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THE UNIVERSITY OF HONG KONGLIBRARIESHong Kong Collectiongift fromHong Kong (China). Labour Dept.


Control of Asbestos at workCode of PracticeLabour DepartmentHong KongDecember 1986


Printed by the Government Printer, Hong Kong.


Control of Asbestos at work—Code of PracticeIntroductionAsbestos deserves its sinister reputation as a health hazard. Its use in theworkplace requires strict control to prevent serious occupational disease.Description'Asbestos' is a generic term for the fibrous forms of several mineralsilicates, namely chrysotile, crocidolite, amosite, anthophylite, tremolite andactinolite. These occur naturally in seams in many rocks and belong to twolarge groups of rock-forming minerals, the serpentines and amphiboles.Chrysotile is one of the serpentine group while the others are amphiboles.Chrysotile is known as white, crocidolite as blue and amosite as brownasbestos.The annual world production of asbestos is over 6 million tons of which 97per cent is chrysotile and the remainder crocidolite and amosite. Commercialproduction of the other three amphiboles has been on a very small scale inthe past but they are found as contaminants of other minerals and soil andmay give rise to atmospheric pollution.Chrysotile is widely distributed with the largest production in Russia,Canada and Africa. Crocidolite is now mined almost exclusively in SouthAfrica and amosite is only exploited in the Transvaal although deposits havebeen confirmed in India.The properties which give asbestos its commercial value are incombustability,strength and effectiveness as a reinforcing or binding agent whencombined with other materials such as cement and plastic. The fibres whenbroken down can be woven.Uses of AsbestosAsbestos has thousands of uses and is widespread throughout industry.Employed in insulation and heat resistant lagging in buildings and ships itis often combined with cement and plastics to produce roof-sheeting, pipesand assorted other products. It is also used in brake and clutch linings,bearing packings and filters. As cloth it is manufactured into fire-proofclothing, curtains and mattresses. Hong Kong is not a producer or amanufacturer of asbestos products on a major scale but some raw asbestosis used and a large amount of asbestos products are consumed. Asbestos maybe found in many local industrial situations such as ship building andrepairing, many construction and insulating processes, some plasticsmanufacturing and ship breaking and building demolition.


Health Hazards of AsbestosAir contamination of the workplace and neighbourhood may occur inhandling asbestos raw material, its mixing and the disintegration of productscontaining asbestos in machining, sawing or scrapping.Inhalation of asbestos fibres causes asbestosis which is a disease of progressivefibrosis of the lungs with increasing respiratory disability. This is oftencomplicated by lung cancer. Mesothelioma affecting the lung and abdominallining membrane is another cancer caused by asbestos and is particularlyassociated with crocidolite (blue asbestos) fibres. There is some evidence ofincrease of gastro-intestinal cancers from ingestion of asbestos but inhalationto the lungs is by far the most important route of entry.Prevention of Health HazardHealth protection depends on preventing the inhalation of asbestos fibres andsafe practice is based on the following principles:—(a) Substitution—Replacement of asbestos by a safer substitute such asman-made fibres for insulation. This is the perfect solution.(b) Segregation—Asbestos work and storage should be isolated frompersons engaged in other work to reduce exposure.(c) Dust control—Depends on enclosing a dust source or providingadequate exhaust ventilation to prevent fibres reaching the breathingzone of personnel. Dust reducing work methods such as wetting canalso help control as can the use of impervious material such as polythenesheeting to confine dust spread and catch debris.(d) Good housekeeping—Regular cleaning of the workplace by vacuum orwet methods is required. The vacuum cleaners used must have high(e)efficiency filter systems capable of trapping asbestos fibres.Respiratory protection—Unless dust can be kept to minimum levelsrespiratory protection for personnel is essential. Depending on the dustconcentration this should be by respirator (mask) fitted with theappropriate dust filter, or more elaborate equipment including air-linebreathing apparatus.(/) Protective clothing—This is required to minimise skin and hair contact,prevent contamination of personal clothing and stop the disseminationof dust outside the working area. Nylon overalls and dust caps aresuitable. Cleaning of protective clothing should be planned to minimisedust dissemination.(g)(h)Ablution and changing facilities for personnel should be provided andarranged to allow the separation of personal from protective clothing.Waste disposal—Waste collection and disposal require attention toavoid increasing the atmospheric concentration of asbestos fibres. Theusual method is sacking in impervious sack and disposing in anapproved manner e.g. burying in a controlled tipping site.


