INHALATION AND RETENTION OF THORIUM DUSTS BY MINERAL SANDS WORKERS
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Ann. occup. Hyg., Vol. 41, Supplement 1, pp. 92-98, 1997
© 1997 British Occupational Hygiene Society
Published by Elsevier Science Ltd. All rights reserved
Printed in Great Britain
0003-4878/97 $17.00 + 0.00
Inhaled Particles VIII
PII: S0003-^878(96)00134-2
INHALATION AND RETENTION OF THORIUM DUSTS
BY MINERAL SANDS WORKERS
G. S. Hewson
Department of Minerals and Energy, 100 Plain Street, East Perth, WA 6004, Australia
INTRODUCTION
Heavy mineral sands consisting of ilmenite, rutile, zircon and monazite, have been
mined and processed in Western Australia since 1956. The industry is a significant
one; in 1995 it mined 37.4 million tonnes of sand and processed 2.6 million tonnes
of heavy mineral concentrate to produce 2.1 million tonnes of the individual
mineral sands, at a value of $550 million. A specific occupational health concern
associated with this industry is radiation exposure arising out of the presence of
thorium and, to a lesser extent, uranium with all the heavy minerals. Monazite, a
rare earth phosphate, is radiologically the most significant mineral, containing
typically between 5 and 7% thorium and 0.1 and 0.3% uranium. Although
monazite is a low volume product, comprising only about 0.5% of total mineral
sand production, it tends to preferentially concentrate in airborne dust because it is
softer than the titanium and zirconium bearing minerals. This is of particular
concern during the processing of mineral sands because the minerals are subjected
to a variety of vigorous physical treatment processes, such as screening and
magnetic, electrostatic and gravity separation. Without the application of appropri-
ate dust control technology, considerable airborne dust (and consequently radio-
activity) concentrations may be experienced by workers who operate and maintain
the separation plant.
While the presence of radioactivity in this industry has been long appreciated,
early protective and regulatory measures were focussed on control of external
radiation exposure in circuits where monazite was being concentrated and bagged.
Intake of radioactive dust was only recognised as a potentially significant source of
exposure in the early 1980s, following national review and acceptance of ICRP
Publication 30 (ICRP, 1979). This resulted in derived air concentration (DAC)
values for thorium an order of magnitude lower than those previously applied to
the industry (Hewson and Terry, 1995).
The issue of intake of radioactive dust prior to the mid 1980s is important
because there are many long-term workers in the industry and it is well recognised
that each of the five dry separation plants were very dusty. In addition, early work
practices, such as use of compressed air to blow down equipment, and sweeping,
banging and brushing of floors and plant equipment, coupled with limited use of
respiratory protective equipment, exacerbated the potential for dust inhalation by
workers operating and maintaining the separation plants. Industry management
92Inhalation and retention of thorium dusts 93
and employees, through a concern about the likely long-term health implications of
exposure to radioactive dust, have both been anxious to ascertain the extent of past
exposure and have supported a range of bioassay research endeavours.
This paper examines the inhalation exposure estimates obtained from bioassay
studies and compares them with estimates from personal air sampling and a
retrospective assessment of intake of radioactivity. One objective of such a
comparison is to assess the veracity of recommended biokinetic models. Another is
to investigate the feasibility of using an alternative, cost-effective exposure
assessment technique, such as exhaled thoron, as a routine monitoring tool.
METHODS
As thorium oxides have a very long half-time of clearance from the lung,
bioassay measurements, apart from analyses of faeces, will reflect long-term
chronic inhalation. To relate the results from bioassay to air sampling requires
knowledge of annual average daily intake over the exposure period.
For each year since 1977 an estimate of the average airborne alpha activity
concentration across the industry was made as follows:
X = (6).(20).(0.06).Dg.SATh.MHMc
where X is the average alpha activity, Bq m~3
6 is the number of alpha particles emitted per decay of 232Th in ore dust
20 is the concentration factor for monazite in airborne dust (Hewson
and Terry, 1995)
0.06 is the average thorium content of monazite
Dg is the arithmetic mean gravimetric dust concentration, mg m~3
SATh is the specific activity of 232Th, 4.1 Bq mg""1
M H MC is the average monazite content of heavy mineral concentrate.
The mean gravimetric dust concentration for separation plant workers was
obtained from the Department's atmospheric contaminant exposure database,
known as CONTAM. The number of personal air samples has varied from 165 in
1977 to in excess of 2581 in 1989 and the number of samples per "designated"
radiation worker is now typically in the range of 6-10 per year. Details of the dust
sampling measurements are summarised in Hewson and Terry (1995).
The monazite content of heavy mineral concentrate feedstock (M HMC) to the dry
separation plant was determined by reviewing historical mineral sands production
data reported to the Department of Minerals and Energy by the mineral sands
companies.
