Appl.Occup.Environ.Hyg.12(12) December 1997
Diesel particulate matter (DPM) in workplaces in Germany
Dirk Dahmann and Hans-Dieter Bauer
Institut für Gefahrstoff-Forschung, Bochum, Federal Repuplic of Germany
Diesel particulate matter (DPM) is regarded as carcinogenic in Germany, with threshold limit values for
underground noncoal mining of 0.6 mg/m³ and for surface workplaces of 0.2 mg/m³. The current German
practice for workplace surveillance is described, and sam-pling and measurement procedures are
discussed. Currently, diesel soot is collected on quartz fiber filters using the respirable dust fraction and
oxidized in an oxygen atmosphere to yield carbon dioxide. This component is analyzed as a direct measure
of DPM by applying coulometry as a detection method. A thermal desorption/decomposition step is used for
analytical determination of elemental carbon as a future measure of DPM exposure. Coulometry is then
applied as a detection technique as well. By using this method, lower detection limits of about 5 µg carbon
per filter may be obtained. The procedure has been used in a variety of workplaces in Germany. The results
are presented in detail. Surface workplace exposures are generally below 0.15 mg/m³, but those in
underground salt and potash mines are generally higher. In these workplaces a further differentiation between different types of workplaces may be performed. Where levels of exposure in repair shops are also
generally below 0.15 mg/m³, the so-called winning range of the mines, characterized by intense operation of
loading and transportation machinery, shows concentrations below 0.3 mg/m³. The change in analyte from
total carbon to elemental carbon in connection with the analytical procedure has occured in Germany in
early 1997.
Introduction
Diesel engines are in widespread use throughout the mining industry in Germany. In salt and potash mining
they have been the most important production factor since their introduction in the early 1960s. Diesel
particulate matter (DPM) has been classified as probably carcinogenic by the International Agency for
Research on Cancer (IARC).(1) The U.S. National Institute of Occupational Safety and Health (NIOSH) has
recommended that it be considered as a possible occupational carcinogen.(2) In Germany, the discussion of
the potential health effects of DPM led to its official classification as carcinogen in 1990 (3) and to the
publication of a corresponding threshold limit value (Technische Richtkonzentration [TRK])(4) 1 year later.
At present, DPM still causes some problems. First of all, very few quantitative exposure data exist.
Additionally, though medical scientists seem to agree that DPM is carcinogenic, the true nature of the
mechanism causing cancer is still under debate.(5)(6)
This article describes the current approach to DPM monitoring, gives a short description of the sampling and
measurement techniques used in Germany for its determination, and presents recent results in surface and
underground workplaces obtained during typical compliance measurements in this country.
Current German Approach
Though in the international scientific community many questions about the true carcinogenic effect, the
relevant component(s) for measurement (depending on that effect), and the sampling/measurement strategy
in general remain unanswered, the situation is quite clear and instructions for occupational hygienists are
very specific for workplaces in Germany. German regulatory bodies have taken a very pragmatic approach.
They started with the assumption that an analyte was needed to correlate high diesel exhaust levels and
personal exposure levels of workers. It had to show as little interference from extradiesel sources as
possible. It had to be measurable under high quality standards and with as few demands on manpower and
technical equipment as possible. It had to provide exposure data as quickly and reliably as possible for
compliance purposes, and allow quick, decisive action to reduce exposure levels. The result was the TRK
value, which is still valid. The analyte was called diesel engine emissions, and it is defined via the sampling
and measurement technique to be used. It should be determined by coulometric analysis of total carbon
(TC) in the respirable dust fraction.(7) Like all other TRK values, it is strictly technically based. For general
workplaces, the limit was set at 200 µg/m³ after an extensive survey of the state of the art of exposure
reduction. In workplaces of underground noncoal mines, this low value may not be complied with, though
fairly elaborate and expensive measures are taken by the respective industry—for example, the use of
specific low-emission engines, the implementation of extensive maintenance protocols, and the application
of special diesel power-related minimum ventilation requirements. A threshold limit of 600 µg/m³ was fixed
for these workplaces. The measurement and sampling technique is described in the literature.(8,9) Eighthour time-weighted average values are compared to the TRK. Sampling of the respirable dust fraction is
performed on pretreated glass fiber filters or, preferably, quartz fiber filters. The samples are basically
burned in an oxygen atmosphere to yield carbon dioxide, which, in turn, is determined coulometrically to
provide a direct measure of the oxidizable carbon on the filter. This carbon is referred to as total carbon by
the official method. As the chosen analytical method is not applicable in these types of workplaces, there are
no measurements, no determinations of the state of the art, and finally, no threshold limits in German coal
mines. German legislation usually fixes an additional threshold for the mandatory triggering of occupational
medical examinations called Auslöseschwelle. That value was set to 100 µg/m³ for all types of
workplaces(10)
Figure 1. Combustion unit of the coulometric device.
