Research may have implications for current EPA exposure
standards. Dr. Karla Thrall (Project
A1) has been using a novel breath analysis system to accurately
measure the amount of volatile organic chemicals that are absorbed
into the body by any route of exposure, and has developed mathematical
models to describe the absorption, distribution, metabolism and
excretion of these chemicals. This allows a more accurate estimation
of the amount of chemical contaminant that has entered particular
tissues.
In 2003, the Thrall laboratory performed a series of controlled
dermal and inhalation studies to assess the bioavailability of
toluene in human volunteers under exposure conditions designed
to mimic bathing scenarios. Their data indicate that human skin
is more than 80 times less permeable to toluene than previous
U.S. EPA estimates suggest. This finding may have important implications
for the re-assessment of current EPA exposure standards and for
improving overall risk assessments for toluene, as well as other
compounds.
Data may help develop better predictive models for
human dermal absorption of volatile organic compounds.
Dr.Thrall’s lab has compared species differences between
rats and human volunteers in the dermal bioavailability of aqueous
xylene. These comparative studies suggest that, while aqueous
xylene is rapidly absorbed through the skin of both rats and humans,
rat skin is approximately 12 times more permeable to aqueous xylene
than human skin. These experiments provide an opportunity to better
understand species differences in dermal bioavailability and may
ultimately aid in developing more predictive models for human
dermal absorption from rodent data.
The development of reliable human-specific data describing the
dermal bioavailability of two commonly encountered solvents (xylene
and toluene) has important implications for scientists’
ability to accurately assess the toxicological risks presented
by exposure to volatile organic compounds. These methods will
ultimately reduce uncertainty in the risk assessment process involving
volatile organic substances.
Research on the microbial
degradation of trichloroethene (TCE)
may lead to improved remediation technologies
at Superfund sites.
Dr. Jennifer Field (Project
B1) is developing methods to enhance
the microbial degradation of TCE to
nontoxic products (e.g., ethene or carbon
dioxide) by manipulating the microbial
growth conditions that exist underground.
Such technology has the potential to
substantially reduce the impact that
TCE and other chlorinated solvents have
on human health and the environment.
This project addresses a critical need,
because TCE is present at many Superfund
sites and is the organic contaminant
most frequently detected in groundwater.
In 2003, approximately 70 field experiments were conducted at
TCE-contaminated sites in Oregon, California, Washington, South
Carolina, and Tennessee. An important result of this work was
the demonstration that fluorinated analogs of TCE can be used
as chemical tracers to indirectly monitor the microbial degradation
of TCE in heavily contaminated environments, where direct monitoring
of TCE degradation is not possible.
Based on this important result, Dr. Field’s group has developed
a suite of chemical tracers to probe each step in the TCE biodegradation
pathway and to assess the ability of microbes to completely degrade
TCE to non-toxic products. At many sites, biotransformation of
TCE was shown to produce the toxic intermediate, vinyl chloride,
which suggests that the microbial community at those sites is
not able to completely metabolize TCE to ethane, a relatively
non-toxic compound. Dr. Field has developed an assay for monitoring
this transformation, and has shown that the biotransformation
of vinyl chloride to ethane can be enhanced and is feasible under
field conditions. The chemical tracers have allowed comparisons
to be made between the effectiveness of non-microbial (e.g. chemical
reduction using iron) and microbial TCE degradation processes,
and has allowed investigations into the use of lactate and hydrogen
to stimulate the growth and activity of biodegradation organisms.
At one field site, it was shown that subsurface addition of iron
produced a more complete chemical transformation of TCE to non-toxic
products than did lactate and hydrogen additions.
Collectively, this work represents a major advance in our ability
to understand and manipulate microbial processes to degrade TCE
and similar contaminants. The tracers that Dr. Field is developing
provide us the ability to probe relevant microbial processes under
actual field conditions and have a variety of applications to
industry (e.g., to perform rapid feasibility assessments and pilot
testing of remediation technologies).
SBRC research presented at international forum.
Dr. Field’s work has generated substantial international
interest. In 2003, she was invited to participate and speak about
her work on TCE bioremediation at the U.S.-Vietnam Scientific
Workshop on Dioxin Screening, Remediation Methodologies and Site
Characterization, held Nov 2-5, 2003, in Hanoi, Vietnam.
These results raise the possibility that a number of other reportedly
chromogenic organic solvents, including substances that are in
very widespread commercial use, have the potential to cause nerve
degeneration. Moreover, chromophores may be able to be used as
biological markers of exposure to aromatic hydrocarbon solvents,
because the chromophore appears in tissues and urine before neurodegenerative
changes appear. Since 2,5-HD is the ultimate metabolite of the
neuropathy-producing aliphatic solvent n-hexane, and 1,2-DAB is
the equivalent of the more potent neuropathy-producing aromatic
solvent 1,2-diethylbenzene (1,2-DEB), permissible limits for human
workplace exposure for 1,2-DEB need to be re-assessed.
