Original Article
Industrial Health 2003, 41, 273–278
Thorium-232 Exposure during Tungsten Inert Gas Arc
Welding and Electrode Sharpening
Hiroyuki SAITO1*, Naomi HISANAGA1, Yukiko OKADA2, Shoji HIRAI2 and Heihachiro ARITO1
1
2
National Institute of Industrial Health, 6-21-1, Nagao, Tama-ku, Kawasaki, 214-8585 Japan
Department of Energy Science and Engineering, Faculty of Engineering, Musashi Institute of Technology, 1-28-1,
Tamazutsumi, Setagaya-ku, Tokyo, 158-8557 Japan
Received July 26, 2001 and accepted May 28, 2003
Abstract: To assess the exposure of welders to thorium-232 (232Th) during tungsten inert gas arc
(TIG) welding, airborne concentrations of 232Th in the breathing zone of the welder and background
levels were measured. The radioactive concentrations were 1.11 × 10–2 Bq/m3 during TIG welding of
aluminum (TIG/Al), 1.78 × 10–4 Bq/m3 during TIG welding of stainless steel (TIG/SS), and 1.93 × 10–
1
Bq/m3 during electrode sharpening, with 5.82 × 10–5 Bq/m3 background concentration. Although
the annual intake of 232Th estimated using these values did not exceed the annual limit intake (ALI,
1.6 × 102 Bq), we recommend reducing 232Th exposure by substituting thoriated electrodes with a
thorium-free electrodes, setting up local ventilation systems, and by using respiratory protective
equipment. It is also necessary to inform workers that thoriated tungsten electrodes contain
radioactive material.
Key words: TIG welding, Tungsten inert gas arc welding, Thorium, sharpening, Tungsten electrode,
Neutron activation analysis
Introduction
Thoriated tungsten electrodes have been used in tungsten
inert gas arc (TIG) welding since around 19511). Thoriated
tungsten electrodes are widely used because of certain
advantages over pure tungsten electrodes, such as easier arc
starting, greater arc stability and less weld metal
contamination in direct current welding1). As thoriated
tungsten electrodes are gradually consumed during welding,
welders need to sharpen the tip of the electrodes at regular
time-intervals on a grinder. There is a possible health risk
for the welders who are exposed to thorium-containing dust
during TIG welding and electrode sharpening operations.
Thorium-232 (232Th) is a major isotope of Th (abundance
ratio: approximately 100%, and is an alpha emitter with a
decay half-life of 1.4 × 1010 years2). Its specific radioactivity
is 4.07 × 103 Bq/g (1.1 × 10–1 µCi/g). In Japan, the derived
air concentration (DAC) and the annual limit intake (ALI)
*To whom correspondence should be addressed.
for inhalation exposure to 232Th are 3 × 10–2 Bq/m3 and 1.6
× 102 Bq, respectively3). As 232Th is an alpha emitter, its
effect on tissue by internal exposure is greater than betaand gamma-emitters4). In the past, liver cancer and leukemia
developed in a number of patients examined by angiography
with thorium oxide as an X-ray contrast medium5).
Despite the wide spread use of TIG welding, until recently
there have been only a small number of researchers who
have paid attention to the health risks of thorium exposure
associated with TIG welding. Breslin and Harris reported
that the radioactivity released from welding fumes during
TIG welding was less than 0.9 distegrations/min/m 3
(equivalent to 1.5 × 10–2 Bq/m3), which was lower than the
maximum allowable concentration in the 1950’s (70
disintegrations/min/m3 = 1.17 Bq/m3)6). Saito and Ishida
measured the radioactivity of thoriated tungsten electrodes
by gamma-ray spectrometry in terms of both dose equivalent
and surface pollution concentration 7). Although the
radioactivity of welding electrodes was lower than that
stipulated for radioactive materials in Japanese regulations8),
274
they pointed out the necessity of measuring airborne Th
concentrations during welding. In previous reports, airborne
232
Th concentration during electrode sharpening were
reported by Vinzents9), Crim10) and Jankovic11), which were
< 0.3 to 53.3 times the DAC provided for in the Code of
Federal Regulations12). Airborne 232Th concentration during
welding were reported by Jankovic11) and Ludwig13) as 0.001
to 0.87 times the DAC. Thus, the documented levels of
232
Th during these operations are not consistent.
