TOXICOLOGICAL PROFILE FOR BARIUM AND COMPOUNDS ...

8
2. HEALTH EFFECTS
associated with more subtle effects in humans or animals (LOAEL) or exposure
levels below which no adverse effects (NOAEL) have been observed. Estimates
of levels posing minimal risk to humans (Minimal Risk Levels, MRLs) may be of
interest to health professionals and citizens alike.
Estimates of exposure levels posing minimal risk to humans (MRLs) have
been made, where data were believed reliable, for the most sensitive noncancer
effect for each exposure duration. MRLs include adjustments to reflect human
variability from laboratory animal data to humans.
Although methods have been established to derive these levels (Barnes et
al. 1988; EPA 1989c), uncertainties are associated with these techniques.
Furthermore, ATSDR acknowledges additional uncertainties inherent in the
application of the procedures to derive less than lifetime MRLs. As an
example, acute inhalation MRLs may not be protective for health effects that
are delayed in development or are acquired following repeated acute insults,
such as hypersensitivity reactions, asthma, or chronic bronchitis. As these
kinds of health effects data become available and methods to assess levels of
significant human exposure improve, these MRLs will be revised.
2.2.1 Inhalation Exposure
Studies evaluating the effects of barium following acute, intermediate,
and chronic inhalation exposure are limited to several case reports of humans
exposed occupationally (Doig 1976; Essing et al. 1976; Seaton et al. 1986;
Shankle and Keane 1988) and to two experimental studies with animals (Hicks et
al. 1986; Tarasenko et al. 1977). These case reports and animal studies are
not adequate for firmly establishing the health effects of barium by
inhalation because of a number of significant study limitations. The case
reports are generally inadequate because data were available for a limited
number of exposed subjects and because exposure conditions (duration,
frequency, dose) were not well characterized (Doig 1976; Essing et al. 1976;
Seaton et al. 1986; Shankle and Keane 1988). One of the two animal studies
was limited in that apparently no control animals were used, an inhalation
chamber providing a controlled dose and environment was not used, and there
was a lack of information regarding the vehicle used, the purity of the test
material, the duration and frequency of exposure, and the number of animals
tested (Hicks et al. 1986). The second animal study consisted of several
experiments but was generally limited in that the authors provided few details
regarding experimental methods, exposure conditions, and test results, and no
information as to the number of animals tested, the purity of the test
material, or the statistical methods used; furthermore, in some experiments it
was not clear whether or not control animals were used (Tarasenko et al.
1977). In view of the major limitations associated with the available case
reports and animal studies, results from these reports should be regarded as
providing only preliminary and/or suggestive evidence that acute,
intermediate, and chronic inhalation exposure to barium may potentially be
associated with adverse health effects. No reliable information was available