Exposure assessment in German potash mining

ABSTRACT


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 NO2 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.

METHODS

General remarks

The concentrations of the following components have been measured during three of the four different measurement campaigns: nitrogen monoxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), diesel particulate matter ("dpm", "elemental carbon"), respirable dust, and inhalable dust, the latter two according to the European Standard EN 481 . Due to experimental limitations in the very first campaign in 1995 only the particulate components ("dpm", respirable and inhalable dust) could be measured.
The measurements have been performed according to the following timetable.

General aspects of measurement strategy

For all the components time weighted 8-h shift values were determined according to the German regulation TRGS 402 [Technische Regeln Gefahrstoffe (TRGS) 402 1997], which is in its procedures in line with the European Standard EN Time of sampling

As it was generally not possible to run the samplers on a whole shift base, i.e. from entry of the miners into the mine to exit, time weighting was done by measuring during time of exposure and numerical calculation including "exposure-free" time periods (e.g. travelling to the worksite).

This is exactly the required procedure for the German compliance process, thus the results give a good description of the exposure situation in legal terms. As a rule of thumb personal sampling was generally preferred, but for several reasons in some cases stationary samplers had to be applied as well. So, in low-exposure situations, for example in the work shops, which were located near the intake shafts of the mines in both cases, the personal samplers for the four gaseous components were generally not suitable because of their insufficient lower detection limits (see below).

As a consequence and because the concentrations especially of the nitrogen oxides were required for comparison purposes in these settings, stationary instruments had to be applied1 as well. In a former study we could show, that under the conditions of underground mining with directed ventilation present in all workplaces, the differences for diesel particulate matter between personal and stationary sampling results tend to be very small, if the stationary sampling is performed "near the source" (as required by German regulation) because of the ultrafine nature of that particulate aerosol.

The same cannot safely be said for respirable dust and inhalable dust. Therefore the latter two components were only measured by personal sampling. 1 It is NOT recommended to use the environmental monitors (chemoluminescence) for these purposes, as the instruments need special maintenance procedures in salt mines and are in severe danger of malfunction after only a few uses underground due to their fragility.

Instruments used


The following instruments have been applied (Table 2).
Whereas the dust sampling equipment (PGP, MPG II) could be used according to well-established standard operation procedures , the same is not true for the person carried gas sensors (Multiwarn). As mentioned above, the chemoluminescence monitors were only used in low-exposure situations of workshops, where otherwise relatively undemanding environmental conditions could be guaranteed. Even under these conditions, the equipment needed special cleaning after each single use underground in order to guarantee its functionality. We do not recommend their use in salt or potash mines, because of their general fragility.

The Multiwarns are characterized by relatively sturdy design but low sensitivity. In addition they need very high quality control efforts to guarantee their proper use. There is clear evidence, that cross sensitivities between some of the components (e.g. NO and CO, humidity) may play a part. Therefore it was necessary to put special emphasis on the calibration procedures applied. Commercially available calibration gases were used to calibrate the instruments in the laboratory. However, because of the special influence of moisture a one-point calibration with calibration gases within the underground mines themselves, i.e. under the specific respective humidity (very low) and atmospheric pressure conditions (variable depending on the site within the mine) was necessary.

Even under these circumstances, we have to comment the measurement results according to our own validation results. Not all of the Multiwarn results can be classified as belonging into the same high validity category. Table 3 reports the analytical properties of the methods and our validation ranking).

As a consequence the results for the component CO2 are not given in this paper. Also, they are of only minor significance for the epidemiological study.

The direct reading instruments were set to a recording time of 1 min. As a consequence all exposure data for the gaseous components are available as a sequence of 1-min values from which shift values have been numerically calculated. This could be of high significance when irritant components like NO2 are discussed. Especially in this case short time exposure and less dose-related effects can be important for eventually detected health effects. We have developed a specific evaluation tool for the discussion of short time values, the so-called workplace-exposure profiles (WEP), which have been reported elsewhere. This procedure is discussed below in detail.

Analytical procedures

The analytical methods of the direct reading instruments have already been described above. Diesel particulate matter was measured as elemental carbon using the coulometric method . This method has been demonstrated to yield well comparable data to the so-called thermo-optical method, standardized in NIOSH 5040 under mining conditions and has been tested extensively in a series of European round robin tests.

Because of the large number of samples only gravimetrically determined respirable dust and inhalable dust results are presented here. A special procedure for the determination of soluble (salt) percentages of the dust was also developed in IGF and has been published in. Because of the considerable additional effort it would have required, and because it only was available in later stages of the study, only few filter samples have been investigated with this method.

RESULTS

Shift value data

Table 4 details the total numbers of measurements for the various different workplaces and mines. The measurements have been performed over a period of almost 10 years, therefore certain changes in the measurement methods, the measurement strategies and the sites of measurements have been inevitable. One example is the change of analyte for diesel particulate matter. Whereas in 1994/1995 dpm has been measured and evaluated as "total carbon", it was determines as "elemental carbon (ec)" since then.

