On the Relationship between Exposure to Particles and Dustiness during Handling of Powders in Industrial Settings.

Exposure to ceramic powders, which is frequent during handling operations, is known to cause adverse health effects. Finding proxy parameters to quantify exposure is useful for efficient and timely exposure assessments. Worker exposure during handling of five materials [a silica sand (SI1), three quartzes (Q1, Q2, and Q3), and a kaolin (K1)] with different particle shape (prismatic and platy) and sizes (3.4-120 µm) was assessed. Materials handling was simulated using a dry pendular mill under two different energy settings (low and high). Three repetitions of two kilos of material were carried out per material and energy conditions with a flow rate of 8-11 kg h-1. The performance of the dustiness index as a predictor of worker exposure was evaluated correlating material's dustiness indexes (with rotating drum and continuous drop) with exposure concentrations. Significant impacts on worker exposure in terms of inhalable and respirable mass fractions were detected for all materials. Mean inhalable mass concentrations during background were always lower than 40 µg m-3 whereas during material handling under high energy settings mean concentrations were 187, 373, 243, 156, and 430 µg m-3 for SI1, Q1, Q2, Q3, and K1, respectively. Impacts were not significant with regard to particle number concentration: background particle number concentrations ranged between 10 620 and 46 421 cm-3 while during handling under high energy settings they were 20 880 - 40 498 cm-3. Mean lung deposited surface area during background ranged between 27 and 101 µm2 cm-3 whereas it ranged between 22 and 42 µm2 cm-3 during materials handling. TEM images evidenced the presence of nanoparticles (≤100 nm) in the form of aggregates (300 nm-1 µm) in the worker area, and a slight reduction on mean particle size during handling was detected. Dustiness and exposure concentrations showed a high degree of correlation (R2 = 0.77-0.97) for the materials and operating conditions assessed, suggesting that dustiness could be considered a relevant predictor for workplace exposure. Nevertheless, the relationship between dustiness and exposure is complex and should be assessed for each process, taking into account not only material behaviour but also energy settings and workplace characteristics.

Inter-comparison of personal monitors for nanoparticles exposure at workplaces and in the environment.

Personal monitors based on unipolar diffusion charging (miniDiSC/DiSCmini, NanoTracer, Partector) can be used to assess the individual exposure to nanoparticles in different environments. The charge acquired by the aerosol particles is nearly proportional to the particle diameter and, by coincidence, also nearly proportional to the alveolar lung-deposited surface area (LDSA), the metric reported by all three instruments. In addition, the miniDiSC/DiSCmini and the NanoTracer report particle number concentration and mean particle size. In view of their use for personal exposure studies, the comparability of these personal monitors was assessed in two measurement campaigns. Altogether 29 different polydisperse test aerosols were generated during the two campaigns, covering a large range of particle sizes, morphologies and concentrations. The data provided by the personal monitors were compared with those obtained from reference instruments: a scanning mobility particle sizer (SMPS) for LDSA and mean particle size and a ultrafine particle counter (UCPC) for number concentration. The results indicated that the LDSA concentrations and the mean particle sizes provided by all investigated instruments in this study were in the order of ±30% of the reference value obtained from the SMPS when the particle sizes of the test aerosols generated were within 20-400nm and the instruments were properly calibrated. Particle size, morphology and concentration did not have a major effect within the aforementioned limits. The comparability of the number concentrations was found to be slightly worse and in the range of ±50% of the reference value obtained from the UCPC. In addition, a minor effect of the particle morphology on the number concentration measurements was observed. The presence of particles >400nm can drastically bias the measurement results of all instruments and all metrics determined.

Review of measurement techniques and methods for assessing personal exposure to airborne nanomaterials in workplaces.

Exposure to airborne agents needs to be assessed in the personal breathing zone by the use of personal measurement equipment. Specific measurement devices for assessing personal exposure to airborne nanomaterials have only become available in the recent years. They can be differentiated into direct-reading personal monitors and personal samplers that collect the airborne nanomaterials for subsequent analyses. This article presents a review of the available personal monitors and samplers and summarizes the available literature regarding their accuracy, comparability and field applicability. Due to the novelty of the instruments, the number of published studies is still relatively low. Where applicable, literature data is therefore complemented with published and unpublished results from the recently finished nanoIndEx project. The presented data show that the samplers and monitors are robust and ready for field use with sufficient accuracy and comparability. However, several limitations apply, e.g. regarding the particle size range of the personal monitors and their in general lower accuracy and comparability compared with their stationary counterparts. The decision whether a personal monitor or a personal sampler shall be preferred depends strongly on the question to tackle. In many cases, a combination of a personal monitor and a personal sampler may be the best choice to obtain conclusive results.

