Barium and barium compounds
25
11.1.3
Sample risk characterization
There are a number of different approaches to
assessing the risks to human health posed by chemicals.
For example, barium sulfate is the most likely substance
of occupational concern and is of very low toxicity.
Since exposure estimates can vary widely, the risk
characterizations below are provided as examples for
illustrative purposes.
11.1.3.1 Ingestion
Dog and rat pharmacokinetic studies (Taylor et al.,
1962; Cuddihy & Griffith, 1972) suggest that gastro-
intestinal absorption of barium may be higher in young
animals than in older ones. Brenniman & Levy (1984)
examined persons 18–75+ years of age living in the
community for more than 10 years. It is likely that this
study included adult residents who were exposed to
elevated barium levels as children, but it may not
account for all of the uncertainty. The barium database
consists of subchronic and chronic toxicity studies in
three species (humans, rats, and mice) and a marginally
adequate first-generation reproductive/developmental
toxicity study. The rat and mouse study (Dietz et al.,
1992) gave no indication that developmental or repro-
ductive end-points are more sensitive than other end-
points; interpretation of the study results is limited by
very low pregnancy rates in all groups, including con-
trols, and examination of a small number of develop-
mental end-points. No modifying factor is proposed for
this assessment.
The US EPA (1998) derived an RfD of 0.07 mg/kg
body weight per day for barium, based on the NOAEL of
0.21 mg/kg body weight per day for no adverse health
effects identified in the Wones et al. (1990) and
Brenniman & Levy (1984) human studies, with an
uncertainty factor of 3 to account for some database
deficiencies and potential differences between adults
and children. The primary route of exposure to barium
appears to be ingestion in drinking-water and food. A
daily intake of 0.03–0.60 mg barium/kg body weight per
day from drinking-water can be estimated using the
drinking-water concentration of 1–20 mg/litre, a reference
consumption rate of 2 litres/day, and a body weight of 70
kg. IPCS (1990) reported several published estimates of
dietary intake of barium by humans, ranging from 300 to
1770 mg barium/day, with wide variations; this is
equivalent to a range of 4–25 mg barium/kg body weight
per day, assuming a 70-kg adult body weight. Hence,
populations consuming high dietary barium levels may
have intakes approximating or exceeding the oral RfD
value of 0.07 mg/kg body weight per day and the
tolerable intake of 0.02 mg/kg body weight per day.
11.1.3.2 Occupational (barium sulfate
)
Another sample risk characterization is based on
occupational exposure primarily to barium sulfate in the
United Kingdom. In general, the highest typical levels of
exposure appear to occur in offshore drilling activities.
The highest exposures for which measured data are
available apparently occur during addition of the barite
ore from the bulk hopper to the mud mixing tank. There
are no concerns for human health with typical exposures
that arise during drilling activities if the machinery is
enclosed and LEV used. However, where the machinery
is not enclosed and appropriate LEV is not available,
modelled data indicate that exposures could be much
higher, on the order of several tens of mg/m
3
of total
inhalable dust. The consequences
for human health of
long-term exposures at such high levels are not clear.
Because of the low exposures involved, there are
no concerns for human health during the processing of
barite ore where LEV is used. A similar conclusion can
be drawn regarding its use in the formulation of plastics
and coatings, although, from modelled data, exposures
could be much higher and the human health picture less
reassuring in these industries if LEV is not used. The
extremely high personal sampling values for total
inhalable dust (55 mg/m
3
8-h TWA) measured at one
factory milling barite ore merit further consideration. At
present, workers wear powered respirators that should
substantially reduce the current level of personal
exposure to levels below those measured in the atmos-
phere.
If occupational exposures are controlled to less
than 10 mg/m
3
(total inhalable dust, which is primarily of
low toxicity) as an 8-h TWA, it would appear that there
are no significant risks to human health.
11.1.4
Uncertainties in the evaluation of health risk
An area of scientific uncertainty concerning the
non-cancer hazard assessment for barium is the identi-
fication of the most sensitive end-point of barium
toxicity in humans. The results of the NTP (1994)
medium-term rat study suggest that renal effects may be
a more sensitive end-point than hypertension. However,
it is not known if a similar relationship would exist
following long-term exposure or in humans. The
Brenniman & Levy (1984) human study examined the
effect of barium on blood pressure but did not inves-
tigate sensitive renal end-points (kidney disease was
assessed by a health questionnaire only). The long-term
rat study (NTP, 1994) did not measure blood pressure.
Another area of scientific uncertainty is whether any
toxicological or toxicokinetic differences exist between
children and adults. Animal data (Taylor et al., 1962;