RESPIRATORY PROTECTIVE EQUIPMENT

RESPIRATORY PROTECTIVE EQUIPMENT SETTING Correspondene to: Dr R M Howie, 12 Morningside Close, Edinburgh EH10 4DB, UK; robin.howie@ btlik.om T RMHowie Oup Environ Med 2005; 62:423 428. doi: 10.1136/oem.2002.004424 his artile is intended to outline the role of respiratory protetive equipment (RPE) in a risk ontrol programme and to provide advie on the steps in setting up and implementing an effetive RPE programme so that oupational physiians an hek the effetiveness of any suh programmes they may enounter. Although the details of suh a programme may depend on the requirements of national legislation, the general priniples to be applied should be independent of loal legislation. In this artile the details of the programme are desribed in terms of UK legislation and guidane. It should be noted that although all duties in the UK to prevent or ontrol exposures to hazardous substanes are to the limit of reasonably pratiable, the orresponding duties under the European Diretives, on whih the UK legislation is nominally based, are to the limit of tehnially feasible and that the English Civil Court has judged that an employee an have a valid ase against the UK government where he/she would have been proteted by duties to the limit of tehnially feasible but not by the limit of reasonably pratiable. It is therefore onsidered that it would be prudent for a reasonable employer to work to tehnially feasible rather than reasonably pratiable where possible as ation to tehnially feasible ould be a valid defene in ivil litigation. Respiratory protetive equipment is widely used to protet wearers against hazardous aerosols, gases, or vapours beause it is pereived to provide effetive and relatively inexpensive protetion whereas preferred tehniques, suh as total enlosure, annot be applied or are pereived to be expensive. The major limitation of RPE is that the antiipated protetion is ahieved only if the equipment is worn orretly. In addition, RPE performane in the workplae is generally muh poorer than suggested by standards or manufaturers literature. RPE should therefore be used only as one omponent of an overall prevention and ontrol programme. Airborne hazards should be ontrolled by substitution of hazardous substanes by safer substanes, total enlosure, et rather than by RPE. In any situation, the ontrol hierarhy of prevention, ontrol, and personal protetion should be applied. The ontrol hierarhy is enshrined in both European and UK legislation; for example, Artile 2(h) of the European Framework Diretive requires that the employer gives olletive protetive measures priority over individual protetive measures, 1 and is reiterated in Regulation 7 of the Control of Substanes Hazardous to Health Regulations 2002. However, there will be situations where preventing or ontrolling risks is not tehnially possible and RPE use is unavoidable; for example, during inidents, between reognising risk and implementing ontrol, or to supplement inadequate ontrol. When used, RPE must provide adequate protetion without imposing unaeptable disomfort on the wearer as unomfortable equipment may be worn inorretly to minimise disomfort. RPE must therefore math eah individual wearer s personal harateristis, job, and working environment, and any other items of personal protetive equipment (PPE), whih may have to be worn simultaneously with the RPE. UP AN EFFECTIVE RPE PROGRAMME An effetive RPE programme should inlude the following steps. Assess risks and identify where ontrol is required The essential first step is an assessment to identify any likely oupational hazards and to quantify any risks. The assessment should identify all unaeptable risks and the individuals at risk and provide the information needed for preventing or ontrolling suh risks and for seleting adequate and suitable RPE. Substitute the hazardous by less hazardous if tehnially possible Determine if it is possible to use a less hazardous substane or the same substane in a less hazardous form; for example, replae a fine powder by a solution or a oarse powder. 423 Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om