[Comparative field study on high flow rate samplers for respirable fraction-A solution to smaller collected masses].

OBJECTIVE: Dust sample mass gain is too smaller to satisfy the limit of detection (LOD) even in most cases during dust sampling at workplaces nowdays, especially for respirable fraction. Therefore, it is aimed to solve the problem by increasing sample load with high flow rate samplers. METHODS: In A and B two shipyards respirable welding fume was sampled by high flow rate cyclone samplers of FSP-10 (10 L/min) for 2-2.5 hours and normal flow rate FSP-2 (2 L/min) for 3-4 hours with a stratigy of parallele sampling at the same workpalce, in order to compare their mass gain, coincidence rate with LOD, and airborn dust concentration. RESULTS: Sample mass gain of 0.97±0.40 mg and 1.61±0.86 mg respectively in the two factories by FSP-10 was significantly higher than that of 0.29±0.12 mg and 0.51±0.27 mg by FSP-2 (t-test, P<0.05 in both cases) , increasing herewith the coincidence rate with LOD from 26.8% (when sampling with FSP-2, calculated together with samples of the two factories) to 89.7%. However there was no significant difference in dust concentrations by the two different samplers, 0.53±1.88 vs 0.73±1.61 mg/m(3) by FSP-2 and FSP-10 in the shipyard A and 1.14±1.78 vs 1.01±1.63 mg/m(3) in the factory B (t-test, P>0.05 in every case) . In addtion, sample loading by FSP-2 was found to be correlated to sampling time (R(2)=0.7906, y=0.002 6x) , therefore, it has to sample for ≥192.3 min to meet the LOD (0.5 mg) in case of normal flow rate. CONCLUSION: By using of high flow rate cyclone FSP-10 the problem of LOD could be solved, along with increased sample mass and similar respirable dust concentration by the two samplers. Some techincal improvements of FSP-10 and increasing of LOD coincidence rate by other methods was also disscussed.

Assessing the protection of the nanomaterial workforce.

Responsible development of any technology, including nanotechnology, requires protecting workers, the first people to be exposed to the products of the technology. In the case of nanotechnology, this is difficult to achieve because in spite of early evidence raising health and safety concerns, there are uncertainties about hazards and risks. The global response to these concerns has been the issuance by authoritative agencies of precautionary guidance to strictly control exposures to engineered nanomaterials (ENMs). This commentary summarizes discussions at the "Symposium on the Health Protection of Nanomaterial Workers" held in Rome (25 and 26 February 2015). There scientists and practitioners from 11 countries took stock of what is known about hazards and risks resulting from exposure to ENMs, confirmed that uncertainties still exist, and deliberated on what it would take to conduct a global assessment of how well workers are being protected from potentially harmful exposures.

Lung cancer among coal miners, ore miners and quarrymen: smoking-adjusted risk estimates from the synergy pooled analysis of case-control studies.

OBJECTIVES: Working in mines and quarries has been associated with an elevated lung cancer risk but with inconsistent results for coal miners. This study aimed to estimate the smoking-adjusted lung cancer risk among coal miners and compare the risk pattern with lung cancer risks among ore miners and quarrymen. METHODS: We estimated lung cancer risks of coal and ore miners and quarrymen among 14 251 lung cancer cases and 17 267 controls from the SYNERGY pooled case-control study, controlling for smoking and employment in other at-risk occupations. RESULTS: Ever working as miner or quarryman (690 cases, 436 controls) was associated with an elevated odds ratio (OR) of 1.55 [95% confidence interval (95% CI) 1.34-1.79] for lung cancer. Ore miners (53 cases, 24 controls) had a higher OR (2.34, 95% CI 1.36-4.03) than quarrymen (67 cases, 39 controls; OR 1.92, 95% CI 1.21-3.05) and coal miners (442 cases, 297 controls; OR 1.40, 95% CI 1.18-1.67), but CI overlapped. We did not observe trends by duration of exposure or time since last exposure. CONCLUSIONS: This pooled analysis of population-based studies demonstrated an excess lung cancer risk among miners and quarrymen that remained increased after adjustment for detailed smoking history and working in other at-risk occupations. The increase in risk among coal miners were less pronounced than for ore miners or quarrymen.