In the very near future, a change is to be expected in the situation just described. Diesel particles consist of
a core of solid elemental carbon; a variety of organic compounds resulting from residual fuel or lubricants,
as well as incomplete combustion products; and a variety inorganic compounds and materials like sulfates
from sulfurous constituents of the fuel or abrasion products. The analyte, total carbon, will reflect all carbon-
containing materials in the filter sample that yield carbon dioxide under the high temperatures of the
combustion process in an oxygen atmosphere. This includes some inorganic compounds like certain
naturally occurring carbonates and all volatile organic compounds which might have been introduced
externally by adsorption to the excellent sampling substrate soot during sampling. While the interference
from inorganic carbonates may be easily eliminated by an acid treatment process developed in our
institute,(8) higher amounts of volatile organics from nondiesel sources will lead to a positive bias in the
current TC determination process. Consequently, the TRK value was revised in 1996, the main change
being the redefinition of the analyte to elemental carbon (EC). EC is basically determined by a combustion
process identical to the one described above. However, it is preceded by a thermal
desorption/decomposition step that should be able to remove volatile organic carbon (OC) compounds from
the sample (see the next section). The current version of the official German method (8) describes all these
steps in detail. An English-language version is included in Reference 9. The special aspects of the method
will be discussed in the next section. The new EC-TRK values are expected to be lower than the old ones,
but they will still differentiate between underground noncoal mining and all other workplaces. The exception
is coal mining, where the new method is not applicable either.
It is also worth noting that the term elemental carbon and organic carbon, used in the official method and in
fact in all of the literature, are not really correct. Nor is elemental carbon pure carbon in the chemical sense
(it also contains the compounds that were simply not volatile at the temperature of the thermal
desorption/decomposition step), nor is all the organically bonded carbon present in the sample determined
in the organic carbon fraction. The analytical procedure only defines the two analytes.
Sampling and Analytical Determination
For sampling according to the official method (ZH 1/120.44), either personal samplers or area samplers
(static samplers) are used. Personal sampling is generally preferred and is performed using cyclone
preseparators (Casella) with a flow rate of 2 L/min. The filter diameter is 37 mm. These samplers are used
with no problems in salt and potash mines. Due to possible interference by droplet aerosols in typical repair
shop environments, ZH 1/120.44 no longer recommends their use in these types of workplaces. For area
samplers, only a device with a horizontal elutriator as a preseparator is used (MPG II; Dr.-Ing. Georg
Wazau, Berlin). It has a flow rate of 46.5 L/min and filters 47 mm in diameter. As the absolute lower
detection limit is nearly identical (about 5 µg carbon per filter) in both cases, the relatively lower detection
limit in area sampling is better by about a factor of 20. The loaded filters are transferred to the laboratory in
the presence of a drying agent (silica gel) because the hygroscopicity of the salt matrix could otherwise lead
to loss of sample mass by "running off" the filter.
Figure 2. Organic carbon as part of total carbon: correlation between OC and TC.
Each loaded filter is then treated with diluted hydrochloric acid to remove "inorganic carbon."(1) By this
procedure, interference from carbonate matrices is removed and the filter blanks are lowered, which, in turn,
produces less blank scattering and lower detection limits. Figure 1 shows the principal construction of the
combustion unit of the coulometric device. The -whole apparatus is now commercially available (Ströhlein,
Kaarst), but it can be constructed in a normal analytical laboratory from existing material. It should also be
noted that different methods of detection of carbon dioxide may be used if they are as sensitive and reliable
as coulometry. This has been proved by interlaboratory tests for infrared detection methods, for
example.(11)
The parameters for the determination of EC in filter samples are as follows: The loaded filters are treated
under nitrogen at a temperature of 500°C for 8 minutes. After this period, the gas supply is switched to
oxygen (without removal of the sample from the quartz tube) after being cooled to room temperature and
heated to 300°C within 0.5 minute and subsequently to 800°C within 4.5 minutes. It should be noted that an
OC determination during the first step of the process is possible by catalytic conversion of the volatile
organic compounds to carbon dioxide. Thus, a comparison of the sum of OC and EC to the corresponding
value of TC is possible on halved filters. This method has obtained recovery rates of about 100%.(9)
It is also possible to plot the OC or EC values versus the corresponding TC values to find the typical
percentage of these fractions in the whole soot sample. A typical plot for about 200 samples from
underground salt and potash mines is shown in Figure 2. The resulting linear regression, with an excellent
correlation coefficient of about 0.9, results in an OC percentage in that set of samples of about 38%. It
should be noted, however, that depending on the nature of the workplace and the engine(s) operated there,
extremely different percentages of OC can be obtained. A constant recalculation factor does not exist.