Elemental carbon is still the currently used analyte and therefore only these results have been included in this paper3. Some of the workplaces where classified with different formerly used denominations in the campaigns A1, A2, and B1. They have been reclassified according to the most recently used notification as of campaign B2. Great care has been taken; however, to guarantee comparability with respect to the real workplace situation of the miners involved.

So, as an example "small transportation vehicle" jobs  are the ones of supervisors or other persons with a more mobile work style using small diesel engine driven cars to move around in the production areas proper. The rationale behind the grouping was to have as many different groups as reasonably possible but to have as many measurement results as necessary behind each group for a statistically meaningful evaluation.

Three main regions have been identified within the mines because a functionally different level of exposure was suspected. They are the main production areas, relatively far-off the intake shafts, where higher exposure levels were expected, the areas near the intake shafts where the work shops are located in both mines, and the area where an underground waste disposal is operated in both mines. In addition the personnel working in both of the first two areas ("mobile repair" have been separately treated as well.

Tables 5, 6, 7, 8, 9 and 10 give the results for the indicated groups. The results of campaign A1 have not been included here because they are already published and are restricted only to particulate compounds. The data are given in the following format. The arithmetic mean of the respective set is followed by its geometric mean, the standard deviation and the highest measured result which includes at least 95% of all results. 

This excludes extremely high and possibly outlying results. For sets of ten or fewer results it is identical with the maximum result. This convention is used because at least in Germany the 95 percentile of measurements is relevant for threshold limit discussions.
Tables 5, 6, 7, 8, 9 and 10 (Respirable dust, inhalable dust, diesel particulate matter ("dpm"), CO, NO, NO2).

At a first glance the provisional grouping into the four mine regions does display in the data set. Therefore the regions are outlined with a differently shaded background to highlight the intrinsically different exposure situation. For the epidemiological study a high correlation between the six analytes displayed here was found. It was impossible to differentiate between selectively high exposure for example to nitrogen dioxide and particulate components. High exposure to one of the components usually means high exposure to the others as well. This is in line with expectation and therefore plausible.

As the results of this study were also meant to produce valid data for a threshold discussion the complete set of results was also compiled for each mine.
A simple two-sided t test showed no difference between the three different campaigns for each component. Therefore Table 11 gives the complete summarized exposure data for the three campaigns which can be taken as representative for the whole potash mining industry in Germany.

Short time exposure data

While for shift exposure the time period of 8 h as a base for averaging the concentrations has been generally accepted, the situation for short time exposure is quite different. Nevertheless, short time exposure is a very significant situation with respect to the health of workers, especially for irritant agents. In the campaigns described above it was decided at an early stage to monitor the gaseous components in a way that would allow for significant information to be obtained in this respect. 

Earlier IGF had developed a novel tool for the description of short time exposure situations in cases where it is not known beforehand at exactly which time period during the shift the periods of high exposures are observed. The tool is called "workplace exposure profile (WEP)" .

For this model the direct reading results (in these cases one-minute-averages) are transferred into a spreadsheet. After the measurements a 15-min sliding average filter is run over the data set to identify the one 15-min-period of highest exposure. The average concentration for this period is recorded and removed from the data set. Then the procedure is repeated. One receives a decreasing set of 15 min average concentrations for the shift, which is guaranteed to identify the peak exposure periods without knowing the relevant moments of high exposure beforehand.

An example of the procedure is shown in Figs. 4 and 5.
Figure 4 shows the typical plot of the carbon monoxide concentration of a loader driver's exposure. The concentration is increasing after the driver has taken up a load of salt and drives it in line with the ventilation air (the relative velocity of the vehicle and the ventilation air are not very different). Then he dumps the salt and drives back into the direction of the fresh ventilation air in lower exposure concentrations. Each driving cycle can so be identified (together with the tea break). Depending on the direction of the fresh air flow the periods of high and low exposure may of course change respectively.

Figure 5 demonstrates which kind of information can be generated by the WEP process. It uses the example of Fig. 4.
A wealth of short time exposure data was received by this process. For the purpose of this study only the highest short time exposure value for the three gaseous components during the shifts which gave rise to the shift-data presented before are reported here. Further evaluation of the WEP data is planned and will be reported elsewhere. For example, in the case of the loader driver as of Fig.4 a short time value of 7.61 ppm would be transferred to the database (in this case Table 12). 

Tables 12, 13 and 14 give the results for the maximum short time exposure values for the components CO, NO and NO2. The respective numbers of measurements are similar to the ones given in Table 4, but not completely identical because of practical reasons.
No difference was found for the three campaigns in the case of short time maximum exposure and therefore only the combined data are given in Table 15 as well.

