears ago most bacteriologists if asked what theymeant by sterility would have been inclined to saythat an object was either sterile or not. This is partlybecause when methods, such as sterilization bysteam under pressure or adequate degrees of dryheat, are used there is usually such an enormoussafety margin in the process that sterility can beguaranteed with almost complete certainty. On theother hand, with some of the newer methods ofsterilization, such as ethylene oxide and perhapsirradiation, it is becoming obvious that the achieve-ment of sterility is largely a statistical process andthat we can set our standard at several differentlevels. With irradiation or ethylene oxide, forexample, we can aim at a kill of 1010 organisms or107 or 105. It becomes therefore necessary to definesterility in terms of the attainment of a certaindegree of killing and we have to make up our mindswhat sort of bacterial population we are likely tofind in different types of material. There is littledoubt that this argument also applies to steamthough less obviously.

6 HOW DO WE KNOW WHEN AN ARTICLE IS STERILE?The whole question of proper sterility tests is com-plex. Practical and effective tests are not easy todevise. They fall into two main groups. In the firsttype we use a test organism whose resistance to thebactericidal agent is known and measure the numberof organisms killed by certain doses or certainperiods of exposure to the sterilizing agent. In theother type of test we simply take samples of thefinished product and see whether or not they con-form to certain accepted standards of sterility. Thesecond kind of test is much more difficult to makereliable. In any laboratory there will always be acertain 'natural' contamination rate and it is noteasy to decide, when a certain rate of contaminationoccurs, whether this represents the best that can beachieved with the existing technique even witharticles which are theoretically sterile, or whetherit represents a genuine breakdown in sterilizationof the object under test.

15

16Robert Knox

With certain types of equipment the interpretationof sterility tests is made even more difficult by thenature of the wrapping in which the material hasbeen sterilized. Pre-packaged syringes, for example,are assembled and then placed inside an envelope ofsome plastic material which is then suitably sealed.The syringes are sterilized and issued ready for useinside these containers or envelopes usually madeof materials which have to be cut by scissors or insome similar way. The very process of removingthe syringe from its container in order to perform asterility test is liable to cause contamination. Howserious this risk is is still uncertain but it is obviouscommonsense that, having taken great care to seethat equipment is sterile, we must also ensure thatthe container can be opened with the least possiblerisk of contamination.

7 HOW SHOULD STERILIZERS BE TESTED ? There arethree main ways in which we can be assured whethera sterilizer is functioning efficiently or not.

i The sterilizer itself must have adequate in-strumentation so that we can see whether the basicconditions necessary for its proper functioning arebeing achieved, for example, in the case of steam,temperature and pressure gauges, in the case ofethylene oxide, proper measurement of humidityand concentration of gas, and in the case of irradia-tion, accurate control of dosage.

ii For determining whether the necessary con-ditions have actually been reached in the materialundergoing sterilization various kinds of tell-taleindicator are essential. The best-known example ofthese is the Browne's tube, which, although notabsolutely ideal, gives information of great valueand can be used in the day-to-day control of steamor dry-heat sterilization procedures. Indicators ofthis kind have the great advantage that they can beput in various positions in the sterilizer inside theloads undergoing sterilization. By preliminary ex-periment it is possible to find out whether there areany areas in the sterilizer or any parts of a loadwhich are likely to escape adequate sterilization butthe great advantage of the Browne's tube is that itcan be used routinely if not inside every load atleast inside a sample number of loads as a routine.The essential feature of an indicator of this kind isthat it should be small. A number of new indicatorsare being developed and it is to be hoped that thesewill soon be available on a commercial basis for allthe main methods of sterilization. It is of coursepossible in the case of steam to measure temperatureactually inside loads by means of thermocouples orresistance thermometers, provided that these aresmall and can be moved about from one position toanother. But clearly a small tube, such as the

Browne's tube, is much more useful for routinepurposes provided it gives sufficiently reliableresults.

iii Bacteriological tests can be carried out fromtime to time provided it is realized that they havesome of the limitations referred to above and thatsterility tests are not quite as easy to carry out as issometimes thought. There is in fact considerabledanger in occasional bacteriological tests beingcarried out in a hospital laboratory unless doneby well-trained people who are well aware of thevarious pitfalls. Growth obtained from a test sampleembedded inside a dressing pack may well indicatenot a failure of sterilization at all but a failure inthe technique employed in the laboratory. Withsome objects, especially if they are large, such failuresare almost impossible to avoid, since it is extremelydifficult to carry out an adequate sterility test on alarge and bulky object without introducing somecontamination in the process. On the other hand,the kind of sterility test which is often carried out inhospital laboratories can be even more dangerousin the opposite direction. For example, if propercontrols are not used and if the organism used is tooeasily killed by heat, then the hospital pathologistmay go on reporting sterile cultures till somedisaster occurs. Only then does a thorough in-vestigation reveal the fact that the sterilizer has notbeen functioning properly for a long time. In suchcases the bacteriological tests are worse than uselessas they give a false sense of security. Adequateinspection and control of the functioning of steri-lizers are far more important than the purely routinemultiplication of bacteriological tests which them-selves may be quite uninformative. On the otherhand, properly conducted bacteriological tests,preferably those in which test organisms of knownresistance to the bactericidal agent are used, are theonly direct method of telling whether the sterilizeris doing its job or not. The situation is very muchthe same as in the testing of water supplies. Bac-teriological tests are the only tests which will tellus directly whether the bacterial content of a watersupply conforms to certain standards but a singleisolated test in inexpert hands is worse than useless.No bacteriological test is of any value at all, how-ever expertly carried out, unless it is viewed inrelation to the background of information aboutthe source from which the water comes, the way inwhich it is treated, and the arrangements availablefor its distribution and maintenance of its freedomfrom contamination.

