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PII: Ann. occup. Hyg., Vol. 44, No. 3, pp. 165±172, 2000 Crown Copyright 7 2000 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain. S0003-4878(99)00085-X 0003±4878/00/$20.00 Mineral Oil Metal Working Fluids (MWFs)Ð development of Practical Criteria for Mist Sampling A. T. SIMPSON$*, J. A. GROVES$, J. UNWIN$ and M. PINEY% $Health and Safety Laboratory, Broad Lane, Sheeld, S3 7HQ, UK; %Health and Safety Executive, Haswell House, 54 Nicholas Street, Worcester WR1 1UW, UK Not all mineral oil metalworking ¯uids (MWFs) in common use form stable airborne mists which can be sampled quantitatively onto a ®lter. This much has been known for some time but no simple method of identifying oils too volatile for customary ®lter sampling has been developed. Past work was reviewed and experiments were done to select simple criteria which would enable such oils to be identi®ed. The sampling eciency for a range of commercial mineral oil MWF were assessed by drawing clean air through spiked ®lters at 2 l. minÿ1 for periods up to 6 h before analysis. The physical properties of MWF are governed by their composition and kinematic viscosity was found to be the most practical and easily available index of the potential for sample loss from the ®lter. Oils with viscosities greater that 18 cSt (at 408C) lost less than 5% of their weight, whereas those with viscosities less than 18 cSt gave losses up to 71%. The losses from the MWF were mostly aliphatic hydrocarbons (C10±C18), but additives such as alkyl benzenes, esters, phenols and terpene odorants were also lost. The main recommendation to arise from the work is that ®lter sampling can be performed on mineral oils with viscosities of 18 cSt (at 408C) or more with little evaporative losses from the ®lter. However, sampling oils with viscosities less than 18 cSt will produce results which may signi®cantly underestimate the true value. Over a quarter of UK mineral oil MWFs are formulated from mineral oils with viscosities less than 18 cSt (at 408C). The problem of exposure under-estimation and inappropriate exposure sampling could be widespread. Further work is being done on measurement of mixed phase mineral oil mist exposure. Crown Copyright 7 2000 Published by Elsevier Science Ltd.. All rights reserved. Keywords: mineral oil; oil mist; metalworking ¯uid; sampling INTRODUCTION Metal working ¯uids (MWF) is the general term given to a range of products used as lubricants and coolants during the machining or treatment of metal components. These liquids consist of complex mixtures of chemicals in either mineral oil or water (known as water mix metal working ¯uids). Both types contain a variety of chemical additives to improve or maintain the performance of the MWF. HSE's Engineering National Interest Group estimates that there are at least 50 000 businesses in the UK using MWFs. These include not only engineering companies but the maintenance departments Received 16 December 1998; in ®nal form 21 July 1999. *Author to whom correspondence should be addressed. Tel.: +44 114 289 2000; Fax: +44 114 289 2500. 165 of large companies and organisations outside of the engineering sector. The number of employees exposed could be anywhere between 100,000 and 200,000. Approximately 1300 tonnes of neat mineral oil MWFs are used in the UK per month, and the amount of water mix MWF concentrate used is approximately 600 tonnes per month (at an average working strength of 5% this equates to roughly 12,000 tonnes). This paper is concerned with aspects of sampling mineral oil MWF aerosol. Mineral oil MWF generally consist of one or more severely re®ned mineral base oils, together with extreme pressure additives (such as esters, chloroparans and sulphurised esters) and possibly odorants, antimist and anti-corrosion additives. During use, aerosols of the oil can be generated, usually referred to as oil mist. These originate from processes where the oil is subject to high sheer forces (such as when it is applied to parts 166 A. T. Simpson et al. rotating at high speeds) or where they are exposed to excess heat (vaporised oil condenses into small droplets as it cools). In some activities such as metal forming (for example, shaping sheet metal by pressing it into a die), the oil is applied in the form of a mist using compressed air (nebulisation) to coat areas quickly and evenly with a thin coat of lubricant. Occupational exposure to oil mist is associated with eye, nose and throat complaints and dermatitis. In addition, there are concerns relating to occupational asthma, allergic alveolitis and other lung diseases, as