Results of Measurements
In a large number of measurements, the current TC measurement procedure and the EC version have been
applied in different workplaces of underground mines and in a wide variety of nonmining workplaces. The
EC results are shown in Figures 3 and 4. As can be seen in Figure 3, which contains a simple frequency
distribution of about 340 EC measurement results, the vast majority of measurements show results below
100 µg/m³, with a significant proportion below 50 µg/m³. Figure 4 shows the frequency distribution of a set of
285 recent measurement results from underground workplaces in salt and potash mines. No differentiation
between different types of workplaces has been made in this figure. As demonstrated, the exposure values
are significantly higher than those of workplaces on the surface. An interesting question is whether there are
different levels of exposure identifiable in underground mines. From the number and power of diesel engines
used in underground salt and potash mines, two different location types can be identified: the so-called
winning range, where powerful loading and hauling machines are operated intensely, and the repair shops
of the mines situated near the intake shafts with relatively uncontaminated ventilation in the mines where we
performed the measurements. Figure 5 shows a frequency distribution of about 80 measurement results
from these repair shops. The majority of results lie below 150 µ/m³. Figure 6 shows the frequency
distribution of about 210 results from the winning range, 10% of which are above 300 µg/m³.
Figure 3. Frequency distribution of results of EC measurements: nonmining activities
Figure 4. Frequency distibution of results of EC underground measurements (unspecified).
Figure 5. Frequency distribution of results of EC measurements: underground repair schops
Figure 6. Frequency distribution of results of EC measurements: underground (winning range
Conclusions
The coulometric measurement of EC after thermal desorption/decomposition of OC can be regarded as
routine in German workplaces, with the notable exception of coal mining. In comparison with nonmining
workplaces, higher results are obtained in underground salt and potash mines, especially in the winning
range. Though from a purely scientific point of view the discussion about what analyte should preferably be
used as a measure of DPM exposure is still not closed, the currently used German method is well
characterized and fulfills the stipulated requirements.
References
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Exhausts and Some Ni-troarenes. International Agency for Research on Cancer, World Health Organization:
Lyon (1989).
2. Current Intelligence Bulletin No. 50: Carcinogenic Effects of Exposure to Diesel Exhaust. DHHS (NIOSH)
Pub. No. 88-116. NIOSH, Cincinnati, OH (1988).
3. MAK-Werte 1990. Maximale Arbeitsplatzkonzentrationen und Biologische Arbeitsplatztoleranzwerte
(TRGS 900), Bundesarbeitsblatt, p. 35. Verlag W. Kohlhammer, Stuttgart (1990).
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(TRGS 900), Bundesarbeitsblatt, p. 36. Verlag W. Kohlhammer, Stuttgart (1991).
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Inhalation Toxicol., 8(Suppl):51 (1996).
6. Mauderly,J.L.; Snipes, M.B.; Barr, E.B.; et al.: Pulmonary Tox-icity of Inhaled Diesel Exhaust and Carbon
Black in Chronically Exposed Rats. Part I, Neoplastic and Nonneoplastic Lung Lesions. Research Rep. No.
68. Health Effects Institute, Cambridge, MA (1994).
7. EN 481 Workplace Atmospheres: Size Fraction Definitions for Measurement of Airborne Particles. CEN
European Committee for Standardization, Brussels (1993).
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Dieselmotor-Emissionen in Arbeitsbereichen. Carl Heymanns Verlag KG, Cologne (1995).
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Germany. Occup. Hyg. 3. (1996).
10. Technische Regeln für Gefahrstoffe, TRGS 554: Dieselmotoremissionen (DME). Carl Heymanns Verlag,
Cologne (1993).
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