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ASBESTOS DUST CONG A PRACTICAL AP P. G. Harries Medical Research Unit, HM Dockyard, Devonpori. . . {Received A January 1911) .Pwriwt--A su/v'oy of the asbestos fibre concentrations associates vdA work invoicing ashes1'- is insulating materials has been undertaken in Devonpori Dockyc ,-d. The results show rm1 '''oiicr-uion 'irn removal oi asbestos materials both create high dust conceura'-tioos. and res to redone Chi health, hazards; associated with such processes arc described. The 'v.'fe. ov sarnpi ;tg after the introduction of preventive measure:, arc Ls-oe.f> presented tmd Lv- resuit?. incAaie tfat the precautions are effective in reducing tD, asrestos dust concerv:.'rcr ir, rhioy; re insuiation processes. - w ' - - - .d-hr mve been few published reports on asbestos dust ..once, occrnuiiG rh>hb building and shiprepairing industry despite the roc,wa;mg mieren in mb' ;cTt'. cawr-T; associated with asbestos. The present poper t part of at; ?xw;u;;ve- ' - cr,A oi the hazards associated with asbestos in Devorroou Dockyard TLo-imel- > ' ' >/Op THE USE OF ASBESTOS IN NAVAL DOCKYARDS AAC'tito. ehrysobk- and crocidoiite asbestos have been used in insulating materials Dz Naval ships since about 1880. Large amounts of high ediidency insuianti havE ; 1 ' Lam vetnuived to p-?;p pace with progressive ship design, and airborac asbestos dusk " \ has been liberated whenever these materials have been applied or removed. From 1940 i:o 1963 environmental insulation was applied mainly in the form of asbestos '.prayed onto deckheads and bulkheads. Most of the asbestos for this purpose was' - ' crocidoiite, but some amosite was also used. Pipes and machinery nave been insulated ' with moulded sections containing from 15-90 per cent amosite asbestos. These fragile sections were covered with a protective layer of chrysovde asbestos doth. During refit periods large amounts of sprayed environmental insulation, and often all machinery insulation were ripped our to facilitate modernisation and repair of the ships. There are many other uses of asbestos, and some of these will be described ' later. Because the full extent of the hazard associated with asbestos was not appreciated, adequate preventive measures were not introduced into Naval Dockyards until 1967, Variable amounts of asbestos debris were left scattered about the ships for most of the refit periods (sometimes up to 3 yr), because there was no defined procedure for clearing it away. As a result very large numbers of men have been exposed to asbestos dust by working with or near other men who were applying or removing asbestos ' 245 242 P. G-. Harries materials, or because they were themselves disturbing asbestos debris and creating their own local dust clouds. Energetic measures have now been taken by the Ministry of Defence (Navy) to reduce the hazards associated with asbestos in Naval Dockyards and these have been described by Harries (1968, 1970), and by the Ministry of Defence (Navy) (1970). This report describes the dust concentrations associated with the use of asbestos materials, and the improvements that have been brought about by the recent preventive methods. METHODS . Dust sampling survey Sampling was planned in an attempt to give some idea of dust concentrations likely to have been present in the ships over the last 25 yr. The revision of design specifications, as well as substitution to reduce the dust hazard, made it necessary to take the bulk of these samples in ships containing the old materials in order to give some idea of past conditions. The opportunity to do this occurred in an aircraft carrier and a cruiser in 1967. The samples during other processes, especially the application of insulation, have been taken in destroyers and frigates, as well as in the larger vessels so as to give an. idea of present working-conditions. Sampling methods . The membrane filter method of sampling and evaluation described by Holmes (1965) was used for this survey. Millipore type GA 20 and 25 mm membrane filters, pore size 045 pm were used in Gelman sampling heads, and air was drawn through them by using Hunt personal samplers (Hunt and Ellison, 1963), Austen-Dymax diaphragm pumps, or Drager hand pumps. The Hunt samplers ran at a flow rate of 11-3 ml/min, and the Austen-Dymax pumps at 200 ml/min. Both were calibrated with a suitable Rotameter flow meter. The Hunt samplers were worn by men to give an estimate of personal exposure, or were placed alongside long-running gravimetric samplers to provide an estimate of the fibre content of the dust; sampling lasted over a working shift. Austen-Dymax pumps were used to take