well as the possibility of cancer (although this may relate to historical uses). Given the complexity and changing nature of formulations and likely contaminants it is unclear what the causative agent(s) might be. HSE is currently reviewing the use patterns and available data for oil mists with the aim of producing industry guidance. In the UK the current occupational exposure limit for mineral oil mist is an Occupational Exposure Standard (OES) of 5 mg mÿ3 8-h time weighted average (TWA), with a short-term exposure limit (STEL) of 10 mg mÿ3 (15-min TWA) (HSE, 1999). A number of methods have been described to measure exposure to oil mist including gravimetric methods (CONCAWE, 1981; HSE, 1991; Menichini, 1986) and infrared and ultraviolet spectroscopic methods (CONCAWE, 1981; Menichini, 1986; NIOSH, 1994). In each case the sample is collected on a ®lter and the assumption is made that there is no sample loss during the sampling period. Mineral oil MWF can be made up of a variety of base oils and other additives or diluents covering a range of volatilities. The aerosol formed will consist of droplets and vapour, and will be dynamic with material moving between the two phases. The phase distribution of such aerosols is vulnerable to changes in ambient conditions (Soderholm, 1988). Variations in temperature and vapour concentration as the newly formed aerosol is diluted will cause further change in the phase distribution. These changes will also aect the oil on the ®lter during sampling. The mass of oil collected on the ®lter will continue to alter if it has not reached equilibrium with the air being sampled. If the person wearing the sampler moves to an area which has air containing less vapour, there will be evaporation from the sample on the ®lter. MWF which contain a high proportion of more volatile components will be aected more than MWF containing less volatile components. It cannot therefore be assumed that simply collecting the oil mist on a ®lter will give an accurate measure of oil mist concentration. This problem has been recognised by others: Menichini (1986) found that when seven oils spiked on ®lters were aspirated at 2±4 l. minÿ1 for 1 h with clean air, most lost less than 1% mass, however a light oil, viscosity 2.8 cSt, initial boiling point 2408C, lost 6%. It was recommended that recovery tests for oils sampled should be performed. McAneny et al. (1995) aspirated ®lters loaded with a new and used oil aerosol sample with clear air at 1.5 l. minÿ1 for 4 h and found that the new oil lost an average of 35% of the collected oil, whilst the used oil lost only an average of 12%, the dierence being put down to the previous loss of the more volatile components in the used oil during its use. What is needed is some simple way of determining which mineral oil MWF can be measured by ®lter sampling without unacceptable sample losses. In this work, methods are developed using easily available information which can be used to predict which oil mists may be subject to such losses during ®lter sampling. EXPERIMENTAL To assess the scale and extent of the sample loss from a mineral oil MWF trapped on a ®lter during sampling, a variety of oils were examined under simulated sampling conditions. Thirteen MWF and one hydraulic oil were included covering a range of viscosities (4±33 cSt (mm2 sÿ1) at 408C). The hydraulic oil was included to ®ll a gap in the viscosity range. Spiked ®lters were produced by pipetting neat oil (1±2 mg) onto pre-weighed 25-mm glass ®bre ®lters. The ®lters were placed in Gelman sampling heads and clean air (0208C) was sucked through at 2 l. minÿ1 to simulate collection of a sample. A guard ®lter was positioned in a second Gelman sampling head fastened face to face with the ®rst to prevent collection of airborne dust. The spiked ®lters were re-weighed at intervals (0.5, 1, 3 and 6 h) during aspiration to determine the weight loss. The ®lters were analysed in groups of ®ve replicates, and blank ®lters were run in parallel. In addition to examining commercial mineral oil MWF products, the losses from a light base oil (mineral seal oil) and deodorised kerosene were also determined. Mineral seal oil is a base oil in many of the low viscosity MWF, and kerosene is reported to be in some products (CONCAWE, 1986). To identify the components lost during aspiration, a Tenax sorbent tube was positioned between the spiked ®lter and the sample pump to collect any vapours released. Air was pumped through at a ¯ow rate of 2 l. minÿ1 for a period of 5 min, and samples were collected on tubes loaded with 55 mg of Tenax TA (a ¯ow of 2 l. minÿ1 could not be achieved with larger