Re-characterization of some factors influencing aerosol sampling in the workplace: results from field studies.

OBJECTIVE: Workplace aerosol sampling is challenged by its influencing factors and methodological limitations. Some factors, including blank setting, electrostatic effect, sample mass gain, and limit of detection (LOD), were studied to characterize them further and thereby to improve the sampling method. METHODS: Through a field sampling in two shipyards and by an analysis of a large amount of dust data from different industries, the influencing factors were comparatively studied with emphasis on their effect on the final result. RESULTS: After calibration with field blanks, the concentrations of most sample types in the shipyards decreased significantly, varying by as much as -24.3% of the final measurements. After laboratory blank calibration, dust concentrations increased or decreased without a definite change trend. With a variation of -1.8%, only the measurements of Chinese "total dust" sampled with polypropylene filters were significantly influenced by the electrostatic effect. The LOD coincidence rate was only 17.3% for American respirable dust in different industries and 12.2% for respirable particles collected by normal flow rate samplers (FSP2) in the shipyards. The latter increased to 73.9% when high flow samplers (FSP10) were used. CONCLUSIONS: It was suggested that field blank calibration was the predominant influencing factor in comparison with electrostatic effect and laboratory blank adjustment. The LOD coincidence rate was too low for reliable sampling, and this might be improved by use of high flow samplers.

Comparability of portable nanoparticle exposure monitors.

Five different portable instrument types to monitor exposure to nanoparticles were subject to an intensive intercomparison measurement campaign. Four of them were based on electrical diffusion charging to determine the number concentration or lung deposited surface area (LDSA) concentration of airborne particles. Three out of these four also determined the mean particle size. The fifth instrument type was a handheld condensation particle counter (CPC). The instruments were challenged with three different log-normally distributed test aerosols with modal diameters between 30 and 180 nm, varying in particle concentration and morphology. The CPCs showed the highest comparability with deviations on the order of only ±5%, independent of the particle sizes, but with a strictly limited upper number concentration. The diffusion charger-based instruments showed comparability on the order of ±30% for number concentration, LDSA concentration, and mean particle size, when the specified particle size range of the instruments matched the size range of the aerosol particles, whereas significant deviations were found when a large amount of particles exceeded the upper or lower detection limit. In one case the reported number concentration was even increased by a factor of 6.9 when the modal diameter of the test aerosol exceeded the specified upper limit of the instrument. A general dependence of the measurement accuracy of all devices on particle morphology was not detected.

A comparative field study on dust measurements by different sampling methods with emphasis on estimating factors for recalculation from chinese 'total dust' measurements to respirable dust concentrations.

In China, dust samplers were originally designed to collect 'total dust' for a short term during production, which is different from the widely adopted sampling strategy for dust. With the aim to provide the conversion factor from Chinese total dust to US and German respirable dust and to look at the influences on conversion factors from environment, production, and instruments, a comparative field study on the dust concentration measurements by different sampling methods was carried out in the same Chinese industries as in the 1989-1990 study and in some other factories. A supplemental experiment was also conducted in a wind tunnel. Dust concentration was measured with a parallel sampling strategy by using the following samplers: 10-mm nylon cyclone for US respirable dust (AR), FSP-Berufsgenossenschaftliches Institut für Arbeitssicherheit (BIA) cyclone for German respirable dust (GR), and samplers for Chinese total dust (CT). Totally, 1434 samples were collected (269 AR, 198 GR, and 967 CT), from which 429 matched sample pairs (249 pairs of AR/CT, 180 GR/CT) were available to calculate conversion ratios. Industry- and job-based conversion factors are presented in this study. The conversion factor of AR/CT was 0.38 for tungsten mines, 0.19 for copper/iron mines, 0.65 for tin mines, and 0.20 for pottery industry, while the factor of GR/CT was 0.69 for tungsten, 0.37 for copper/iron, and 0.52 for pottery. In the job category, AR/CT factors varied from 0.16 to 0.96 and GR/CT from 0.12 to 0.72. For the industries studied in 1988-1989, the AR/CT and GR/CT factors were 0.29 and 0.45, respectively. Both factors were definitely influenced by production, CT dust concentration, sample gain, and variation of dust concentration. Moreover, the respirable dust concentration by FSP-BIA was significantly higher than that by 10-mm cyclones, 63.27-73.10% more as showed also by the wind tunnel experiment. Meanwhile, the GR/CT ratio was significantly larger than the AR/CT in every industry or job with only few exceptions. The GR/CT estimates should be considered as independent ones. Following these results, there is a need to use 'ideal samplers' (consistent with the internationally accepted respirable fraction) in practice and to assess the existent samplers in order to homogenize the exposure data situation.