424 Abbreviations APF: assigned protetion fator BA: breathing apparatus COSHH: Control of Substanes Hazardous to Health Regulations 2002 HSC: Health and Safety Commission HSE: Health and Safety Exeutive MEL: maximum exposure limit NPF: nominal protetion fator OEL: oupational exposure limit OES: oupational exposure standard PF: protetion fator PPE: personal protetive equipment RPE: respiratory protetive equipment Implement all tehnially possible ontrols If hazardous substanes or proesses must be used, all tehnially possible means of redution of risk at soure should be onsidered before adopting RPE; for example, to enlose any proess involving hazardous substanes or to apply loal exhaust ventilation to suh proesses. It may also be neessary to minimise the number of persons who may be exposed and/or to redue the duration of exposure. Identify who needs residual protetion From the assessment of likely risks and of the effetiveness of the measures applied to prevent or redue these risks, all persons still potentially at risk should be identified and the level of residual protetion still required should be quantified. Inform wearers of onsequenes of exposure To ensure that all employees fully utilise all ontrol measures, they should be made fully aware of the risks to their health and safety in the workplae and the potential onsequenes if these risks are not adequately ontrolled. If the orret use of ontrol measures involves inonveniene or redution in produtivity, partiularly for those on pieework, ontrol measures may not be orretly used unless those at risk pereive some benefit to themselves. Sine many types of RPE are inherently unomfortable, some exposed persons may refuse to wear suh equipment unless onvined that the imposed disomfort an be justified in terms of redued risk to themselves. To ensure that wearers are aware of the benefit of wearing the RPE provided, it is important to ensure that all persons exposed to risk in the workplae have a pereption of the risk(s) to whih they may be exposed. Selet RPE adequate to ontrol residual exposure RPE should be seleted whih redues any risks to aeptable levels. Given the general redution in OEL and the inreasing number of substanes assigned a maximum exposure limit (MEL) rather than an oupational exposure standard (OES), it is onsidered prudent to limit personal exposures to,oel. In the ase of a substane assigned an OES, it is onsidered prudent to redue personal exposure to,25% of the OES or in the ase of a substane assigned an MEL or a ontrol limit, to,10% of the MEL or ontrol limit. RPE should be seleted on the basis of demonstrated workplae performane. If a manufaturer annot supply workplae data or written assurane as to the level of performane whih an realistially be ahieved in the workplae, his RPE should not be used. Involve wearers in the RPE seletion proess Many types of RPE impose some level of disomfort on the wearer and sine the level of disomfort may reflet the degree to whih the RPE and the wearer s fae have to mutually deform to ahieve adequate fit, wearers should be fully involved in the seletion proess to minimise the imposed disomfort. Involving wearers in the seletion proedure and in all aspets of the RPE programme gives them a stake in ensuring the programme s overall effetiveness. The importane of suh involvement is reognised in Artile 8 of the Use Diretive, Commission of the European Communities, 1 whih requires the onsultation and partiipation of workers and/or their representatives. Math RPE to eah wearer RPE wearers an vary substantially in size and faial harateristis so that a piee of equipment whih fits one person may not fit another. RPE must therefore be seleted to fit eah individual wearer. Carry out fit tests Current UK guidane requires that fit tests should be used to ensure that eah wearer is provided with a faepiee whih fits his/her fae. Details on arrying out suh tests is provided in HSE (2004). 2 However, it should be appreiated that suh tests identify gross misfits only and should not be used to infer likely performane in the workplae. However, fit tests are an extremely valuable training aid and an illustrate the onsequenes of inorret fitting or faial hair. Ensure that RPE does not reate risks Full faepiees an redue downward vision so that desending steps an be dangerous, partiularly in poor lighting onditions. If RPE is worn together with hemial protetive lothing, the lothing an redue the body s ability to lose metaboli heat, so ausing heat strain. In addition, sweating an redue the protetion afforded by RPE. 3 Faepiees should be seleted whih are suitable for the intended use. Any potential for heat strain must be very arefully addressed and avoided; for example, by providing personal ooling and/ or by limiting wear periods. Ensure RPE are mutually ompatible Wearers often have to wear more than one type of PPE; for example, RPE may need to be worn with a safety helmet and/ or protetive lothing et. If not seleted with are, the different types of PPE an interat to redue the protetion provided by one or both items or generate a new risk for the wearer. For example, when safety helmets are worn with fullfaepiee RPE, the front head-harness bukle may redue the spae between forehead and helmet, so that an impat on the front of the helmet an fore the bukle into the forehead. If the helmet is struk on the brim, the helmet an be fored downwards onto the faepiee and ause it to be displaed. It is therefore essential that eah item of PPE provides the required level of protetion without affeting the effetiveness of any other item of PPE. Care must therefore be taken to ensure that all items of PPE whih may have to be worn together are mutually ompatible. It should also be appreiated that a ombination of two or more omfortable items of PPE may be unomfortable when worn simultaneously. Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om