amounts of Tenax). The Tenax tubes were analysed on a Perkin Elmer ATD 400 automated thermal desorber connected to a Hewlett Packard 5970 Mass Selective Detector (ATD-GC-MS). MWFsÐdevelopment of practical criteria for mist sampling 167 Table 1. Results of evaporative weight loss experiments Analyte Viscosity (cSt at 408C) Flashpoint (8C) Boiling point range (8C)a Kerosene 02 Mineral seal oil 3.7 MWF A 4.4 MWF B 5 MWF C 7 MWF D 9.2 MWF E 11.1 MWF F 13 MWF G 13.2 MWF Hb 15 MWF I 15.5 MWF J 18.3 MWF K 19.0 MWF L 19 MWF M 23 MWF N 33 65±85 122 115 115 160 152 135 140 138 180 > 180 168 200 185 202 190 197±236 258±330 205±400 260±330 IBP 350 IBP 260 284±449 205± > 400 337±439 IBP >320 % Cumulative weight loss at: 30 min 60 min 180 min 360 min 98.0 9.2 22.0 4.5 1.4 3.1 2.4 2.8 1.3 0.8 0.2 0.7 0.3 0.1 0.1 ÿ0.1 99.6 16.8 34.3 7.9 2.5 5.6 4.2 4.6 2.4 1.4 0.2 1.1 0.4 0.4 0.3 0.3 100.0 36.7 56.9 17.2 5.4 11.5 9.7 8.8 5.4 3.4 0.4 1.9 0.9 0.8 0.5 0.4 99.7 57.2 71.2 27.5 8.9 16.9 16.0 13.2 9.1 5.7 0.6 2.7 1.5 1.4 0.6 0.4 a Estimated from data on base oils obtained from oil producers: IBP=initial boiling point. Hydraulic oil. b RESULTS The results of the weight loss experiments are shown in Table 1. The data are presented with the manufacturers' ®gures on the viscosities and ¯ash points of the mineral oil MWFs tested. In addition, where obtainable, an estimated boiling point range is included. This is not the measured boiling point range for the MWF, but is based upon information obtained via manufacturers on the base oils known to be present, and contains no allowance for other constituents. An example of the change in weight with time during aspiration is shown in Fig. 1 (MWF G). The compounds identi®ed on the Tenax back up tubes are shown in Table 2. Only prominent peaks in the sample chromatograms were identi®ed. There was often a hydrocarbon envelope of unresolved peaks, predominantly branched aliphatic hydrocarbons, which were not investigated. DISCUSSION All the oils tested showed some weight loss after 6 h of aspiration. For those oils showing signi®cant weight loss it was apparent that material continued to evaporate from the samples throughout the 6 h. The loss measured varied from less than 1% for the heavier oils (MWF M and MWF N), to 71% for the lighter oils (MWF A). A simple way of identifying those mineral oil MWF unsuitable for ®lter sampling was sought. Such a method should link the weight loss of the Fig. 1. The rate of loss of material from MWF G during aspiration: weight loss (%), time aspirated (min). C10±C18 Alkanes (C12±C16 n-alkanes major components) n-Octane 1,2,3,4-tetra-hydro-naphthalene C13±C18 Alkanes (C13±C15 n-alkanes major components) C4 Alkyl benzenes C5 Alkyl benzenes C11±C18 Alkanes (C13±C15 n-Alkanes major components) Trimethyl benzene tr Decane tr Ethyl, methyl benzene tr 6-methyl,-1,2,3,4-tetrahydro naphthalene C13±C18 Alkanes (C18 trace amounts) Unidenti®ed aromatics Diethylene glycol 2,6-bis(1,1-Dimethyl ethyl)-4-methyl phenol Methyl dodecanoate Methyl tetradecanoate Menthyl hexadecanoate C14±C19 Alkanes (C14 and C19 trace amounts) C4 Alkyl benzene C5 Alkyl benzenes C10±C18 Alkanes (C13±C15 n-alkanes major components) MWF A MWF B Mineral seal oil Kerosene MWF M MWF N MWF L MWF K MWF H MWF I MWF J MWF G Analyte Major component: signi®cantly above instrument detection limit; tr: trace levels detected, close to instrument detection limit. a MWF F MWF E MWF D MWF C Possible identi®cation of components Analyte Table 2. Identity of the materials evaporated from the MWFa Decahydro naphthalene Methyl decahydro naphthalene C11±C18 Alkanes (C12±C17 n-alkanes major components) C5 Benzenes C14±C18 Alkanes (C18 at trace level) 3-Methyl, butyl acetate tr 1-methyl, butyl acetate tr Decane tr 1,8 Cineole tr Undecane tr Bornyl acetate tr C10±C16 Alkanes (C11±C15 n-alkanes major components) C6 Alkyl cyclohexane C10±C18 Alkanes Hydrocarbon envelope with no outstanding feature C13±C18 Alkanes (C13, C14, C15 and C18 at trace level) C12±C17 Alkanes (C12±C15 n-alkanes major components) Unidenti®ed aromatics C15±C18 Alkanes tr Unidenti®ed phenol compound tr ± Possible identi®cation of components 168 A. T. Simpson et al. MWFsÐdevelopment of practical criteria for mist sampling oil during aspiration with a physical property of the oil, such as boiling point range, vapour pressure, viscosity or ¯ashpoint. The data must be easily obtainable so that analysis of the mineral oil MWF is not required before sampling of the oil mist can begin. The most obvious sources of this kind of information are the manufacturers' product information sheets. The boiling point range and vapour