The aim of the present study was to measure the airborne
232
Th radioactivity concentrations in the breathing zone of
TIG welders during welding and electrode sharpening, as
well as in the workplace air of the welding operations. The
annual 232Th intake was estimated in order to assess 232Th
exposure to workers during TIG welding.
Methods
TIG welding and electrode sharpening operations,
simulating regular work and environments, were performed
at the welding shop of a factory manufacturing chemical
plant in Kawasaki-city in Japan. The operations were carried
out under windless conditions without local exhaust
ventilation in a capacious building (90 m × 140 m × height
15 m). The welder was outfitted with protective clothing,
including a hood, gloves, dust respirator, and welding helmet.
Welding
Airborne samples were collected in the breathing zone
during two kinds of welding: TIG welding of aluminum (TIG/
Al) using a 2% thoriated tungsten electrode (φ 3.2 mm) under
conditions of 15 L/min flow rate of shielding argon gas and
a welding current of 110 amperes AC; and TIG welding of
stainless steel (TIG/SS) under the same conditions, except
for a welding current of 115 amperes DC. Both operations
were performed continuously for one hour.
Electrode sharpening
The welder manually sharpened the 2% thoriated tungsten
electrode (φ 3.2 mm) on a wheel bench grinder (G13, Hitachi
Co., Ltd., Japan), without local exhaust ventilation. The
sharpening operation was performed continuously for 10
min, at a point 5 m from the welding area.
Sampling
Respirable particles (smaller than 7.07 µm) and nonrespirable particles (larger than 7.07 µm) near the breathing
zone were separately collected using the “Roken T.RSampler” (Sibata Scientific Technology, Japan)14). Non-
H SAITO et al.
respirable particles were collected on an impact plate coated
with silicone grease by inertial impact, and respirable particles
were collected on a Teflon-binding glass fiber filter (T60A20,
Palflex Co., Ltd., USA, φ 35 mm). The sampling points
during TIG welding and electrode sharpening operations
were less than 20 cm from the operator’s nose. The massconcentration of total particles was calculated from the massconcentrations of respirable and non-respirable particles.
A background air sample was collected in the same factory
using a Teflon-binding glass fiber filter (T60A20, φ 110 mm)
attached to a high-volume aerosol-sampling device (HVC500, Sibata Scientific Technology, Japan) with a flow rate
of 500 L/min. Background sampling was performed
continuously for two hours at two points over 10 m from
the welding and electrode sharpening areas. The dust
generated during the electrode sharpening operation was
collected as sediment on a paper laid on a desk to the right
of the grinder.
Neutron activation analysis
The 232Th in the airborne dust samples collected on the
filters and in the sediment dust collected on the paper were
analyzed by neutron activation analysis. The samples were
placed in a double polyethylene bag and inserted into an
irradiation capsule, then irradiated in a nuclear reactor
(TRIGA-II, Rikkyo University, Japan). 232Th in the irradiated
samples absorbed neutrons to become 233Th, and 233Th
decayed to 233Pa. After six hours of irradiation and nine
days of cooling, the gamma activity of 233Pa (312 keV) was
measured by gamma-ray spectrometry using a germanium
detector at the Musashi Institute of Technology. The weight
of 232Th (µg) was converted to the radioactivity of 232Th (Bq)
using the specific radioactivity (4.07 × 10–3 Bq/µg). The
radioactivity of total dust samples collected in the breathing
zone was calculated from the radioactivity of the filter sample
collected by the TR-sampler and the percentage of respirable
dust in total dust measured by the same sampler. The airborne
radioactive concentration (Bq/m3) was calculated with the
radioactivity of 232Th (Bq) and sampling volume (m3).
Time spent in welding work
The length of time that operators spend in TIG/Al and
TIG/SS welding and electrode sharpening, amongst other
kinds of work in a regular workday, was investigated via a
questionnaire on welders’ daily work assignments in the same
factory.