8 WHAT PART DOES STERILIZATION PLAY IN THE

CONTROL OF HOSPITAL INFECTION? There is greatconcern at the present time about the increasing

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A survey ofsome problems in sterilization

frequency of staphylococcal and other infections inour hospitals (Williams, Blowers, Garrod, andShooter, 1960). At the same time, as has beenmentioned already, there have been many reportsof breakdowns in sterilization and it is clear thatfailure to sterilize is widespread. It is natural thata connexion should have been suggested betweenhospital infection and breakdowns in sterilization.It has even been implied that if we could put our

sterilization in order we should eliminate hospitalinfections or at any rate a large part of them. Thereis little to justify this attitude. Some infections whichoccur in hospitals are undoubtedly due to a break-down in sterility: some cases of tetanus, for example,probably some cases of hepatitis, and an unknownnumber of other infections. But the staphylococcuswhich is causing most concern at the present time isquite easily killed by heat and there is not oftenenough evidence to suggest that a breakdown insterilization is a major cause of staphylococcaloutbreaks, though it may have been from time totime. Hospital infection spreads by many channelsand efficient sterilization is only one of the ways bywhich its incidence may be reduced.The true frequency of hospital infection is difficult

to estimate but there is now general agreement thatalthough the rate of post-operative sepsis is far lowerthan it was before the use of antibiotics, it is now a

good deal higher than it was in the years imme-diately following their general introduction.Although in many ways surgical technique has madetremendous advances it is generally agreed thatthere is a real need for a tightening up of standardsof operative and other techniques intended to reducethe risk of infection. One of these is the provisionof sterile equipment. Here there is no doubt thatstandards have fallen to dangerously low levels. Onthese grounds alone it is essential that we shouldcompletely reorganize our sterilization procedures.

CONCLUSION

Two established methods of sterilization have stoodup to the test of recent criticism, namely, the use

of steam under pressure and dry heat. Otherestablished methods such as boiling, steaming, andthe use of liquid disinfectant are to be regarded nolonger as methods of sterilization but rather asmethods of disinfection. Of the newer methods,ethylene oxide and irradiation are the most promis-ing. It is likely that for many years ethylene oxidewill have a place for sterilizing certain special typesof equipment, but though it is risky to prophesy itseems that irradiation is going to play the mostuseful part in the future. This inevitably meansincreasing centralization, increasing use of dis-posable equipment, and increasing industrializationof the whole process. We seem to be moving into anera in which sterilization will be much less a hospitalresponsibility but will be in fact an industrial processcarried out by industrial methods and with adequatemonitoring and methods of control on an industrialscale.

REFERENCES

Allison, V. D. (1960). Brit. med. J., 2, 772.Bowie, J. H. (1955). Pharm. J., 174, 473, 489.

(1957). Hosp. Engng, 11, 74 and 98.Darmady, E. M., and Brock, R. Barrington (1954). J. clin. Path.,

7, 290.Howie, J. W., and Timbury, M. C. (1956). Lancet, 2, 669.Kelsey, J. C. (1961). J. clin. Path., 14.Knox, R., and Penikett, E. J. K. (1958). Brit. med. J., 1, 680.Medical Research Council Working Party's Report (1959). Lancet

1, 425.Second Report (1960).

Ibid,, 2, 1243.Nuffield Provincial Hospitals Trust (1958). Present Sterilizing

Practice in Six Hospitals. London.Penikett, E. J. K. (1960). Ph.D. thesis. London University.Perkins, J. J. (1956). Principles and Methods ofSterilization. Thomas,

Springfield, Illinois.Public Health Laboratory Service (1958). Report of Committee on

Formaldehyde Disinfection: Disinfection of fabrics withgaseous formaldehyde. J. Hyg. (Lond.). 56, 488.

Sav3ge, R. M. (1937). Quart. J. Phar., 10, 445.--(1944). Ibid., 17, 165.Underwood, W. B. (1934). A Textbook of Sterilization. American

Sterilizer Co., Erie, Pa.Walter. C. W. (1948). The Aseptic Treatment of Wounds. Macmillan,

New York.Wells, C., and Whitwell, F. R. (1960). Lancet, 2, 643.Williams, R. E. O., Blowers, R., Garrod, L. P., and Shooter, R. A.

(1960), Hospital Infection. Lloyd-Luke, London.

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