Train wearers in the orret use of their PPE Wearers and supervisors should be trained in how to fit the RPE orretly, how to assess that it is orretly fitted, how to inspet the RPE to ensure it has been orretly manufatured and is omplete, and, if relevant, has been adequately leaned and maintained. Wearers should be aware that any hair whih lies between the faepiee and the fae may ause leakage. Male RPE wearers should be instruted to shave prior to any RPE wear period. RPE wearers should be informed that RPE is the first equipment fitted when wearing ensembles of two or more different types of PPE and that the head harness straps and faepiee must be worn below any lothing. If the lothing beomes ontaminated, for example with asbestos, the faepiee must not be removed before the lothing otherwise the wearer may inhale fibres released off the ontaminated lothing. Figure 1 shows an inorretly worn ensemble with the faepiee and the head harness straps worn over the hood of the protetive lothing. The hood will interfere with the seal between the fae and the faepiee and the faepiee must be removed before the ontaminated lothing an be removed. Figure 2 shows a orretly worn ensemble. Personnel serviing reusable RPE must be trained how to lean, servie, and inspet the equipment and how to ensure that they are not put at risk by any ontamination on unleaned or inadequately leaned RPE. Supervise wearers to ensure the orret use of RPE The overall effetiveness of any RPE programme an be ritially dependent on the ations of supervisors who should Figure 1 Inorretly worn ensemble. atively enfore orret usage of RPE. Supervisors should ensure that all RPE wearers are lean-shaven, wear their RPE orretly when required, and that the equipment is lean and properly maintained. Unshaven wearers should be instruted to shave or be exluded from the job. Note that employees have a duty under the Health and Safety at Work et At 1974 to ooperate with the employer in ensuring health and safety and also have a duty to ensure their own and others health and safety. Minimise wear periods Many types of RPE are inherently unomfortable. Aeptability of a given level of disomfort an derease with inreasing wear time; for example, wearing shoes two sizes too small is bearable for a short time but not for a long walk. Wear times should therefore be redued as far as possible. If most exposure ours during a short period, it may be possible to ahieve adequate exposure redution by wearing RPE only during suh proesses. However, are must be taken to ensure that ontamination of the wearer, RPE, or other PPE does not onstitute a signifiant exposure soure if the RPE is removed. Maintain RPE in effiient and hygieni ondition Reusable RPE will need to be leaned, servied, and maintained. Suh leaning, serviing, and maintenane should be extended to any so-alled low maintenane equipment. RPE maintenane is the legal responsibility of the employer, not the employee. Wearers may be unwilling to wear obviously dirty or faulty equipment. The persons responsible should ask themselves would I be prepared to wear the RPE provided?. Inspet RPE to ensure it is orretly maintained Regular inspetion and testing of servied RPE is required by UK regulations, suh as the COSHH Regulations, to ensure that the equipment is maintained in good ondition. Monitor programme to ensure ontinuing effetiveness The RPE programme should be ontinually monitored to ensure its ongoing effetiveness. If any shortomings are observed, additional training of wearers and/or supervisors and/or maintenane personnel may be required. Audit programme to ensure ontinuing effetiveness The RPE programme should be regularly audited to ensure its ongoing effetiveness. Suh auditing should preferably be arried out by an independent person who is not involved in the day-to-day running of the programme. CLASSES OF RPE There are two main lasses of RPE: breathing apparatus and filter devies. Breathing apparatus (BA) supplies the wearer with breathing gas independent of the wearer s immediate environment. Breathing gas may be supplied from ylinders worn by the wearer, fed from a remote loation by an umbilial, or generated hemially on the wearer. The simplest form of BA is a faepiee fitted with a hose whose distant end is loated in lean air. BA an be worn in onditions of potential oxygen defiieny. Filter devies use filters to remove ontaminants from air in the wearer s immediate environment. Filter devies may be fitted with partiulate, gas, and vapour, or ombined gas and vapour and partiulate filters. Filter devies do not supply oxygen and 425 Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om