Development of an exposure measurement database on five lung carcinogens (ExpoSYN) for quantitative retrospective occupational exposure assessment.

BACKGROUND: SYNERGY is a large pooled analysis of case-control studies on the joint effects of occupational carcinogens and smoking in the development of lung cancer. A quantitative job-exposure matrix (JEM) will be developed to assign exposures to five major lung carcinogens [asbestos, chromium, nickel, polycyclic aromatic hydrocarbons (PAH), and respirable crystalline silica (RCS)]. We assembled an exposure database, called ExpoSYN, to enable such a quantitative exposure assessment. METHODS: Existing exposure databases were identified and European and Canadian research institutes were approached to identify pertinent exposure measurement data. Results of individual air measurements were entered anonymized according to a standardized protocol. RESULTS: The ExpoSYN database currently includes 356 551 measurements from 19 countries. In total, 140 666 personal and 215 885 stationary data points were available. Measurements were distributed over the five agents as follows: RCS (42%), asbestos (20%), chromium (16%), nickel (15%), and PAH (7%). The measurement data cover the time period from 1951 to present. However, only a small portion of measurements (1.4%) were performed prior to 1975. The major contributing countries for personal measurements were Germany (32%), UK (22%), France (14%), and Norway and Canada (both 11%). CONCLUSIONS: ExpoSYN is a unique occupational exposure database with measurements from 18 European countries and Canada covering a time period of >50 years. This database will be used to develop a country-, job-, and time period-specific quantitative JEM. This JEM will enable data-driven quantitative exposure assessment in a multinational pooled analysis of community-based lung cancer case-control studies.

Association between lymph node silicosis and lung silicosis in 4,384 German uranium miners with lung cancer.

This study investigates the association between lymph node-only and lung silicosis in uranium miners with lung cancer and exposure to quartz dust. Tissue slides of 4,384 German uranium miners with lung cancer were retrieved from an autopsy archive and reviewed by 3 pathologists regarding silicosis in the lungs and lymph nodes. Cumulative exposure to quartz dust was assessed with a quantitative job-exposure matrix. The occurrence of silicosis by site was investigated with regression models for exposure to quartz dust. Miners with lung silicosis had highest cumulative quartz exposure, followed by lymph node-only silicosis and no silicosis. At a cumulative quartz exposure of 40 mg/m(3) × years, the probability of lung silicosis was above 90% and the likelihood of lymph node-only silicosis and no silicosis do not differ anymore. The results support that lymph node silicosis can precede lung silicosis, at least in a proportion of subjects developing silicosis, and that lung silicosis strongly depends on the cumulative quartz dose.

Quantitative crystalline silica exposure assessment for a historical cohort epidemiologic study in the German porcelain industry.