Industrial Health 2003, 41, 273–278
275
THORIUM-232 EXPOSURE DURING TIG WELDING
Table 1. Airborne mass-concentrations of dust during TIG welding and electrode sharpening,
and workshop background
Measurement
number
Respirable dust
(mg/m3)
Total dust
(mg/m3)
Ratio of respirable
dust to total dust (%)
1
1
1
2
0.725
0.358
0.600
–
0.992
0.483
2.85
0.175–0.210
73.1
74.1
21.1
–
TIG/Al welding a)
TIG/SS welding a)
Electrode sharpening a)
Background b)
a) Respirable dust and non-respirable dust were collected using a TR-sampler (Roken type) near the
breathing zone of workers. Concentration of total dust was calculated from concentrations of respirable
dust and non-respirable dust. b) Total dust was collected using a high-volume dust sampler.
Table 2. Radioactivity concentrations of 232Th and percentages in airborne dust during TIG welding and electrode sharpening,
and workshop background
Sampling
period
TIG/Al welding
TIG/SS welding
Electrode sharpening
Background
1 hour
1 hour
10 min
2 hour
Measuremen
number
1
1
1
2
Radioactivity concentration of 232Th (Bq/m3)
232
Total
Respirable
Non-respirable
Th percentage in
airborne dust (w/w %)
1.11 × 10–2
1.78 × 10–4
1.93 × 10–1
4.97 × 10–5
6.67 × 10–5
8.14 × 10–3
1.32 × 10–4
4.07 × 10–2
–
–
2.96 × 10–3
4.60 × 10–5
1.52 × 10–1
–
–
2.8 × 10–1
1.6 × 10–2
1.7
5.81 × 10–3
9.37 × 10–3
Results
Table 1 shows the airborne concentrations of total dust
and its respirable components during TIG/Al and TIG/SS
welding and electrode sharpening operations as well as the
background levels. The level of respirable dust was highest
for TIG/Al welding (0.725 mg/m3), followed by electrode
sharpening (0.600 mg/m3) and TIG/SS welding (0.358 mg/
m3). The levels of total dust during electrode sharpening
(2.85 mg/m3) were higher than during TIG/Al welding (0.992
mg/m3) and TIG/SS welding (0.483 mg/m3). The ratio of
respirable dust to total dust for both TIG welding operations
was relatively high (73.1–74.1%) compared to electrode
sharpening (21.1%). This indicates that less respirable dust
is contained in the dust produced during electrode sharpening.
The background concentrations of total dust were found to
vary from 0.175 to 0.210 mg/m3, with an arithmetic mean
of 0.193 mg/m3.
Table 2 shows the radioactivity concentrations of 232Th
and mass concentration of 232Th in airborne dust during TIG/
Al and TIG/SS welding and electrode sharpening, as well
as the background levels. The level of respirable 232Th was
highest for electrode sharpening (4.07 × 10–2 Bq/m3), followed
by TIG/Al welding (8.14 × 10–3 Bq/m3) and TIG/SS welding
(1.78 × 10–4 Bq/m3). The level of total 232Th, calculated
from the radioactive concentration of 232Th in the respirable
dust and the percentage of the respirable dust in the total
dust, was highest for electrode sharpening (1.93 × 10–1 Bq/
m3), followed by TIG/Al welding (1.11 × 10–2 Bq/m3) and
TIG/SS welding (1.78 × 10–4 Bq/m3). The background 232Th
radioactivity concentrations were found to vary from 4.97
× 10–5 to 6.67 × 10–5 Bq/m3, with an arithmetic mean of 5.82
× 10–5 Bq/m3.
The mass concentration of 232Th in airborne dust varied
from the highest in electrode sharpening (1.7%) to much
lower values during TIG/Al welding (0.28%), TIG/SS
welding (0.016%) and in the background (an arithmetic mean
of 0.0076%).
The mass-concentration of 232Th in the sediments collected
from the electrode sharpening workplace was 1.4%.
Discussion
The nominal percentage of ThO2 in the electrode is 1.7–
2.2% according to JIS Z 3233, equivalent to 1.5–1.9% Th15).
The observed percentage of 232Th in the sediment (1.4%)
approximates to the nominal value by JIS. Similarly, the
percentage of 232Th in the airborne dust collected on the filter
during electrode sharpening (1.7%) was almost equal to the
nominal content of Th in the electrode. These two points of
276
H SAITO et al.