For the years 1975, 1976 and the period prior to 1975, it was assumed that the
mean personal dust concentration was 15 mg m~3, based on measurements in the
late 1970s. This assumption maybe overly conservative as the two new plants which
commenced operations in the mid to late 1970s were larger than the three existing
plants and were considered very dusty by the inspectorate. An average M H MC
content prior to 1975 of 0.40% was calculated by examining total production figures
to that time.
The estimates derived from this analysis were used to assign pre-1986 values for94 G. S. Hewson
the annual intake of radioactive dust (and hence internal radiation dose) for those
workers who had participated in bioassay studies. This required knowledge of the
employment history of the individual and average annual working hours; informa-
tion which was readily available from company records. The intakes were
subsequently converted to committed (over 50 years) effective doses using conver-
sion factors derived from the new ICRP 66 lung model (ICRP, 1994). Procedural
details are described by Hewson and Terry (1995) and Terry and Hewson (1994).
Results of recent bioassay studies were reviewed, including: (1) the concentra-
tion of thorium in blood serum and urine (Hewson and Fardy, 1993); (2) the
concentration of thorium in faeces (Terry et al., 1995); (3) the concentration of
thoron exhaled in breath; and (4) the amount of thorium decay chain radionuclides
deposited in lungs and other organs (Terry and Hewson, 1994, 1995).
The predicted concentration of thorium in urine, blood serum and lungs was
calculated using the ICRP 66 lung model (ICRP, 1994) and ICRP 30 biokinetic
model for thorium (ICRP, 1979) and assuming workers were chronically exposed to
relatively insoluble thorium ore dust with a median particle size (or activity median
aerodynamic diameter—AMAD) of 10 urn.
RESULTS
At the five individual separation plant sites the average monazite content was
found to vary between less than 0.1% and about 2.0%. The average monazite
content across the industry has varied from 0.11% in 1994 to 1.13% in 1983, with
the latter figure corresponding to annual monazite production of 15 606 tonnes.
The value for 1994 is biased low as the industry substantially curtailed monazite
production in 1994 as a result of a drastic fall off in demand. Thus, the above
retrospective analysis cannot be applied to the 1994 and 1995 dust data.
CONTAM records show that mean dust concentrations in the dry separation
plants have progressively declined from 16.8 mg m~3 in 1977 to 1.4 mg m~3 in 1991,
although a substantial level of non-compliance with the general dust standard of 10
mg m~3 was evident up to the mid 1980s. The average alpha activity derived from
the retrospective assessment procedure for the period < 1975-1993 is shown in Fig.
1, together with the average monazite content and the actual average alpha activity,
routinely determined by the mineral sands industry since 1986. It can be seen that
there is reasonably good correlation between the actual and estimated alpha
activities since 1986, which provides confidence in the reasonableness of the
estimates in the period before 1986. The increased measured alpha activity in 1989
corresponds to a period when large scale engineering control work was being
undertaken in some of the plants and it is assumed that the additional maintenance
activity was resulting in increased resuspension of settled dust.
Depending on the site (or monazite content of feed material), pre-1986 doses to
separation plant workers were estimated to vary between 9 and 90 mSv per annum
(cf. current ICRP recommended dose limit of average 20 mSv per annum). For
other workers, annual doses of one-tenth or one-third these values were chosen,
depending on nature of task and extent of dust exposure (Hewson and Terry,
1995). It is emphasised that the estimates relate to a "typical" plant producing
about 2000-3000 tonnes monazite per annum or with about 0.5-0.7% monazite inInhalation and retention of thorium dusts 95
•*- Estimated Alpha Activity
—% Monazite
• Actual Alpha Activity
9
CD
1
Q.
' •
N, • • \ II
0 ! 1 i 1 1 1 1 1 1 1 1 ! 1 1 1 1 1'96 G. S. Hewson
Urine - n=34, 1-20y
Blood - n=25,1-20 y
Lung - n = 19, 6-35 y
TIB - n =62,4.5-35 y
TIB - n =47, 7-35 y
Faeces - n=2
PAS -
0.5 1 1.5 2.5
-1
Thorium Intake (Bq d
232
Fig. 2. Estimated intake of Th by mineral sands workers using various bioassay techniques, normalised
against personal air sampling measurements. The longer thoron in breath (TIB) bar refers to 62 workers
with employment between 4.5 and 35 years, and the shorter to a subset of 47 workers with employment
longer than 7 years.
from the start of intake (and is in equilibrium with daily inhalation rate) and
reaches negligible amounts in about 7 days from the cessation of intake.
The thoron in breath results show that, for those employees above the minimum
detection limit (MDL), the thorium lung burdens are significantly higher than
predicted on the basis of applying intake data (from personal air sampling data) to
the new ICRP 66 lung model (ICRP, 1994). However, the results are biased by the
large number of workers (81 out of 145 workers tested) below the MDL of the
technique. Of interest is that 75% of short-term (employment duration < 7 years)
workers are below the MDL and that the average intake of short-term and
long-term workers is 0.15 and 0.54 Bq d"1 232Th, respectively. Thus, it is evident
that there has been improvement in the exposure status of mineral sands workers
over the last 7-10 years.