shorter samples. A com parison between the results of gravimetric sampling and fibre counting is the subject- of a separate report. - . .. Many samples were taken with Drager hand pumps to obtain 200-2000 ml samples. The disadvantages of small, quick samples are recognised, but it is considered that these are valuable as a method of indicating the fluctuation in the local dust concentrations of various processes. The samples were taken in the general environ ment as well as in the breathing zone of the workers. Small samples are also useful in showing the change in dust concentrations at varying distances or the rate of change after work has stopped. These methods now form the basis of dust monitoring to be used in Naval Dockyards. The method of fixing the samples, clearing the membrane and the counting technique has been fully described by Holmes (1965). The samples were counted under phase contrast microscopy at x400 magnification. Fibres were counted 5-100 pm in length with an aspect ratio of 3 : 1 or over. Asbestos dust concentrations in ship repairin, 243 RESULTS OF SAMPLING Sprayed asbestos insulation No results for spraying are available from the present study because the process ceased in 1963. Records of dust sampling during spraying in 1951 show asbestos fibre counts of 173-322 fibres/cm3 (fibres 2-10 urn length only) and 3121-5957 particles/cm3 of other dusts (particles 0-5-5 pm dia.). This was obviously a most hazardous operation. The results shown in Table 1 are the concentrations found during the removal of Table 1. Asbestos dust concentrations during removal of sprayed crocidolite asbestos Mean concentration, fibres/cm8 Range Number of samples Aircraft hangar Adjacent areas Sample location Small compartments Adjacent areas Sweeping and bagging debris 334-0 117-484 5 56-5 19-131 4 219-9 35-384 15 82-6 43-177 4 353-0 213-493 2 sprayed crocidolite asbestos from the deckheads and bulkheads of an aircraft carrier. Sampling periods were for 1-2 hr of the working shift as the samples were too dense to count if the sampler was run for the whole shift. Very high concentrations (> 200 fibres/cm3) occurred at the stripping areas, slightly lower concentrations (mean > 50 fibres/cm3} were recorded for rooms and passageways adjacent to the stripping zone. Sweeping and bagging debris created mean concentrations of 353 fibres/cm3, but small samples taken with the Drager hand pump in the breathing zones of men bagging crocidolite debris showed concentrations of 1000-2000 fibres/ cm3. The widespread dispersion of dust throughout the ship is seen from the results in Table 2. Long running samples showed the dust concentration at the top of a ladder running up from the stripping area to the deck above to be 109 fibres/cm3 and at the top of the next ladder two decks above the stripping area the concentration was Table 2. Asbestos dust concentration associated with removal of sprayed crocidolite ASBESTOS Concentration, fibres/cm3 7 Deck stripping area 311 Hatchway 6 deck leading from 7 deck Sample location Hatchway 5 deck leading from 6 deck Passageway to shower Removing suits after shower Workmen's dining room 109 30 25 16 0-05 Long-running Hunt samplers were used. 244 P. G. Harries 30 fibres/cm3, This shows the importance of sealing off large parts of the ship when this work is being done and allowing access only to those men wearing adequate respiratory protection. Special facilities for washing, changing and eating were available on the- ship for these men. The results in Table 2 show that although showering when wearing special impervious overalls did not fully remove the fine dust, after the men had removed the contaminated clothing and entered their dining room the dust levels they were exposed to were below 0-1 fibre/cm3. Removal ofpipe and machinery insulation Long running samples, were taken with Hunt samplers during the removal of pipe lagging in engine rooms and boiler rooms of aircraft carriers, cruisers and frigates. The materials removed included pipe sections of 90 per cent amosite asbestos, sections of calcium silicate and 15 per cent asbestos, asbestos cloth and cement. Samples were also taken in small compartments, the brick stowage space for example, to show the effect of removing large amounts of insulation in a small confined space. General atmosphere samples were taken at convenient points in the compartment near gravimetric samplers, and breathing zone samples were taken as near as possible to the men's faces. The results in Table 3 show higher mean general sample levels in Table 3. Asbestos dust concentrations during removal of pipe and machinery lagging Location Boiler rooms Engine rooms Brick stowage space General atmosphere No of samples Mean, fibres/cm3 Range 153 171 0-04-1062 45 88 0 16-3021 13 257 9-592 Breathing zone No of samples Mean, libres/cm3 Range 20 97 25-220 25 91 2-490 -- ---- boiler rooms.