pressure of the mineral oil MWF were not used since neither property is usually measured by manufacturers. The information which is available on boiling points generally refers to measurements on the base oils by the base oil producers, and no allowance is made for the eect of blending base oils and additives; over half of a mineral oil MWF can consist of additives. The boiling point range of the base oils could be used as a guide, but the information is hard to obtain as it is not always known by the MWF manufacturer, and it is questionable how applicable it would be for mineral oil MWF which contain a considerable portion of additives. A similar situation exists for vapour pressure measurements; any data available refers to the base oil, not the MWF, and additives can change the vapour pressure of the MWF. Generally, when information is quoted it is in the form of an all encompassing maximum vapour pressure (for example, <0.1 kPa at 208C). The kinematic viscosity (at 408C) and the ¯ashpoint of the mineral oil MWF are usually quoted on the product data sheets, and there is a clear relationship between both parameters and the ®lter weight loss for the oils in this study, illustrated in Fig. 2 and 3. The error bars represent +/ÿ1.96 times the standard error of the data. Figure 2 shows that the oils at the top end of the viscosity range (23 and 33 cSt) lost very little material during aspiration, while those in the middle 169 and bottom end of the range show an increase in sample loss with decreasing viscosity. Those at very low viscosity (MWF A and B) had very high evaporative losses from the ®lter. The mineral seal oil sample, which has a similar viscosity to those very low viscosity mineral oil MWF and is probably a base oil present in the MWF formulation, had a similar weight loss. MWF C does not appear to ®t the trend, having a lower than expected weight loss. This MWF is exceptional in that 75% of it is additive and the remaining 25% is made up of two base oils. Unsurprisingly the kerosene sample was fully evaporated in the ®rst half hour. There should be very little contribution to gravimetric results from any kerosene present in samples trapped on glass ®bre ®lters. Figure 3 shows similar behaviour when the weight losses were plotted against ¯ashpoint. Oils with high ¯ashpoints unsurprisingly lost little material during aspiration, whilst oils with lower ¯ashpoints showed increasing sample losses. There is little dierence between the trend within the graphs, with each showing a similar pattern. Although a degree of caution should be taken in analysing the data (each MWF is relatively unique, and data at levels of sample loss near to zero could be expected to skew some regression models), a line of best ®t was determined for both graphs. Using an exponential model, the viscosity data gave the line y = e4.24x ÿ 0.183, correlation coecient ÿ0.88 (Fig. 2), and the ¯ashpoint data gave the line y = e9.43x ÿ 0.0488, correlation coecient ÿ0.91 (Fig. 3). The graphs were used to try and estimate a cut-o point, below which oils would be liable to unacceptable losses. From inspection of the graphs in Fig. 2 and 3, the oils which had less than 5% losses were generally those with viscosities greater than 18 cSt and ¯ashpoints of 1808C and above. If losses of 5% after 6 h of aspiration are deemed acceptable then Fig. 2. The relationship between weight loss and MWF viscosity: weight loss after 6 h (%), MWF viscosity (cSt at 408C). 170 A. T. Simpson et al. these results would suggest that 8-h sampling of oil mist onto glass ®bre ®lters is a suitable sampling method for mineral oil MWF with viscosities of 18 cSt and above or ¯ashpoints of 1808C and above. Although both ¯ashpoint and viscosity are capable of identifying these oils, it was considered that the 18 cSt viscosity criterion was a clearer cut-o point and could be more usefully applied. Mists from oils below these limits may be liable to suer losses during sampling, giving erroneous oil mist measurements. An enquiry to the British Lubricant Federation revealed that analysis of neat cutting oil sales shows that 38% by number, and 28% by volume are less than 18 cSt at 408C. This represents a signi®cant proportion of oils used where occupational exposures may, to some extent, be underestimated. The evaporative losses from ®lters when sampling the aerosol from very light mineral oil MWFs (for example, <5 cSt) could be large, with losses of up to 71.2% recorded here. In these cases collecting the oil mist on a ®lter could give very misleading results since the sample collected on the ®lter could evaporate easily during sampling. Oil