Table 3. Summary of past reports of airborne radioactivity concentration of 232Th during electrode sharpening
Author
Kind of
grinder
Vinzents 9)
Crim
Jankovic 11)
Radioactivity concentration (Bq/m3)
Measurement
number
Total
Breathing zone a)
Respirable
Non-respirable
4.5 × 10–1 d)
1.15 d)
2
1.6 d)
Bench
Belt
a)
Breathing zone
Breathing zone a)
1
1
7.0 × 10
< 1 × 10–2 c)
< 1 × 10
< 1 × 10–2 c)
–
–
Bench, belt
Breathing zone b)
4
8.1 × 10–2 d)
–
–
Belt
10)
Measurement
position
–2
–2 c)
a) No description of detailed position. b) Near collar element of worker. c) Below detection limit. d) Arithmetic mean. –: Not
determined.
Table 4. Summary of past reports of airborne radioactivity concentration of 232Th during welding
Author
Jankovic
Current
11)
Ludwig 13)
Local ventilation
system
Measurement
position
DC
DC
DC
DC
None
None
Installed
Installed
Outside helmet
Inside helmet
Outside helmet
Inside helmet
DC
DC
AC
None
Installed
None
Inside helmet
Inside helmet
Inside helmet
Measurement
number
Radioactivity concentration (Bq/m3)
Total
Respirable Non-respirable
4
2
4
5
1.5 × 10
2.2 × 10–4
3.7 × 10–5
1.1 × 10–4
–
–
–
–
–
–
–
–
14
2
8
4.6 × 10–4
1.8 × 10–4
2.6 × 10–2
–
–
–
–
–
–
–3
–: Not determined.
evidence indicate that the airborne 232Th originated from
the electrode.
The radioactivity concentration of 232Th in the total dust
during electrode sharpening was 1.93 × 10–1 Bq/m3; higher
than that during both welding operations. Although this is
a spot measurement, this value is six times the DAC (3 ×
10–2 Bq/m3), stipulated in the pertinent ministerial ordinance3).
Compared with previously published values (Table 3), the
airborne radioactivity concentrations of 232Th obtained in
the present study were one tenth of those reported by
Vinzents9), and 2.4–2.8 times those reported values by Crim10)
and Jankovic11).
The airborne radioactivity concentration of 232Th in the
total dust sampled during TIG/Al welding was 1.11 × 10–2
Bq/m3. Although this was a spot measurement, this level
was one third of the DAC. The level obtained in the present
study was half the highest airborne 232Th concentration
reported by Ludwig13) (Table 4). The airborne 232Th during
TIG/SS welding was 0.1–4.8 times that reported by
Jankovic11) and 0.007–1.27 times the concentration of that
reported by Ludwig13) (Table 4). In the present study, the
airborne radioactivity concentration of 232Th during TIG/
Al welding was found to be 62–64 times that during TIG/
SS welding. The reason for this is that TIG/Al welding is
done with an alternating current and that increases the
consumption of the electrode compared to direct current
welding6). It has also been reported that the airborne 232Th
concentration during alternative current welding was higher
than during direct current welding 13) . The airborne
radioactivity concentration of 232Th during electrode
sharpening reported by Vinzents et al. was 8 times that found
in the present study, and 53 times the DAC (3 × 10–2 Bq/
m3)9). The 232Th radioactivity concentration during TIG
welding with alternating current reported by Ludwig was
2.4 times that found in TIG/Al welding of the present study,
and close to the DAC level12). The difference in the airborne
232
Th radioactivity concentrations between Vinzents 9),
Ludweig13) and the present study would be derived from
the differences in the work space, welding devices including
the electrode-sharpening grinder, electric current and welding
materials, as well as the installation of a local exhaust
ventilation system.
To assess the exposure to 232Th, the annual intake (AI) of
232
Th was estimated by the following equation:
AI = CshpRwTshpDwP / PF + CalRwTalDwP / PF
+ CssRwTssDwP / PF + CweRwTweDwP
+ CenvRr P {(24-Tshp-Tal-Tss-Twe) Dw + 24(365-Dw)}.