(171 fibres/cm3) than engine rooms (88 fibres/cm3) but similar breathing zone levels in boiler rooms and engine rooms (91-97 fibres/cm3). This is because there is more insulating material in a boiler room than in an engine room, and because work proceeds on at least two levels in boiler rooms so that a lot of debris falls 3-4 m to the deck and creates more dust in the general atmosphere. The very high dust levels in the brick stowage space (mean 259 fibres/cm3) are the result of a lot of insulation being removed in a small space. Dust concentrations recorded during the application of pipe insulating materials are shown .in Table 4. The materials being applied included pipe sections of calcium silicate and 15 per cent asbestos, asbestos rope, asbestos cloth and asbestos cement. The dust concentrations were much lower than for the stripping procedures. As in the stripping operations, and for the same reasons, higher values for general and Asbestos dust concentrations in ship repairing 245 Table 4. Asbestos dust concentrations during the application op ripe and machinery lagging Location Boiler rooms Engine rooms Accumulator room General atmosphere No of samples Mean, fibres/cm3 Range 17 224 3-61 28 2-1 0-1--14 5 16-5 2-5-46 Breathing zone No of samples Mean, fibres/cm3 Range 14 36-8 0-1-68 16 7-3 0-04-40 17 9-6 1 -47 breathing zone samples were found in the boiler rooms and the small accumulator room than in the engine rooms. The results shown in Table 5 indicate that considerable dust concentrations (30-68 fibres/cm3) were created by sawing calcium silicate sections, or by removing them from their boxes. Fitting the sections to pipes created high local concentrations (43 fibres/cm3), but low general concentrations (4T fibres/cm3). Clearing calcium silicate debris (134-155 fibres/cm3), "blowing down'5 the debris from inaccessible ledges with an air hose (480 fibres/cm3) and sweeping amosite debris (564 fibres/cm3) created very high dust concentrations; previously men doing this type of work would not have been protected. Mixing asbestos cement in buckets and removing the dry material from a bag created high general concentrations (167/199 fibres/cm3) and even higher breathing zone levels (217-256 fibres/cm3). Wrapping amosite asbestos rope around small bore pipes gave dust concentrations over 100 fibres/cm3. Ripping untreated asbestos cloth is a vigorous procedure and produced general dust concentrations of 33 fibres/cm3 and breathing zone levels of 7 fibres/cm3. When the cloth had been contaminated with asbestos debris the mean breathing zone levels in ripping the cloth rose to 20 fibres/cm3.' Ripping asbestos cloth treated with a dust suppressant by the manufacturers produced dust concentra tions which were all less than 1 fibre/cm3. Removal of asbestos sheets usedfor acoustic insulation This friable material used to soundproof certain compartments of the ship was applied and. removed by joiners. The dust concentrations are shown in Table 6. The general atmosphere concentrations were high because the debris was thrown down on to the deck creating intense local dust clouds and the samplers were 1 m above deck level. The mean breathing zone concentrations were lower (130 fibres/cm3) since the samplers were carried by the men working up at the deckhead on staging. Other processes using asbestos materials Relatively low dust levels (mean 2-43 fibres/cm3) were seen in sawing perforated asbestos cement sheets (Table 7); these levels were reduced (mean 0-1 fibre/cm3) when the sheets were guillotined instead of being cut with a handsaw. Asbestos materials fitted into galley equipment produced mean dust levels of 1-8 fibres/cm3, but higher dust levels (mean 20-8 fibres/cm3) were found when men fitted calcium silicate slabs into boiler casings. The use of asbestos cloth for maintaining heat in metal to be 246 P. G. Harries Table 5. Asbestos dust concentrations in miscellaneous processes associated with pipe lagging General atmosphere Breathing zone Progress No of samples Mean, fibres/cm3 Range No of samples Mean, fibres/cm3 Range Sawing calcium silicate sections Removing calcium silicate sections from box Fitting calcium silicate section to pipe Cleaning calcium silicate debris Fitting amosite rope Removing asbestos `plastic mix' from container Mixing asbestos `plastic mix' with water in bucket Ripping cloth (untreated) Ripping cloth (contaminated) Ripping cloth (treated) Stitching cloth Fitting cloth over