(f)Education—All personnel exposed to asbestos dust must be instructedon the health hazard involved and the necessary protective measures.Code of PracticeThe aim of the code is to protect the health of people at work by controllingtheir exposure to asbestos. It applies to all works which expose people to therisk of inhaling asbestos dust and contains the fundamental requirementsneeded to prevent disease.The key to control is the requirement for the employer to assess the natureand extent of exposure to asbestos so that on the basis of that assessment hemay determine and implement the measures which will be adequate toprevent disease.The basic measure to protect employees in the prevention of the escape ofasbestos dust into the workplace and ensuring that the amount of asbestos inair in the breathing zone of any person is below the accepted level. Becauseof particular carcinogens properties crocidolite (blue) or amosite (brown)asbestos should not be used.The spraying of asbestos on walls and pipes generates very high dust levelsduring the operation itself and such coatings have poor durability and areeasily damaged in such a way as to allow the release of respirable fibres.Because of the particular health risks asbestos or any products containingasbestos should not be applied by a spraying process.If circumstances arise when asbestos cannot be maintained below theacceptable levels adequate personal protection equipment should beprovided and worn and complemented by adequate changing, cleaningand laundry facilities as necessary. Good housekeeping methods requireattention.Employees should use the equipment and facilities provided and co-operatewith the employers in restricting the spread of asbestos contamination.The spread of pollution from the workplace is related to both process controlmeasure and the standards of general cleanliness. Adequate steps should betaken to avoid spread of contamination to protect those in adjacentworkplaces and also people in the neighbourhood.Assistance from Labour DepartmentThe Factory Inspectorate and the Occupational Health Division of theLabour Department would be pleased to give advice to any employer whoencounters problems in the use and control of asbestos. The OccupationalHealth Division can give assistance in determination of asbestos in the airand advice on medical supervision of persons who are exposed to asbestos.


ContentsApplication of CodeDuties of employerAssessment of work which exposes persons to asbestos dustHygiene standards for asbestosSamplingMeasurement of airborne dustInformation, instruction and trainingNotificationUse of blue and brown asbestosSpraying of asbestosControl measures for material, plant and processesPersonal protective equipmentCleanliness of premises and plantStorageDisposal of wasteProtection of young personsMedical supervision of asbestos workersAppendix I —Hygiene Standards for Asbestos DustAppendix II —SamplingAppendix III—The Measurement of Airborne Asbestos DustAppendix IV—Notification of Processes involving AsbestosParagraph1234591011-1819-242526272829


Code of Practice forControl of Asbestos at workApplication of Code1. This code applies to work which exposes persons, both employeesand others to the inhalation of asbestos dust. In practice this meansany work in which raw asbestos fibres are used (e.g. lagging, manufactureof asbestos cement pipes, etc.) or any work on asbestos containingmaterial which generates dust (e.g. delagging, sawing asbestos board,etc.).Duties of Employer2. The employer has a duty to ensure that he protects not only his ownemployees but also other persons who are engaged in any work at thepremises where the employer's work with asbestos is carried out and whichexposes them to asbestos dust. These persons "would include for examplemaintenance contractors engaged in maintenance of plant or other personssuch as contract cleaners not directly engaged on asbestos work but who areexposed to dust. Any employer of these persons will also have duties as theiremployer to satisfy himself in his own right that adequate precautions aretaken to protect their health.Assessment of work which exposes persons to asbestos dust3. Where any work may expose persons to the inhalation of asbestosdust the employer should determine the nature and degree of the exposure.The assessment should be revised if there is any reason to suspect thatit is incorrect or where there is material change in the process. Theobject of assessment is to ensure that appropriate preventive measures aretaken.Hygiene standards for asbestos4. In determining exposure the Hygiene Standards for Asbestos Dust asdetailed in Appendix I should be applied.Sampling5. Sampling should follow the guidelines as detailed in. Appendix II.Measurement of airborne dust6. In determining airborne asbestos fibres the method approved by theAsbestosis Research Council and detailed in Appendix III should befollowed.7