A time-dependent quantitative exposure assessment of silica exposure among nearly 18,000 German porcelain workers was conducted. Results will be used to evaluate exposure-response disease risks. Over 8000 historical industrial hygiene (IH) measurements with original sampling and analysis protocols from 1954-2006 were obtained from the German Berufs- genossenschaft der keramischen-und Glas-Industrie (BGGK) and used to construct a job exposure matrix (JEM). Early measurements from different devices were converted to modern gravimetric equivalent values. Conversion factors were derived from parallel historical measurements and new side-by-side measurements using historical and modern devices in laboratory dust tunnels and active workplace locations. Exposure values were summarized and smoothed using LOESS regression; estimates for early years were derived using backward extrapolation techniques. Employee work histories were merged with JEM values to determine cumulative crystalline silica exposures for cohort members. Average silica concentrations were derived for six primary similar exposure groups (SEGs) for 1938-2006. Over 40% of the cohort accumulated <0.5 mg; just over one-third accumulated >1 mg/m(3)-years. Nearly 5000 workers had cumulative crystalline silica estimates >1.5 mg/m(3)-years. Similar numbers of men and women fell into each cumulative exposure category, except for 1113 women and 1567 men in the highest category. Over half of those hired before 1960 accumulated >3 mg/m(3)-years crystalline silica compared with 4.9% of those hired after 1960. Among those ever working in the materials preparation area, half accumulated >3 mg/m(3)-year compared with 12% of those never working in this area. Quantitative respirable silica exposures were estimated for each member of this cohort, including employment periods for which sampling used now obsolete technologies. Although individual cumulative exposure estimates ranged from background to about 40 mg/m(3)-years, many of these estimates reflect long-term exposures near modern exposure limit values, allowing direct evaluation of lung cancer and silicosis risks near these limits without extrapolation. This quantitative exposure assessment is the largest to date in the porcelain industry.

Comparative evaluation of the dustiness of industrial minerals according to European standard EN 15051, 2006.

A range of industrial minerals was tested using the rotating drum and the continuous drop methods, the two methods proposed by the published European standard EN 15051 [CEN. (2006) EN 15051 Workplace atmospheres-measurement of the dustiness of bulk materials-requirements and test methods. Brussels, Belgium: European Committee for Standardization], to evaluate and compare their dustiness. The assessment of bulk materials dustiness can help to develop less dusty products and to reduce dust exposure to the workers by improving the processing of minerals. The European standard EN 15051 (CEN, 2006) proposes a classification system that was developed with the intention to assist in the labelling of products in the future. This paper presents a comparison of both test methods in classifying industrial minerals. The correlation between the dustiness measured by the two methods for the inhalable and respirable fractions is given. The results show there is no unambiguous dependence of the dustiness on the grain size of an industrial mineral. Although dustiness can significantly be affected by product moisture, the influence of this parameter is not studied in detail as the industrial minerals were tested in the conditions they are sold, as the standard requires. Especially, the classification of substances with respect to different classes of dustiness was found to be problematic, as the two methods are by no means yielding identical classification groups for all the substances. In any use of the standard (EN 15051; CEN, 2006) for labelling purposes, a revision of the present classification system provided in the standard is required for industrial minerals.

Exposure assessment for nitrogen oxides and carbon monoxide in German hard coal mining.

OBJECTIVE: The exposure situation of German hard coal miners with respect to the components nitrogen monoxide and nitrogen dioxide. Carbon monoxide was measured additionally and the results are displayed but not discussed in detail in this paper. The data were used to estimate personal long-term exposures in an inception cohort. METHODS: For all three components, time weighted 8-h shift values were determined for typical groups of coalminers according to the European measurement standard. An expert panel from the coal mining company made an effort to estimate major potential changes in the exposure situation. RESULTS: The main sources of nitrogen oxides and carbon monoxide in hard coal mining were the diesel engines. Blasting fumes contributed only to a lesser degree and with different exposure characteristics, e.g. much reduced NO(2) levels compared to the mines' rear areas. As rough 8 h-shift averages describing the current exposure situation, we found 1.35 ppm NO and 0.21 ppm NO(2) for the diesel engine drivers. Blasting specialists were more difficult to evaluate but rough 8 h-shift averages of 0.84 ppm NO and 0.014 ppm NO(2) could be estimated from our measurement series. By applying these data and the estimates of experts about the retrospective exposure situation to a cohort of 1,369 coalminers, we derived mean (max) cumulative exposures in ppm x number of shifts of 1,748 (5,928) for NO and 19.6 (1,013) for NO(2) when summarizing over the follow-up period from 1974 until 1998. CONCLUSIONS: Especially for the diesel engine drivers, exposure can be regarded as rather high, in particular, when compared to recommended limits by SCOEL and MAK, though the exposures have been in line with the enforced German occupational exposure limits. Whether this exposure situation has caused adverse health effects will be investigated epidemiologically.

Assessment of exposure in epidemiological studies: the example of silica dust.