mist from these oils could be associated with very high levels of oil in vapour form. Although the vapour is considered less harmful than the mist, it may become a signi®cant form of exposure at high levels. It should be noted that these results could be used as an indicator of the potential for error during sampling, but should not be used to back-calculate the true mist concentration from existing data. It has been reported that sampling oil mist using personal electrostatic precipitators will retain more sample than ®lter sampling (Leith et al., 1996), however, such devices are highly specialised and not widely available. Filter sampling using inhalable samplers such as those recommended by HSE (1997) is currently the only feasible method to collect the particle size fraction of the mist required for comparison with the OES, but as can be seen, ®lter samples can be subject to evaporative losses. The experimental methods used here do not fully model the conditions during normal sampling in the workplace. The ®lters were spiked by pipetting neat mineral oil onto the ®lter; real oil mist samples would have covered a larger surface area of the ®lter which would have increased the rate of evaporation, thus the results here should be seen as the minimum losses under the test conditions used. Increasing the air temperature would also have the eect of increasing the rate of evaporation. Conversely, the clean laboratory air used for aspiration did not contain any signi®cant amount of hydrocarbon vapour which would have reduced or prevented evaporation. In the workplace, sta may enter areas of clean air during the sampling period, but when operating the relevant machinery will be exposed to air containing either high or even saturated levels of vapour from the oil. In addition, the spiked ®lters tested were aspirated for 6 h, but during actual sampling, oil would accumulate on the ®lter over the whole sampling period, and only a small portion of the ®nal sample will have been aspirated for longer than 6 h; most would have been aspirated for a shorter period. Nevertheless, the experimental method was thought to represent the best practical way of estimating the losses. Because there are many mineral oil MWF formulations available, the results reported here are necessarily based upon a limited number of products, but these are thought to be representative of the range available. All the oils used were new oils, but it is possible Fig. 3. The relationship between weight loss and MWF ¯ashpoint: weight loss after 6 h (%), MWF ¯ashpoint (8C). MWFsÐdevelopment of practical criteria for mist sampling that the composition of oils which have been used for a period of time may change. They could contain fewer volatiles because of preferential evaporation of lighter components, or become contaminated by tramp oil (for example, lubricating and hydraulic oil). The loss of volatiles would raise the ¯ashpoint, increase viscosity and reduce the amount of material liable to loss from the ®lter during sampling. The hydraulic and lubricating ¯uids used for metalworking machines are predominantly neat oils with higher viscosities than MWF. Machine lubricating oil is typically 68 cSt and hydraulic oil 32 cSt. These oils contain heavier, less volatile components than MWF, and would generally increase viscosity, raise the ¯ashpoint and reduce the volatility of the oil. A range of compounds from the base oils and additives in the mineral oil MWF were identi®ed amongst the material lost from the ®lters during sampling aspiration. It is dicult to estimate the amount of material trapped on the Tenax tubes without doing quantitative work, but from comparing chromatographic peak heights it was apparent that more material was detected in the analyses of the lower viscosity MWF than in those of the higher viscosity MWF. This is not unexpected considering the results in Table 1. Aliphatic hydrocarbons in the range C10±C19 were detected; 10 of the 14 samples contained C14±C18 n-alkanes. This is consistent with other research into the evaporation of oil droplets, where Raynor et al. (1996) found C14±C18 n-alkanes in vapour originating from an oil containing C14±C20 alkanes. Little material was detected from the four most viscous mineral oil MWF (MWF K, L, M and N). `MWF' H (the hydraulic oil), despite having a hydrocarbon envelope in the chromatogram, did not contain any prominent peaks. The base oil was not a paranic oil unlike the MWF analysed. Paranic base oils would appear to be favoured over naphthenic base oils for use in MWF because of their greater stability at high temperatures and because they are cheaper. Some