lagged pipes '`Blowing down" asbestos debris Sweeping and bagging amosite debris 7 7 9 9 4 7 3 2 -- 12 -- _ 7 10 68 0-7- 158 n 30 2-78 10 4-1 0-23 20 134 32 - 372 7 318 1 - 280 13 199 48 - 328 13 167 53 - 3774 12 33 23 - 43 5 -- -- 12 >1 >1 ---- 12 12 7 489 140 - 932 -- 564 76-3-3191 55 7 -152 52 16 -136 43 1 -329 155 90 -277 112 5 -340 257 48 -470 256 7 20 >1 3-4 24 -579 0-3- 36-5 5-5- 43 >1 0-10 22 0-3- 43 ____ _ Asbestos dust concentrations in ship repairing 247 Table 6. Asbestos dust concentration during removal of fibrous asbestos acoustic panels No. of samples 6 General atmosphere Mean, fibres/cms Range 413-5 30-684 Breathing zone Mean, fibres/cm8 Range 131 48-271 welded produced mean dust concentrations of 8-7 fibres/cm3. Brushing welding slag off asbestos cloth which had been used to protect equipment during welding and burning was accompanied by mean concentration of 76-6 fibres/cm3. Repeated sampling during the pre-heat welding technique when dust suppressed asbestos cloth was used showed counts below 1 fibre/cm3. Asbestos mattress shop A new purpose built mattress shop was erected during the course of the survey and the results in Table 8 show that there is little difference between it and the old shop. The high counts in each shop were associated with sweeping up amosite asbestos fibre around the filling booths. Table 7. Asbestos dust concentrations in other miscellaneous processes Process No. of samples Mean fibres/cm3 Range Sawing and fitting perforated asbestos board 9 2-4 0-10 Guillotining perforated asbestos board 8 01 0-04- 0-5 Fitting asbestos board in ship's galley 6 1-8 0-11 Use of asbestos cloth for pre-heating welding technique 9 8-7 0-30 Asbestos cloth used to protect equipment from welding slag 18 . 76-6 0 -660 Fitting calcium silicate slabs in boiler casing 18 20-8 3-1 - 7-3 Table 8. Asbestos dust concentrations in ASBESTOS MATTRESS SHOPS Genera! atmosphere Breathing zone Location No of samples Mean, fibres/cms Range No of samples Mean fibres/cm3 Old shop New shop 12 12-7 0-126 15 11 16-3 2- 83 25 1-5 3-7 Range 0- 7 0-17 248 P. G. Harrus PRECAUTIONS TO REDUCE ASBESTOS HAZARDS During this survey the Ministry of Defence (Navy) took steps to reduce the hazai Js associated with asbestos. These steps included a review of the use of asbestos, and where possible the introduction of substitute materials. The current uses of asbestos and the progress in finding substitute materials are summarized below. Thermal insulation in machinery spaces Calcium-silicate sections andplastics. Now supplied asbestos-free and recognizable by its yellow or pink colour. Ail calcium-silicate should be cut off-ship so far as is practicable. (Calcium-silicate containing up to 14 per cent asbestos is coloured white). Amosite-asbestos sections and plastics. No longer used, replaced by calciumsilicate. A large number of existing ships are insulated with amosite; de-lagging operations will involve the full implementation of Naval Asbestos Regulations. Self-setting asbestos finishing cement for high-temperature insulation An asbestos-free substitute is now specified. Asbestos cloth. Glass-cloth, is now specified. Dust emitted from this doth is an irritant but not a health hazard. Work is continuing to make this doth less objection able to handle. Asbestos rope and twine. This has been deleted from, specifications. Glass is nowspecified. Asbestos mattresses. Mattresses of rocksil fibre covered with giass-cloth are now Local protection against fine Asbestos millboard. This is now very little used and has been deleted from speci fications. Asbestos cloth. This is more frequently used, and a dust-suppressed cloth is now specified. No prospect is seen of finding a substitute fire-resistant material. Main applications are hangar fire-curtains, curtains in magazines, and protection in welding and burning operations. Magazines will be separately compartmented in new con struction and at long refits of existing ships. Glass-cloth is not acceptable for these usages. Fire protection in magazines where jet efflux presents a potential hazard Resin-bonded asbestos fibre. No substitute material suitable for this purpose. Material is supplied by manufacturer to exact sizes required. There is therefore no health hazard in shipyard application. Thermal insulation of hull structure Sprayed limpet asbestos. Deleted from specifications. Mineral fibre marine board, which is asbestos-free, is now specified. Any ships still insulated with sprayed limpet Asbestos dust concentrations in ship repairing 249 asbestos will require full implementation of Naval Asbestos Regulations for de-lagging operations. Blue-block asbestos or amosite board. Deleted