Information, instruction and training7. Every employer should ensure that adequate information, instructionand training is given to his employees or others who are liable to be exposedto asbestos dust so that they are aware of the health hazards and theprecautions necessary to prevent.Notification8. Before a process involving asbestos, including the removal of asbestoscontaining lagging is undertaken, the employer should give at least 28 daysnotice to the Commissioner for Labour. The notification form shown inAppendix IV should be used.Use of blue and brown asbestos9. Raw crocidolite (blue) or amosite (brown) asbestos should not be usedfor any purpose or in any circumstances.Spraying of asbestos10. Asbestos or any product containing it should not be applied by sprayprocess.Control measures for material, plant and processes11. Asbestos should only be used when there is no suitable and safersubstitute.12. When used the process should be planned to prevent environmentalcontamination and complete enclosure when possible should be the methodof choice.13. When total enclosure is not reasonably practical the processes should asfar as possible be isolated from other working areas.14. Areas in workplace where dust concentration is likely to exceed thecontrol limit should be suitably marked and a scheme of restricting access tothese areas should be instituted.15. Wet working methods to reduce dust production should be considered.16. When dust is not otherwise prevented an effective exhaust systemdesigned to keep asbestos fibres out of the exposed persons' breathing zoneshould be installed and regularly inspected and serviced to ensure maximumefficiency.17. The amount of asbestos in air in the working area should be regularlymonitored and records maintained.18. Control measures for materials, plant and processes should be acceptedas adequate only when they can prevent exposure to asbestos dust whichexceeds the Hygiene Standards as specified in Appendix I.


Personal protective equipment19. When asbestos in air is above the accepted standards respiratoryprotection should be provided and used. The type of protection isdetermined by the dust level and may be either the cartridge type respiratorwith control designed to filter off asbestos fibres or more elaborateequipment including air-line fresh air supply if dust concentrations are high.20. Protective clothing should be provided and worn to preventcontamination of skin, hair and personal clothing. Nylon is a suitablematerial for protective clothing. Use of suitable disposable clothing isrecommended.21. Adequate washing and changing facilities should be provided andshould be sited so as to preventing asbestos from contaminating areas outsidethe workplace. This should be achieved by separation of changing andstorage accommodation for protective clothing from personal clothing bymeans of separate rooms or one large room divided into clean (personalclothing) and dirty (protective clothing) areas. The washing and showeringfacilities should be located between the areas to enable the person to removeasbestos contaminated clothing in the dusty area, then pass to the washingarea and finally enter the clean area to put on personal clothing.22. Protective clothes should be cleansed by vacuum cleaner after theworking shift. They should be stored separately and sent for laundry insealed sacks labelled 'Asbestos Contaminated'. Laundry personnel shouldbe acquainted with the dust hazards and the need to avoid shaking. Sacksshould not be opened until they can be tipping into the washing machinesso that the contents are immediately wetted. Asbestos contaminated laundryshould be kept separate from other items and the machines thoroughlyflushed after use.23. Eating, drinking or smoking should not take place in the presence ofasbestos dust.24. Because of the synergestic effect between tobacco and asbestos in theproduction of cancer all persons whose work is liable to expose themregularly to asbestos dust should be advised not to smoke.Cleanliness of premises and plant25. Special attention should be paid to good housekeeping using vacuumor wet methods to ensure that asbestos dust is not allowed to accumulate inthe workplace. Only vacuum cleaners fitted with high efficiency filtersystems capable of trapping asbestos fibres should be used.Storage26. All loose asbestos in use should be kept in suitable closed receptacleswhich prevent escape of dust and all asbestos waste should be suitablycontained for disposal.


Disposal of waste27. Asbestos waste disposal methods should ensure that the dust doesnot contaminate the environment. Sealing in polythene sacks or otherimpervious container and burying at the tipping site are recommendedmethods of disposal.Protection of young persons28. Young persons under 18 years of age should not be employed in anyprocess which entails exposure to asbestos dust.Medical supervision of asbestos workers29. Persons regularly exposed to asbestos dust should be under medicalsupervision with examination annually.10


HYGIENE STANDARDS FOR ASBESTOS DUSTAPPENDIX 11. Exposure to all forms of asbestos dust should be reduced to theminimum that is reasonably practicable.2. In any case, occupational exposure to asbestos dust should not exceed:For crocidolite:0.2 fibres/ml.For amosite:0.2 fibres/ml.For other forms of asbestos: 0.5 fibres/ml.Note:Fibres means particles of length greater than 5 micrometres, adiameter of less than 3 micrometres and having a length to breadthratio of at least 3:1, observed by transmitted light under phasecontrast conditions at a magnification of approximately 500x.11