compounds were identi®ed which may be additives to the MWF. These included alkyl benzenes, diethylene glycol, an alkylated phenol, fatty acid methyl esters, iso pentyl acetate, bornyl acetate and 1,8 cineole. The last three listed are most probably odorants and were detected at low levels. During the Tenax back up tube analysis, about 25% of the kerosene sample was lost during the ®rst 5 min; the mineral seal oil sample lost less than 5% in the ®rst 5 min (but 57% over 8 h). All of the components of kerosene were identi®ed, and most of the components of mineral seal oil were identi®ed (C19±C23 were not detected). The rate of sample loss re¯ects the volatility of the components present on the ®lters; kerosene lost each of its components quickly, whilst mineral seal oil lost most 171 components at a slower rate. Given that mineral seal oil or similar oil is the main base oil in very low viscosity mineral oil MWF, it is not surprising that MWF of this type lost a large proportion of sample during aspiration. Little material was lost from mineral oil MWF with viscosities around 20 cSt during aspiration. When a base oil of similar viscosity (known as `100 solvent neutral') was analysed by GC±MS it was found to contain very little material below C19 (nonadecane), the heaviest component lost from the MWF in detectable quantities. This is consistent with a CONCAWE (the oil companies internal study group for conservation of clean air and waterÐEurope) report (1986) which states that oil mists from lubricants containing hydrocarbons greater than C20 are not normally associated with signi®cant levels of vapour. If the proportion of a mineral oil MWF more volatile than nonadecane is very low, then it appears that little material will be lost from the ®lter during the sampling of its aerosol. In the NIOSH mineral oil mist sampling method (NIOSH, 1994) the sampling procedure for mineral oil mist is said to be applicable to mineral oils with the formula Cn H2n2 , where n is greater than or equal to 16. This approach may identify those oils which can be sampled without signi®cant losses from the ®lter, but would be dicult to apply in practice because of the data being unavailable. In 1975 the Institute of Petroleum Occupational Hygiene Subcommittee stated that the threshold limit value (TLV) used as a guide at the time only referred to oil mist and not vapour, and warned that accurate sampling of particulate mist from oils containing a signi®cant proportion of relatively volatile components was dicult, but did not oer any further guidance or criteria on this problem (Turner et al., 1975). CONCAWE warn that the methods described in their report may not be applicable to more volatile products, and cite low boiling range oils and products containing a signi®cant proportion of relatively volatile components (CONCAWE, 1981), but do not attempt to nominate a limit. They suggest that in such products vapour concentrations are more likely to be the appropriate criteria for assessing exposures. Little has been reported on the concentrations and eects of the vapour found with oil mists, but it is considered to be present in potentially much higher concentrations than the particles, and also to be much less toxic (CONCAWE, 1981). It was considered less toxic because it was thought that, although the vapour penetrates the lungs to the alveoli and a portion will dissolve in the mucous membrane, it is then exhaled, whilst the aerosol will be deposited on the walls of the lungs as droplets (Beviz, 1975). Once deposited, oil particles may be removed by clearance mechanisms or absorbed into body ¯uids and therefore are capable of causing 172 A. T. Simpson et al. both local and systemic toxic eects (CONCAWE, 1986). As a consequence it was considered that measured oil mist concentrations should refer to particles alone, and contain no contribution from the vapour. The problem of which phase to sample and which standard to apply is not particular to airborne mineral oil but applies to other relatively high boiling liquids. Thus, separate mist and vapour standards (Occupational Exposure Standards, 10 and 60 mg mÿ3 respectively) are listed for ethane1,2-diol (boiling point 196±1988C), whereas diethylene glycol (boiling point 2458C) has only a single limit (23 ppm or 101 mg mÿ3). The case of oil is complicated by it being a variable mixture of compounds which cover a range of volatilities, resulting in signi®cant variation in the distributions between liquid and vapour phases encountered. The traditional method of estimating exposure is via sampling of the mist phase onto a ®lter, with no account taken of the