from specifications. Mineral fibre marine board, which, is asbestos-free, is now specified. Asbestos fibre and cloth for pipe covering. Deleted from specifications. Replaced by polyurethane or mineral fibre preformed sections with canvas covering, which are all asbestos-free. Acoustic insulation of hull structure Asbestos fibre board. Deleted from specifications. Existing stocks are being kept for small repairs only. Replaced by mineral fibre resin bonded slab, which is asbestosfree. Amosite asbestos. Deleted from specifications. Replaced by mineral fibre marine board. Limpet asbestos board. Deleted from specifications. Replaced by perforated PVC sheet. High-temperature jointing and packing materials Asbestos fibre and compressed asbestos fibre. No substitute heat-resistant material is available. No health hazard in forms used in shipyard applications. Bearing materials and brake-linings Asbestos reinforced plastics. No substitute wear-resistant material is available. No health hazard except if these materials are ground or worked (no known require ment for this). Bathroom and galley deck-coverings Neoprene terrazzo. Asbestos-free neoprene terrazzo is now available. Dockyards and overseers have been informed that this is the only acceptable material. Partition bulkheads Compressedasbestos sandwiched between metal, plywood, plastic sheets, etc. Deleted from specifications, which now state that materials containing asbestos are not to be used. Steel, aluminium or plywood will generally be used, pending investigations into alternative asbestos-free materials. Covers to cushions and mattresses in submarines Asbestos cloth (not dust-suppressed). Deleted from specification. Fire-retardant foam mattresses now approved. Code ofpractice The precautions to be observed by all Dockyard employees working with asbestos were published as a Code of Practice and have been strictly enforced. They were formulated after careful study of working methods and were based largely on the experience gained from this environmental study. 250 P. G. Harries These precautions are: To isolate asbestos work, and to restrict entry to those properly protected. To reduce the amount of dust created by asbestos work by improving work methods and the materials themselves. To protect all workers whether they work directly with asbestos or not. Protection is based on the degree of risk, and workers are'sub-divided into (a) Registered Asbestos Workers, those who work directly with, asbestos, (b) Neighbourhood Workers, those not directly involved with asbestos, but employed near or with asbestos workers, (c) Management Visitors, those who pay short supervisory visits to places where asbestos work is proceeding. To keep a register of all men directly employed on asbestos work. To ensure that Registered Asbestos Workers have regular medical supervision. These include sprayed asbestos strippers, laggers, asbestos storemen, and nominated boilermakers, joiners, shipwrights, sailmakers, plumbers, smiths and. labourer cleaners. To provide protective clothing adequate for the degree of risk involved. To provide changing and washing arrangements. To carry out regular dust sampling whenever asbestos is handled to ensure that the proper precautions are taken. The regulations are summarized in Table 9 which shows that the protective measures are related to the degree of risk involved in various processes. IMPROVEMENTS IN WORKING METHODS WHICH HAVE REDUCED ASBESTOS EXPOSURE Removal of sprayed asbestos and pipe and machinery insulation will continue to be very hazardous as long as asbestos is present. There seems to be no practical way to wet the material without causing other disadvantages associated with removing and clearing away the debris. Efforts have therefore been concentrated on isolating the work and allowing access only to those men who are properly protected. Large areas of the ship surrounding the work point need to be closed, to unprotected workers because the dust easily spreads from one compartment or deck to another. Protection of the worker includes impervious overalls, an air-fed respirator, or at least a positive pressure power respirator, and adequate changing and washing facilities. Because very high concentrations of dust are to be expected when these materials are removed, dust sampling should be concentrated on the surrounding parts of the ship while the work proceeds to ensure that unprotected men are not at risk. Sampling should also be carried out after all the debris has been removed and after cleaning with industrial vacuum cleaners has taken place. .It is important to continue sampling after other men have started working in the