426 must never be worn in onditions of potential oxygen defiieny. Table 1 shows the different types of filters available and the relevant markings and filter anister olours. Combined partiulate and gas and vapour filters arry both partiulate and gas and vapour markings; for example, an A3P3 is a high apaity A filter fitted with a P3 partiulate filter. Combination filters are available; for example, an ABEK2P3 filter ombines the performane of the A, B, E, and K filters and is fitted with a P3 filter. Combination filters are marked with the relevant olours for eah filter element; for example, an ABEK2P3 filter is marked with brown, grey, yellow, green, and white stripes. Full referenes to the relevant European Standards is given in BSI (2001). 4 Most ontamination inside orretly funtioning BA and filter devies is from leakage between the wearer s fae or body and the RPE. To minimise suh leakage, the breathing gas may be blown into the faepiee. The breathing gas supply an be ontinuous or ontrolled by a demand valve so that gas is supplied only on inhalation. Positive pressure demand devies maintain gas pressure higher inside the faepiee than outside. Suh devies provide the highest performane Table 1 RPE filter types Figure 2 Corretly worn ensemble. of any RPE type. Both BA and filter devies are available with ontinuous flow and positive pressure demand breathing gas supply. Quantifiation of RPE performane There is muh debate about the how to define the RPE performane whih an realistially be ahieved in real workplaes. RPE performane is quantified by the protetion fator (PF), whih is the ratio between the ontaminant onentration outside the faepiee to that inside the faepiee that is, a devie with a PF of 50 maintains the infaepiee onentration a fator of 50 lower than the outside onentration. RPE is tested in the laboratory to demonstrate ompliane with European Standards suh as BS EN 136, whih overs full fae masks. 5 These tests inlude inward leakage measurement. The minimum PF required to meet the standard for a given lass of RPE is alled the nominal protetion fator (NPF); for example, power assisted filter devies fitted with faepiees and P3 filters have an NPF of 2000. Until reently, RPE in the UK was assumed to ahieve the NPF in the workplae. This assumption is still made in most European ountries. However, there is extensive evidene that workplae performane is substantially lower than the Substanes Filter type; omment Casing Partiulate P1 Low effiieny White P2 Medium effiieny P3 High effiieny Organi gases with boiling points.65 C as speified by manufaturer A1, A2, A3* Brown Inorgani gases and vapours as speified by manufaturer, B1, B2, B3* Grey exluding arbon monoxide Sulphur dioxide and other aid gases and vapours as speified by manufaturer E1, E2, E3* Yellow Ammonia and organi ammonia derivatives as speified by manufaturer K1, K2, K3* Green Merury Must inorporate a P3 filter Red white Oxides of nitrogen NO must inorporate a P3 filter, Blue white single use only Organi gases with boiling points,65 C as speified by manufaturer AX Brown Filters against speifi substanes as speified by manufaturer SX, marked with name of hemial Violet *Gas and vapour filters are available with three apaities: 1, lowest; 3, highest. Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om