Exposure to crystalline silica ranks among the most frequent occupational exposures to an established human carcinogen. Health-based occupational exposure limits can only be derived from a reliable dose-response relationship. Although quartz dust seems to be a well-measurable agent, several uncertainties in the quantification of exposure to crystalline silica can bias the risk estimates in epidemiological studies. This review describes the silica-specific methodological issues in the assessment of exposure. The mineralogical forms of silica, the technologies applied to generate dust, protective measures, and co-existing carcinogens are important parameters to characterize the exposure condition of an occupational setting. Another methodological question concerns the measurement of the respirable dust fraction in the worker's breathing zone and the determination of the quartz content in that fraction. Personal devices have been increasingly employed over time, whereas norms for the measurement of respirable dust have been defined only recently. Several methods are available to analyse the content of crystalline silica in dust with limits of quantitation close to environmental exposure levels. For epidemiological studies, the quartz content has frequently not been measured but only calculated. To develop a silica-dust database for epidemiological purposes, historical dust concentrations sampled with different devices and measured as particle numbers have to be converted in a common exposure metric. For the development of a job-exposure matrix (JEM), missing historical data have to be estimated to complete the database over time. Unknown but frequently high-exposure levels of the past contribute largely to the cumulative exposure of a worker. Because the establishment of a JEM is crucial for risk estimates, sufficient information should be made accessible to allow an estimation of the uncertainties in the assessment of exposure to crystalline silica. The impressive number of silica dust measurements and the evaluation of methodological uncertainties allow recommendations for a best practice of exposure assessment for epidemiological studies.

Exposure assessment in German potash mining.

OBJECTIVE: Between 1995 and 2003 a longitudinal study on miners in two German potash mines was planned and performed by BAuA in collaboration with K+S and IGF. Aim was to correlate exposure data to the results of medical examination of the miners in the respective mines' workforce and to use the detailed exposure investigation as a tool for risk assessment by the company. In this time period a discussion about health effects and the corresponding necessity to lower the existing threshold limits for the components NO and NO(2) was started as well. Whereas the epidemiological aspects of this study are reported elsewhere (Lotz et al., in Int Arch Occup Environ Health, 2006), we discuss the exposure situation in detail in this paper. METHODS: In two potash mines in Germany the shift and short time exposure for the components respirable dust, inhalable dust, diesel particulate matter, nitrogen monoxide, nitrogen dioxide, and carbon dioxide was investigated in four separate campaigns. The results are reported and discussed. RESULTS: The miners especially in the production areas of the mines are exposed to a highly correlated mixture of the components though the exposure situation can be regarded as state of the art and representative for the industry. CONCLUSION: All dose-response discussions must take into account that never only one of the components can be made responsible for an eventually occurring respiratory effect. For reasons of availability of proper measurement equipment limitations for a possible lowering of indicative limit values of the EU are to be observed.

Biomonitoring of polycyclic aromatic compounds in the urine of mining workers occupationally exposed to diesel exhaust.

Diesel exhaust is considered a probable human carcinogen by the IARC. Biomonitoring of workers occupationally exposed to diesel exhaust was performed to determine their internal burden of diesel associated aromatic compounds. Personal air sampling also allowed to determine the exposure of the miners at their work place towards several polycyclic aromatic hydrocarbons (PAH) and nitro-arenes, the latter of which are thought to be specific constituents of diesel exhaust. For biomonitoring the urine of 18 underground salt miners was collected during and after their shift for 24-hours. half of the 18 miners were smokers. The urinary levels of 1-hydroxypyrene and hydroxylated phenanthrene metabolites were determined as biomarkers of PAH exposure, whereas urinary levels of some aromatic amines were chosen to monitor exposure towards specific nitro-arenes from diesel exhaust like 1-nitropyrene and 3-nitrobenzanthrone and to monitor the human burden by these compounds from inhaled cigarette smoke. Non-smoking workers exposed to diesel exhaust excrete an average level of about 4 micrograms phenanthrene metabolites, whereas the urinary levels in smokers were up to 3-fold higher. In summary the results indicate that (i) diesel exposure led to an increase of PAH metabolism in the workers examined, most probably by an induction of cytochrome P450 (ii) smokers could be identified in accordance with earlier studies by their increased ratio of phenanthrene metabolites derived from 1,2- and 3,4-oxidation and their higher amounts of excreted 1-naphthylamine, and (iii) the excreted amounts of aromatic amines found as metabolites of the nitro-arenes were about 5- to 10-fold higher as one might expect from the levels determined by personal air sampling at the workplace of the individuals.