cleaned compartments so as to ensure that the cleaning has been efficient, and that no residues of asbestos remains to be made airborne again by the air-driven tools so widely used in ship repair work. This sampling is done routinely in Naval Dockyards, and workmen are not allowed to work unprotected in compartments in which the asbestos fibre count is more than T able 9. Precautions tor asbestos w ork in naval dockyards 252 P. G. Harriks 2 fibres/cm-5. Similar sampling strategy is employed for the application of pipe insulation, ana great emphasis is laid on efficient cleaning of debris both during and after work has finished. Compartments which are shown to produce dust concen trations of over 2 fibres/cm3 are cleaned again until satisfactory sampling results are obtained. For a 6-month period (May-October 1969) 343 samples in engine rooms showed less than 1 fibre/cm3, while 17 samples showed a mean of 2-9 fibres/cm3. In boiler rooms there were 273 samples less than 1 fibre/cm3, and 9 (mean 2-7 fibres/ cm3) more than 1 fibre/cm3. In smaller compartments there were 327 samples less than 1 fibre/cm3 and 25 samples with mean concentration 31-8 fibres/cm3. These figures suggest that it is possible to reduce the asbestos dust concentrations to very low levels if energetic cleaning methods are employed, although these results also reflect the reduction in the amount of asbestos that is now being used in Naval ships. Most of the existing insulation in ships, and some of the new materials still contain asbestos, and there will still be the problem of men working in compartments in which the insulation is incomplete and liable to be damaged. Until the protective layer of glass fibre cloth h.as been applied over the insulating sections, they are vulnerable to damage by tools, other equipment, or merely by men crawling over them to reach other work. Measurements have been made of asbestos dust concen trations created by men working in a boiler room in which the lagging had not been completed. Long running samples over the working shift showed general atmosphere concentrations of less than 2 fibres/cm3. The values for breathing zone long period samples were between 2-5 fibres/cm3, but some short samples taken when men were crawling over partly insulated pipes showed dust concentrations of 6-144 fibres/cm3. Men are therefore required to wear dust respirators when they are working on or close to pipes on which the friable insulating material is still exposed. Table 6 shows that asbestos is no longer used for acoustic insulation so that joiners will no longer be exposed to the dust except when they remove existing material. They will wear air-fed respirators to do this work. Welders and burners will still use asbestos cloth to protect equipment from molten metal, but by using dust-suppressed cloth, and by disposing of it instead of chipping off slag, they will no longer create high dust concentrations. Sampling has shown that levels of less than 1 fibre/cm3 are produced if the cloth is used in this manner. The same applies to the use of dustsuppressed cloth for pre-heat welding techniques; welders are now instructed in the correct use of dust-suppressed asbestos cloth. Asbestos mattresses are no longer used for insulation, so that the provision of dust extracting units in the mattress shop is no longer required. Mattresses are now made of glass fibre cloth filled with mineral rock wool. The problems of skin and mucous membrane irritation associated with the use of glass fibre cloth has been overcome by improvements in the weave of the doth. DISCUSSION This survey has shown that very high concentrations of amosite, crocidolite, and chrysotile asbestos dust have existed for variable periods of time in ships being refitted in Naval Dockyards. High concentrations of asbestos dust have spread to Asbestos dust concentrations in ship repairin. 253 parts of the ship in which unsuspecting men would have worked without respiratory protection. The data give some indication of what the dust concentrations in Navai Dockyards are likely to have been over the last 25 yr, and help to explain the prevalence of asbestos disease in Dockyard workers (Sheers and Templeton, 1968). The dust concentrations in US Naval shipyards reported by Fleischer e,i al. (1946) and by Murphy and Ferris (1966) were very high, but these investigators sampled total dust and the proportion of asbestos fibre was small. The processes studied were mainly concerned with the application of pipe lagging. Ferris (1968) reported dust concentrations of 7-130 mppcf during the removal of amosite asbestos insulation: this dust level is very high but the asbestos content was