APPENDIXIISAMPLINGBulk sampling1. The object of bulk sampling is to determine the presence of asbestos andits type and so the precautions which need to be taken. Old insulation willfrequently contain asbestos, including crocidolite, in varying proportionswith other materials and care should be taken to ensure that the samples arerepresentative.Evaluation of bulk samples2. The type of asbestos used cannot be readily identified by its appearanceor colour, particularly if it is mixed with other materials. Crocidolite (blueasbestos) may be significantly changed in appearance by the action of heat.The techniques employed should be capable of positively identifying all ofthe types of asbestos that are present in the sample. Optical microscopy, X--ray diffraction or electron microscopy coupled with X-ray probe or electrondiffraction are suitable techniques. In many cases, the use of polarized lightoptical microscopy with immersion refractometry is sufficient to identify theindividual forms of asbestos and provide an indication of the amount present.X-ray diffraction techniques supplement optical microscopy though thepresence of silicate materials other than asbestos can make the results difficultto interpret. Electron microscopic techniques coupled with X-ray probeanalysis enables single fibres to be identified but the results may not berepresentative of the whole sample.Environmental sampling3. The purpose of environmental sampling is to measure the concentrationand to identify the type of airborne asbestos dust in the breathing zone of theworker.4. Sampling is normally carried out over a ten-minute or four-hour periodin order to determine ceiling concentrations and time weighted averageconcentrations respectively.5. The membrane filter method is normally used for measurement ofairborne asbestos dust. Automatic particle counters, calibrated against themembrance filter method, provide more rapid results than the membranefilter method but they are of little use in mixed dust atmospheres because oftheir inability to distinguish asbestos fibres when mixed with other dusts.6. The method involves drawing a known volume of dust laden airthrough a membrance filter using either a battery, or hand-operated pump.The collected fibres are subsequently counted and examined under a lightmicroscope.12


7. It is important that the fibres to be counted under the microscope are notso numerous that they overlap. However, it is not possible to indicate thevolume of air to be drawn through the sampling head or the number oftimes which the filter should be changed during the sampling period as thesewill depend on the dust concentration in the environment being measured.The presence of dusts other than asbestos dust should also be taken intoconsideration as these tend to obscure the presence of fibres on the filter.8. Personal samplers, that is sampling devices, fixed to the upper lapel orshoulder of the worker's clothing w r ithin 300 mm of his nose are able to givea more accurate indication of personal exposures although in continuousprocesses background samplers fixed at head height may provide a goodindication of exposure levels.9. Before sampling is undertaken air sampling pumps must be calibratedagainst an external flow meter with the sampling head and filter connected.A clean filter should, of course, be fitted and the flow rate readjusted beforesampling takes place. Flow rates should be checked at least every hour duringthe sampling period and readjusted when necessary.Ten-minute sampling10. Ten minutes is the minimum continuous sampling period needed toassess the concentration of asbestos dust in the worker's breathing zone.Ideally the sample should be taken during a period of maximum dustemission so as to determine peak exposure level. Experience has shown thatfor ten-minute sampling periods the optimum rate for drawing air throughthe filter is 2 litres/minute for most asbestos operations.Four-hour sampling11. A four-hour continuous sample will give a reasonable indication ofthe average exposure to asbestos dust. An air flow rate through the filter of200-500 ml/minute has been found to be most suitable for this period ofsampling although at processes giving rise to high levels of dust it wouldprobably be necessary to replace the filter during the sampling period so asto avoid counting difficulties arising from overlapping fibres on the filter.Evaluation of airborne samples12. The method used for counting asbestos fibres is that given in TechnicalNote No. 1 The Measurement of Airborne Dust by the Membrance FilterMethod' published by the U.K. Asbestosis Research Council Counting isundertaken by means of an optical microscope fitted with a 4 mm/40xobjective at a magnification of 450-500 using transmitted light and phasecontrast techniques. The diameter of asbestos fibres found in airborne dustsamples obtained from asbestos processes varies considerably, some beingsmaller than the limit of resolution of the optical microscope. Meaningfulresults can only be obtained if the size range of fibres to be counted is defined13


and only fibres having length greater than 5 micrometres, breadth less than 3micrometres and length to breadth ratio of at least 3:1 should be countered.The Technical Note is reproduced at Appendix III.14