Here, C shp , C al , C ss , C we and C env are the observed
Industrial Health 2003, 41, 273–278
277
THORIUM-232 EXPOSURE DURING TIG WELDING
Table 5. Annual intake of 232Th for the operator estimated from the observed airborne radioactivity
concentrations of 232Th and hours spent welding
Case number
1
Dust respirator
2
3
4
Without (PF=1)
5
6
With (PF=10)
Electrode Sharpening (hours/day)
TIG/Al welding (hours/day)
TIG/SS welding (hours/day)
Other kind of work (hours/day)
0.17
7
0
0.83
0.17
0
7
0.83
0.17
3.5
3.5
0.83
0.17
7
0
0.83
0.17
0
7
0.83
0.17
3.5
3.5
0.83
Estimated Annual Intake (Bq)
Ratio to Annual limit intake (160 Bq)
33.2
0.21
10.2
0.06
21.7
0.14
3.3
0.02
1.0
0.006
2.2
0.014
radioactivity concentrations of 232Th in total dust during
electrode sharpening, TIG/Al and TIG/SS welding, other
work and in the non-working time period, respectively. The
airborne radioactive concentration of 232Th in the atmosphere
of Kawasaki-city (Cenv) was 3.23 × 10–7 Bq/m3 16). Rw and Rr
are the average hourly respiration volumes during light
physical work (1.2 m3/hour) and in the non-working time
period (1.0 m3/hour), respectively. Tshp, Tal, Tss and Twe are
the working time periods (hours/day) spent in electrode
sharpening, TIG/Al and TIG/SS welding, and other
operations, respectively. The daily work period of an operator
spent in each of the assigned jobs is shown in Table 5. These
time periods were investigated via a questionnaire of daily
work assignments in the subjects’ factory. P is the pulmonary
absorption coefficient of 232Th and is assumed to be 100%.
Dw is the annual number of workdays (days/year), and is
assumed to be 250 days/year. The annual number of work
hours was assumed to be 2000 hours/year (8 hours/day ×
250 days/year). PF is the protection factor of the dust
respirator 17) , defined as the ambient contaminant
concentration divided by the contaminant concentration
inside the face piece18). In the present study, we assumed
two possible cases for TIG welding: with a dust respirator
(PF=10) and without (PF=1). As shown in Table 5, the annual
intake of 232Th was estimated on the basis of the assigned
working hours that the welder spent in welding and electrode
sharpening jobs, with and without a dust respirator. When
the operator worked without the dust respirator (PF=1), the
annual intakes of 232Th were estimated to accumulate to 10.2–
33.2 Bq (one twentieth – one fifth of the ALI), depending
on the time spent in the assigned welding jobs. Wearing
the respirator (PF=10) effectively reduced the annual intake
of 232Th to 1.0 – 3.3 Bq (0.006–0.02 of the ALI). Although
the number of measurements was small in the present study,
the estimation of the AI can be taken to indicate that the
annual intake resulting from occupational exposure of
welders to 232Th during the regular combined operations of
TIG/Al welding, TIG/SS welding and electrode sharpening
does not exceed the ALI. It should be noted, however, that
the present welding and electrode sharpening operations were
carried out in a spacious compartment, which allowed
hazardous fumes and dust to effectively dissipate, although
no local exhaust ventilation system was installed. In many
factories, TIG welding and electrode sharpening are carried
out in a narrow space without the use of a dust respirator.
Moreover workers may handle carelessly the sediment
including 232Th after electrode sharpening operation. There
would be a possibility that airborne 232Th concentrations in
such workplaces elevate close to the DAC.
In conclusion, though the 232Th level during welding and
sharpening was lower than the DAC, we recommend that
thoriated tungsten electrodes be replaced with thorium-free
electrodes such as lanthanum-tungsten or cerium-tungsten
electrodes for safety purposes. If a thoriated tungsten
electrode is used, it is of the upmost importance to inform
workers that the electrode contains radioactive material.
Furthermore, it is necessary to reduce the risk of exposure,
using a local exhaust ventilation system and a dust respirator,
and ensure face and hand washing after work and the sanitary
handling of clothes.
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