not stated. Balzer and Cooper (1968) also reported that ripping out insulating material caused high dust concentrations; 8-5 fibres/cm3 was the mean concentration for this process, and it is not clear in their report where this work was performed. It is likely that higher concentrations occur in the congested spaces aboard ship than in machinery instal lations ashore. These reports mention the difficulty of making an accurate assessment of asbestos exposure for insulating workers. This difficulty is due to the variety of materials used, each containing a different amount of asbestos, and due to the intermittent nature of the work. Both of these factors help to produce widely different dust concentrations, not only between different processes, but also during the course of a. particular process. Without continuous dust monitoring of every process it will not be possible to establish accurate estimates of time weighted dust exposures for insulating workers. The British Occupational Hygiene Society (1968) sub-committee report on Hygienic Standards for Chrysotile Asbestos Dust suggests that if exposure to chrysotiie asbestos is limited to a time-weighted average concentration of 100 fibre yr/ cm3 then it should be possible to reduce the risk of developing the earliest sign of asbestosis over a working lifetime to less than 1 per cent. That is an exposure of 2 fi.bres/cm3 for 50 yr, or 10 fibres/cm3 for 10 yr. Not enough is known about the effect of high exposures for short periods, so that it would not be acceptable to expose a person to 100 fibres/cm3 for 1 yr. The report advises that dust concentrations over 10 fibres/cra3 require the use of an approved dust mask, and that a higher standard of respiratory protection is required for concentrations over 50 fibres/cm3. Technical Data Note 13, published as a supplement to the Asbestos Regulations by the Department op Employment and Productivity (1970), suggests that 2 fibres/cm3 for chrysotile and amosite asbestos is the dust concentration manu facturers and users of asbestos should aim to achieve if they are not to provide their workpeople with respiratory protection. For crocidolite asbestos this limit is 0-2 fibre/cm3 because of the association between this type of asbestos and mesothelial tumours. From the present survey it is clear that all processes involving work with asbestos insulating materials in Naval Dockyards give rise to asbestos dust concentrations of more than 2 fibres/cm3. Many processes have dust concentrations of 50 fibres/cm3 or more. Despite the improvements which have been made towards reducing the asbestos dust concentrations in ship repairing it is obvious that personal protection for the 254 P. G. Harries workers will have to he provided for many years, and that constant vigilance will be required to maintain, and improve, on those standards of asbestos hygiene that have now been achieved. Acknowledgements--This work forms part of the study of asbestos hazards in Naval Dockyards undertaken by the Medical Research Unit, HM Dockyard, Devonport, which is supported jointly by the Institute of Naval Medicine, Alverstoke, Gosport, Hants, and the Medical Research Council Pneumoconiosis Unit, Penarth. I am indebted to the Directors of both institutions, and their staff for help and advice in this study, and the Medical Director General (Naval) for permission to publish this report. Mr. D. Sweet of Devonport Dockyard was responsible for the evaluation of the dust samples. Table 9 is adapted from "Working with Asbestos" Ministry of Defence (Navy) 1970. The data in this paper are from part of a thesis approved by the University of London for the degree of Doctor of Medicine. REFERENCES Balzer, J. L. and Cooper, W. C. (1968) Am. ind. Hyg. Ass. 29, 222. British Occupational Hygiene Society (1968) Ann. occup. Hyg. 11, 47. Department oe Employment and Productivity (1970) Technical Data Note 13, HMSO, London. Ferris, B. G. (1968) Personal communication. ,Fleischer, W. E., Viles, F. J,, Gade, R. L. and Drinker, P. (1946) J. ind. Hyg. 28, 9. Harries, P. G. (1968) Ann. occup. Hyg. 11 135. Harries, P. G. (1970) The effects and control of diseases associated with exposure to asbestos in a Naval Dockyard. A thesis accepted by the University of London for the degree of Doctor of Medicine. Holmes, S. (1965) Ann. NY Acad. Sci. 132, 288. Hunt, R. and Ellison, M. (1963) Lab. Pract. 12, 148. Ministry of Defence (Navy) (1970) Working with Asbestos. Ministry of Defence Labour Division (Naval). Murphy, R. L. H. and Ferris, B. G. (1966) Proceedings of XVInternational Congress on Occupational Health, Vienna, p. 229. Sheers, G. and Templeton, A. R. (1968) Br. Med. J. 3, 574.