The breath measurements revealed that 25 workers (17%), including three short
term workers were estimated as receiving an average annual dose of 20 mSv or
more over their total employment period. One of the highest lung burdens was
measured on a retiree, who had been out of the industry for 10 years and other
substantial burdens were measured on long-term workers who had not been
working in dusty jobs for about 10 years. These results underline: (1) that
considerable intakes of radioactive dust were incurred by some workers in the
1960s and 1970s; and (2) that once thorium ore dust lodges in the lungs it is avidly
retained.
The elevated lung burdens recorded against the three short-term workers are of
concern, as their predicted intake is significantly above their work category
colleagues. Differences in an individual's results arise through personal factors such
as work habits, use or non-use of a respirator, mode (i.e. mouth or nose) and rateInhalation and retention of thorium dusts 97
of breathing and smoking habit. Such factors may result in the actual exposure
being either higher or lower than estimated from personal air sampling. The
discrepancy may also be due to a combination of incorrect intake data, inappropri-
ate metabolic data or incorrect assumptions about dust characteristics, such as
particle size. As the metabolic data reflect the latest scientific knowledge it is more
likely that the air sampling data is not providing representative coverage of high
exposure tasks. However, the discrepancy could be explained if the AMAD was 5
urn (not 10 urn) or if the worker was a mouth breather.
The range of ratios of predicted thorium lung burden (from air sampling) to
estimated burden (from breath measurements) varied from 0.43 to 13.5, confirming
that there is substantial inter-worker variability in intake, even for workers with
similar employment periods in the same work category. It is interesting to note,
however, that the average ratio of measured to predicted lung burdens for long
term workers is within a factor of 2, which provides some confidence in the
reasonableness of the retrospective assessment. Many health physicists might
consider this as good agreement.
The results indicate that thoron in breath measurements provide data that are
complementary to air sampling data and are able to identify workers whose work
habits require closer investigation. One disadvantage of the breath measurements
is that the MDL is relatively high and therefore the technique is primarily restricted
to long-term workers or short-term workers with significant intake. The Depart-
ment of Minerals and Energy has recently developed and commissioned an
alternative breath measurement device, utilising an electrostatic collection cham-
ber and this has resulted in a 50% improvement in the MDL. The Department has
provided this device to industry and it is currently being trialled on selected workers
using a measurement strategy based on tests at the start, middle and end of the
year. Preliminary results indicate that the technique is sufficiently sensitive and cost
effective and could replace the comprehensive and expensive regime of personal air
sampling that is currently used by the industry. The major advantage of the
technique over air sampling is that it identifies individual workers at elevated risk
and enables suitable investigation and intervention strategies to be implemented.
While the air sampling regime is considered comprehensive, in comparison to
sampling conducted in other industry sectors, it should be recognised that the
number of air samples on an individual worker represents only about 5% of the
worker's annual working shifts.
CONCLUSIONS
Reasonably good correlation between estimates of thorium intake derived from
air sampling and retrospective assessment and estimates from bioassay measure-
ments was found for long-term workers. Bioassay measurements on recent
workers, although higher than expected, confirm the increasing improvement in
industrial hygiene conditions and work practices since the early 1980s. Periodic
thoron-in-breath measurements could replace the comprehensive regime of person-
al air sampling which currently exists in the industry, thereby providing a relatively
sensitive and cost effective diagnostic monitoring tool.
Continued study of the mineral sands workforce could yield important informa-98 G. S. Hewson
tion about the inhalation and retention of insoluble dust particles. Mineral sands
dust has the advantage of a "radioactive signature", which allows direct in vivo
measurements of the dust in the lungs and other organs and the measurement of
thoron exhaled in breath. Periodic measurements of workers who are no longer
employed in dusty tasks will enable an assessment of the clearance of dust from the
lungs.
REFERENCES
Hewson, G. S. and Fardy, J. J. (1993) Thorium metabolism and bioassay of mineral sands workers.
Health Phys. 64, 147-156.
Hewson, G. S. and Terry, K. W. (1995) Retrospective assessment of radioactivity inhaled by mineral
sands workers. Radiat. Prot. Dosim. 59, 291-298.
International Commission on Radiological Protection (1979) Annual limits on intake for workers. ICRP
Publication 30, Pergamon Press, Oxford.
International Commission on Radiological Protection (1994) Human respiratory tract model for
radiological protection. ICRP Publication 66, Pergamon Press, Oxford.
Terry, K. W., Hewson, G. S. and Meunier, G. M. (1995) Thorium excretion in faeces by mineral sands
workers. Health Phys. 68, 105-109.
Terry, K. W. and Hewson, G. S. (1995) Thorium lung burdens of mineral sands workers. Health Phvs.
69, 233-242.
Terry, K. W. and Hewson, G. S. (1994) Thorium lung burdens of workers in the mineral sands industry.
Report of research commissioned by the Titanium Minerals Committee of the Chamber of Mines
and Energy. Department of Minerals and Energy, Perth Western Australia.You can also read