vapour phase. If both vapour and particulate were to be taken into account then they could be sampled together simultaneously and expressed as exposure to total airborne oil, avoiding the problem of evaporation of mist trapped on the ®lter. However it seems likely, given the dierences in deposition and uptake and the potential dierences in health eects which may result, that the measurement of mineral oil exposure should involve the separate (possibly still simultaneous) measurement of aerosol and vapour. From a practical point of view, distinguishing between the mist and vapour may be dicult, especially for the very low viscosity oils. CONCLUSION Mineral oil mist samples lose material during sampling because of evaporation of volatile components. The longer a sample is subject to aspiration the more material is likely to be lost. The material lost is mostly aliphatic hydrocarbons in the range C10±C19, but can include additives in the oil. The relationship between the amount of material which could be lost during sampling and MWF composition can be predicted from the MWF viscosity or ¯ashpoint data regularly found on the product data sheet provided by the manufacturers. Viscosity was the favoured method as it was considered to be a clearer and more useful indicator of whether the mist from an oil was suitable for ®lter sampling. Oil mist samples of mineral oil MWF with viscosities greater than about 18 cSt at 408C (or ¯ashpoints greater than 01808C) can be expected to lose less than 5% during 8-h sampling because of evaporation from the ®lter. Below 18 cSt, losses are more variable and generally greater. It is proposed that 18 cSt could be taken as a guide value below which there may be losses during sampling for oil mist. Over a quarter of MWF for- mulations in the UK have viscosities below 18 cSt. Filter sampling may underestimate mist exposure from such MWFs. For very low viscosity MWF (<5 cSt) any occupational exposure measurements taken may seriously underestimate mist concentrations and will also not indicate the high levels of oil vapour likely to be present. The current UK mineral oil OES applies to exposure to oil mist and takes no account of simultaneous exposure to hydrocarbon vapour from the mineral oil. For low viscosity oils it may be appropriate to measure both the mist and vapour. Currently there is no validated method of measuring mixed phase exposure to mineral oil mist but further experimental work is planned. AcknowledgementsÐWe acknowledge the comments and suggestions of Dr R. Gardner, HSE Oshore Safety Division. REFERENCES Beviz A. (1975) Oljedimma: EgenskaperÐMaÈtningÐ Skydd (Oil Mist: PropertiesÐMeasurementÐ Protection). Arbetsmiljo 8/75 (available from HSE Translation Services, HSE transl. no. 15128). CONCAWE. (1981) Guidelines for the Determination of Atmospheric Concentrations of Oil Mists. Report no. 1/ 81, CONCAWE, Madouplein 1, B-1030, Brussels, Belgium. CONCAWE. (1986) Health Aspects of Worker Exposure to Oil Mists. Report no. 86/69, CONCAWE, Madouplein 1, B-1030, Brussels, Belgium. HSE (1991) Metalworking Fluids: Health Precautions, Guidance Note EH 62. HSE Books, ISBN 0 11 885667 7. HSE (1997) General Methods for Sampling and Gravimetric Analysis of Respirable and Total Inhalable Dust, Methods for the Determination of Hazardous Substances MDHS 14/2. HSE Books, ISBN 0 7176 1295 3. HSE (1999) Occupational Exposure Limits 1999, Guidance Note EH 40/99. HSE Books, ISBN 0 7176 1660 6. Leith, D., Leith, F. A. and Boundy, M. G. (1996) Laboratory measurements of oil mist concentrations using ®lters and an electrostatic precipitator. American Industrial Hygiene Association Journal 57, 1137±1141. McAneny, J. J., Leith, D. and Boundy, G. B. (1995) Volatilization of mineral oil mist collected on sampling ®lters. Applied Occupational and Environmental Hygiene 10, 783±787. Menichini, E. (1986) Sampling and analytical methods for determining oil mist concentrations. Annals of Occupational Hygiene 30, 335±348. NIOSH (1994) Mineral oil mist. In Manual of Analytical Methods, Method 5026, 4th ed. National Institute of Occupational Safety and Health, Cincinatti, OH. Raynor, P. C., Cooper, S. and Leith, D. (1996) Evaporation of polydisperse multicomponent oil droplets. American Industrial Hygiene Association Journal 57, 1128±1136. Soderholm, S. C. (1988) Aerosol instabilities. Applied Industrial Hygiene 3, 35±40. Turner, D., Baxter, H. G., Davies, G. M., Eyres, A. R., Lees, G. L., Mitchell, J., Sanderson, J. T., Steel, J. and Moore, J. P. (1975) Methods for the determination of the atmospheric concentration of oil mist. Annals of Occupational Hygiene 18, 293±297.