Table 2 APF Assigned protetion fators Devie 4 K masks with P1 filters, FFP1, FFGasP1, FMP1 10 K masks with P2 filters, FFP2, FFGasP2, FFGasP3, FFGas, FMP2, FMGP3, FMGas K masks with gas and ombined filters Full masks with gas and vapour and ombined filters Powered hoods or masks TH1 or TM1 K mask fresh air hose BA Light duty airline LDH1 BA 20 K masks with P3 filters, FFP3, FMP3 Full mask P3 and ombined P3 Powered hoods or masks TH2 or TM2 Light duty airline mask or hoods LDM1, LDM2, LDH2 BA K mask ontinuous flow airline BA 40 Full masks with P3 filters Powered hoods, blouses, or masks TH3 or TM3 Full masks or hoods, fresh air hose BA Light duty airline hoods LDH3 BA Constant flow airline hoods, blasting helmets, or full masks BA Negative pressure demand full mask BA 200 Air fed suit BA 2,000 Positive pressure demand full mask BA For details on devies see BSI 4 or HSE. 6 NPF; for example, Howie et al reported workplae protetion fators of 42 for power assisted P3 devies as against the NPF of 2000. 3 Although the redution in PF from 2000 to 42 is large, most devies ahieve laboratory PF substantially higher than the NPF. For the devies tested by Howie et al, the laboratory PF was.100 000. That is, the workplae performane was about a fator of 2000 lower than in the laboratory. Given the reality of workplae performane, RPE assumed performane in the UK is now quantified using assigned protetion fators (APF) derived from measured performane in real workplaes. APF is defined as that level of respiratory protetion that an realistially be ahieved in the workplae by 95% of adequately trained and supervised wearers using a properly funtioning and orretly fitted respiratory protetive devie. 4 The APF assigned to the different lasses of RPE are given in BS 4275. 1 Table 2 summarises the APF for BA and filter devies. The three major diffiulties of testing RPE in real workplaes are ost, availability of suitable test sites, and lak of standardised test methods. To some extent the first two problems an be addressed by arrying out simulated workplae protetion fator studies; for example, in the UK, HSE has reently funded three suh studies, for example, Johnston et al, 7 and in the USA suh a study was reported by Cohen et al. 8 The validity of suh simulated studies an be assessed from table 3 whih ompares the available results of real and simulated workplae studies. The very muh higher PF in simulated studies suggests to this author that suh studies do not provide a safe basis for prediting RPE performane in real workplaes. RPE SELECTION RPE seletion proedures should address all the following aspets. Legal requirements Only EC marked RPE may be used in the workplae to ahieve ompliane with health and safety regulations. Suh equipment has either been shown to omply with the Table 3 Comparison of PF from real and simulated workplae studies Devie Real WPF Simulated WPF Full-mask powered R2* 55À 11000` 200 R3* 49À 22500` 459 R4* 8.4À 998` 119 Air fed blasting helmet 28701.400001.14 * Respirators as identified in Howie et al. 3 ÀHowie et al. 3 `Johnston et al. 7 1Parker et al. 9 Ratio simulated:real relevant harmonised standard or with the essential health and safety requirements of the PPE Produt Diretive. Seleting BA or a filter devie Filters devies must never be worn unless there is lear evidene that oxygen defiieny is very unlikely. Gas and vapour filters are effetive against speified gases or vapours only; for example, filters for use against solvents provide no protetion against arbon monoxide. Unless it is known that no gaseous ontaminants are likely to be present or that the given filter will provide protetion against all likely gaseous ontaminants, BA should be seleted. Filter seletion The apaity of gas and vapour filters an vary with ontaminant onentration and wearers breathing rates and an be redued in high humidities. In multiple gaseous hallenges, retention of a strongly absorbed substane an ause release of a less strongly absorbed substane previously retained. It is therefore diffiult to predit gas and vapour filter lifetimes in many workplaes. Manufaturers should be provided with omprehensive information about the intended use and asked for reommendations regarding the most suitable filter type and likely filter lifetimes. 427 Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om

428 Summary RPE should be only one omponent of a omprehensive programme to prevent and redue risks to health, unless in emergeny or minimal risk situations. If RPE must be used, the assumed protetion should be based on information derived from tests in real workplaes only. Those planning RPE programmes should ask themselves would I wear that RPE doing the job they are doing for the time they are doing it?. The reality of RPE is best summed up by a omment in the 1988 draft Approved Code of Pratie for Carinogeni Substanes: but PPE, partiularly RPE, depends for its effetiveness on the wearer s willingness to wear it. 10 Key point For routine ongoing exposures to airborne hazardous substanes, RPE should be used only after all tehnially possible means of prevention and/or ontrol have been applied. RPE seletion The APF an be regarded as the maximum multiple of the aeptable exposure onentration of any airborne ontaminant is whih a given lass of RPE may nominally provide adequate protetion and are used