APPENDIXTHE MEASUREMENT OF AIRBORNE ASBESTOS DUST1. IntroductionThe methods adopted by the Asbestosis Research Council for thedefinition of airborne asbestos fibre concentrations have now beengenerally accepted. This test method describes in detail the membranefilter method of sampling air and the procedure to be followed foranalysing the dust deposit obtained from the sample. It is generally onlysuitable for measuring concentrations encountered occupationally infactories, workshops and on constructional sites.2. Definitions2.1 FibresFibres are defined as being of a length greater than 5 //m andhaving a length/breadth ratio of at least 3:1. There is no upper limitfor the length of the fibres, but a maximum diameter of 3 jum isdefined. Airborne asbestos dust concentrations are expressed infibres per millilitre of air (f/ml).2.2 Continuous SampleA continuous sample is defined as a sample which is collected overa period and therefore represents the average dust concentrationduring that period. This type of sample is suitable where theconcentration does not vary widely over the period of sampling.2.3 Snap SampleA snap sample is defined as a sample taken over a very short period,generally of a few seconds and not exceeding 30 seconds. This typeof sample is suitable for situations where dust emission isintermittent. A series of such samples can be used to indicate thedegree of variation in environments where the dust concentrationis fluctuating.2.4 Personal SampleA personal sample is defined as a sample collected on a membranefilter which is attached to the operator. The filter holder must bepositioned in the operator's breathing zone.3. Equipment3.1 Membrane FilterWhite gridded membrane filters of 25 mm diameter, with a poresize of up to 5 ^um (0.8 ^m is recommended).3.2 Sampling HeadA leakproof open filter holder to take a 25 mm membrane.15III


3.3 Pumping SystemsThe pumping technique recommended is to draw the dustladenair through the filter by means of a suitable sealed pump. Theflow-rate can be controlled by a needle valve. The Draeger handpump is suitable for short period and snap samples. For personalsampling the Casella portable battery driven pump has been foundsatisfactory.3.4 FlowmeterFor continuous samples the flow rate can be monitored by meansof a suitable flowmeter (see Figure 1). If this is not practicable (e.g.personal sampler) the flow rate can be checked using a tripodflowmeter as provided by Casella.needlevaivefilter head-aipumpflowmeterFigure 1 — An assembly for collection of static membrane filter samples4. Sampling4.1 The filters must be placed into the filter holders with the gridsfacing the open end. The holder should be washed in warm waterbefore use, especially after sampling in dusty environments.4.2 Continuous SamplesThe sampling orifice will usually be positioned at head height.The filter surface should not be facing upwards to ensure that thelarge particles and airborne aggregates are not collected. Samplingperiods and rates should be adjusted to suit the anticipated dustlevels. The sampling flow rate should not exceed 2 litres perminute. The sample volumes given below are suggested asconvenient:—16


Anticipated Asbestos Dust Concentration Sample Volume(f/ml)(ml)less than 2 10,000-20,0002-4 5,000-10,0004-12 2,000- 5,000greater than 12pro rata4.3 Short Period or Snap SamplesThe Draeger pump, with the sampling head fitted, is held at therequired position. With each stroke the pump draws in a volumeof 100 + 5 ml air through the membrane. The number of strokesrequired, and consequently the total volume of the sample, willvary according to the anticipated dust concentration. Naturally thelower the concentration, the larger should be the volume ofsample.4.4 Personal SamplesPersonal samples may be collected with the membrane filterholder attached to the person. The air is drawn through the filterat a rate of 2 litres per minute by means of the Casella portablepump.4.5 Whichever method is used, immediately after the sample has beentaken the sampling orifice should be covered by a plastic cap orother convenient means, and the filter holder should then beplaced in a suitable box with the filter surface facing upwards. Thesample should be fixed before transit.5. Fixing the Sample5.1 The dust deposit is fixed on the membrane surface whilst the filteris in the sampling head. The following method is preferred:—(a)Several drops of polymethyl methacrylate (perspex) solution(0.025% in chloroform) are applied to the membrane, whileclean air is being drawn through it by means of a low velocitypump (e.g. water pump).(6) An alternative method of fixing for use in the field is to spraythe filter surface with a cytological fixative from an aerosoldispenser. The filter holder is held with the filter surfacefacing upwards and the aerosol dispenser held some 18" awayso that the spray falls in an arc gently on to the membranesurface for at least 5 seconds. Care should be taken not to directthe aerosol spray onto the filter causing the dust deposit to bedisturbed.If this method of fixing is used the filter holders requirewashing more often than usual.17


6. Mounting the Sample6.1 Filtered triacetin (Glycerol Triacetate) is placed on a clean 76 mmx 25 mm (0.8 to 1.0 rnm thick) microscope slide. Sufficienttriacetin should be used to enable it to be spread out into a circle ofthe same diameter as the filter (25 mm). The triacetin may bedispensed through a solvent resistant membrane filter held in asyringe filter holder. Filtration of the cleaning agent is particularlyimportant if asbestos contamination is possible and if relatively lowdust levels (i.e.


7.5 If desired, the fibres are divided into the length ranges 5-10 /mi,10-20 /mi, 20-50 /xm and greater than 100 /mi. The sizing is carriedout by reference to a suitable graticule in one of the eyepieces. Acontinuous size distribution may be obtained using the methodrecommended in BS3406. The graticule is calibrated against a stagemicrometer.7 6 5 4 3 2 1ft **•*••Figure 2 — Illustration of field of view and BS 3625 Graticule8. Evaluation of Results8.1 Dust concentration is determined from the total fibre count above5 jam in length.8.2 If the diameter of the dust deposit=D and the diameter of eachfield of view =d, then the dust concentration=19


x — x — (fibres per ml)d 2 n Vwhere V = volume of sample (ml),N = number of fibres counted,n = number of fields examined.If the graticule grid is used to delineate the counting areas, then thedust concentration =TiD 2 N 1x — x— (fibres per ml) where A = grid area4A n V V 5* '8.3 Where there is a possibility of relatively high background fibrecounts on the membrane filters, due to instrument or laboratorycontamination, unexposed control samples are counted to evaluatethe significance of this possible source of error.20


Annex iSPECIFICATION FOR PHASE MICROSCOPE SUITABLEFOR COUNTING ASBESTOS DUST PARTICLESLight SourceA built in substage illuminator is preferred, but an external lamp with acondenser and a plane mirror is also satisfactory. A variable intensity controlis necessary for both. A green light filter is also recommended.Substage AssemblyAn achromatic condenser incorporated into a substage unit containing theannular diaphragms is recommended. There should also be means ofcentering the diaphragms with respect to the corresponding phase plates inthe objectives.StageA built in mechanical object stage, fitted with verniers and side clamps, isrequired.ObjectivesA rotating nose-piece fitted with lOx and 40x parfocal phase achromaticobjectives is suitable. It is recommended that the dry 4 mm, 40x objective beused for routine counting and it is an advantage if this is of the flat-field typeincorporating a phase ring of higher than normal absorption (e.g. 95%).EyepiecesBinocular eyepieces of the compensating type are recommended, theirmagnification to be such as to give a total magnification of around 500x.GraticuleThe following graticules have been found satisfactory:(a) Patterson Globe and Circle.(b) Graticule to BS 3625.(c) Microscope eyepiece micrometer.The graticule is calibrated against a stage micrometer (100 jam divided into2 iim parts). A different micrometer (1 mm divided into 10 /mi parts) may beused to measure the diameter of the field of view.Auxiliary Viewing TelescopeThis is essential for correct phasing. Modern microscopes are normallyprovided with suitable equipment (e.g. focusable Bertrand lens).21


11APPENDIXIVNOTIFICATION OF PROCESSES INVOLVING ASBESTOS(Thisform is to be completed in duplicate)To: The Commissioner for Labour,Hong KongName of industrialundertakingAddress of industrialundertakingCorrespondence/registeredofficeName of Proprietor/ManagerApproximate total number ofemployees in the industrialundertakingApproximate number ofasbestos workersForms of asbestosused/involvedMaleMaleTel. No.Tel. No.FemaleFemaleProcess using/involvingasbestosSignaturePosition...Date.......22


X1S3123DTHKP 363.1791 C76 LControl of asbestos at workcode of practiceHong Kong : Labour Dept.,[1986]Date Due

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