to identify RPE nominally adequate to provide adequate protetion in any given situation. For example, selet adequate RPE for a situation where a worker may be exposed to 0.6 fibres/ml of amosite asbestos fibres. The UK ontrol limit assigned to respirable amosite fibres is 0.2 fibres/ml. For a ontrol limit of 0.2 fibres/ml, it is prudent to limit in-faepiee onentrations to 10% of the ontrol limit that is, 0.02 fibres/ml. The minimum required APF is therefore given by 0.6/0.02 = 30. As there is no likelihood oxygen defiieny a filter devie may be used. From table 2 either a full mask unpowered or powered partiulate filter devie would provide an APF of 40. However, unpowered devies provide less seure protetion than powered devies and impose greater breathing resistane to breathing. A powered devie should always be seleted when wear periods are likely to be longer than a few minutes. If safety fators.10 are onsidered prudent, positive pressure demand equipment should be used. Competing interests: none delared REFERENCES 1 Commission of the European Communities. Counil Diretive of 21 June 1989 on the Introdution of Measures to Enourage Improvements in the Safety and Health of Workers at Work. 89/391/EEC. Brussels: CEC, 1989. Framework Diretive is the basis for all urrent European Diretives relating to health and safety in the workplae. The most interesting text in any Diretive is the introdutory whereas setion whih identifies the objets the Diretive hopes to ahieve. 2 Health and Safety Exeutive. Fit testing of respiratory protetive equipment faepiees, Information Doument 282/28. Sudbury: HSE Books, 2003. Gives guidane on how to arry out fit tests of RPE. 3 Howie RM, Johnstone JBG, Weston P, et al. Effetiveness of RPE during asbestos removal work. HSE CRR No. 112/1996. Sudbury: HSE Books, 1996. Desribes the major UK workplae study on power assisted filter devies. The results obtained led diretly to BS 4275. 4 British Standards Institution. Guide to implementing an effetive respiratory protetive devie programme. BS 4275:1997. London: BSI, 2001. Gives the bakground to the setting of APF and outlines the limitations of the data whih were available. Basis of HSG, 53:1998.5. 5 Commission for European Normalisation/British Standards Institution. Respiratory protetive devies Full fae masks Requirements, testing, marking. BSEN 136:1998. Brussels: CEN, 1998. 6 Health and Safety Exeutive. Respiratory protetive equipment, a pratial guide for users. Health and Safety Series Booklet HSG 53:1998 Sudbury: HSE Books, 1998. HSG 53 is based on BS 4275:2001 but very substantially less expensive to buy. 7 Johnston AM, Miller BG, George JPK, et al. Investigation of fators affeting the performane of power-assisted full-faepiee respirators in use. HSE CRR No. 282/2000. Sudbury: HSE Books, 2000. 8 Cohen HJ, Heker LH, Mattheis DK, et al. Simulated workplae protetion fator study of powered air-purifying and supplied air respirators. Am Ind Hyg Asso J 2001;62:595 604. 9 Parker JA, Crump KE, King JH, et al. Protetion fators studies of abrasive blasting respirators in simulated and atual workplae environments. In: Abstrats for 1997 Amerian Industrial Hygiene Conferene and Exposition. Abstrat No. 55. Fairfax, VA: AIHA, 1997. 10 Health and Safety Commission. Draft Approved Code of Pratie for the Control of Carinogeni Substanes. London: HMSO, 1988. QUESTIONS (SEE ANSWERS ON P 362) (1) Identify the main lass(es) of respiratory protetive equipment: (a) Breathing apparatus (b) Filtering faepiee devies () Filter devies (d) Power assisted devies (2) Identify the type of respiratory protetive equipment that an be used in oxygen defiient atmospheres: (a) Full mask gas filter devies (b) Breathing apparatus () Power assisted filter devies (d) Full faepiee fitted with arbon monoxide filter (3) What is the performane index urrently used in the UK for seleting respiratory protetive equipment? (a) Nominal protetion fator (b) Protetion fator () Workplae protetion fator (d) Assigned protetion fator (4) What type of respiratory protetive equipment gives the highest performane? (a) Full mask power assisted devies (b) Fresh air hose devies () Positive pressure demand devies (d) Blasting helmets (5) What is the orret funtion of quantitative fit testing in the seletion of respiratory protetive equipment? (a) To predit likely performane in the workplae (b) To determine if a given devie an fit the wearer () To quantify the effet of faial hair on fit (d) To identify gross misfits only Oup Environ Med: first published as 10.1136/oem.2002.004424 on 18 May 2005. Downloaded from http://oem.bmj.om/ on 6 September 2018 by guest. Proteted by opyright. www.oenvmed.om