Date of Award

12-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Toxicology

Advisor

Johnson, Alan R.

Committee Member

Bowerman , William W.

Committee Member

Rodgers , John H.

Committee Member

Fjeld , Robert A.

Committee Member

Carraway , Elizabeth R.

Abstract

A large and/or long-term released source of a toxic chemical(s) can cause adverse effects to humans and non–human species inhabiting all over the region. The overall goal of this dissertation was to improve practices in risk assessment concerning a regional risk posed by a prospective source. A generic conceptual framework for performing regional prospective human health and ecological risk assessment was developed. The formulation of a priori considerations was used to identify the possible sources of uncertainties in the prospective risk assessment. Physiologically based pharmacokinetic (PBPK) modeling was utilized to reduce uncertainty in a toxicologically safe dose used in risk assessment.
The toxicologically safe doses for methylmercury to neurological impairment for dugongs (Dugong dugon) and human populations in Thailand were derived based on a no-observed adverse effect level (NOAEL) in rats (0.1 mg/kg/day). The intra&ndash and inter&ndash extrapolations by the PBPK models resulted in the oral NOAELs of 9.75 μg/kg/day for the dugongs and ranging from 3.0 to 3.9 μg/kg/day or 111 to 197 μg/day for Thai males and females of age 11 to 100 years old. The NOAELs for humans of age 1 to 10 years old to neurodevelopmental effects, extrapolated from the NOAEL of 0.05 mg/kg/day in rats, were ranging from 1.9 to 3.7 μg/kg/day or 34 to 61 μg/day. By applying the uncertainty factor 3, for the intra&ndashspecies variability in pharmacokinetics and pharmacodynamics, the oral safe doses for methylmercury were suggested to be 3.25 μg/kg/day for dugongs, 1.0 μg/kg/day for non–pregnant humans of age 11 to 100 years old, and 0.5 μg/kg/day for humans of age 1 to 10 years old and the pregnant adults.
A demonstrated case of a regional prospective risk assessment of mercury from a hypothetical coal&ndashfired power plant in Thailand was conducted. The assessment returned the following predictions. (1) The current contaminations of mercury in the Map Tha Phut (MTP) Bay, located at the upper part of the Gulf of Thailand, may not cause the bay likely to be at risk from mercury toxicity, based on the inorganic mercury (Hg2+) risks to the phytoplankton (the hazard quotient, HQ = 0.1) and the juvenile fish (HQ = 0.01). (2) The prospective hypothetical coal&ndashfired power plant of capacity 1,400 megawatts may cause the deposition of the emitted Hg2+ approximately 50 μg/m2/yr to the bay and may increase about to 100 and 150 μg/m2/yr once the plant expands its capacity to 2,800 and 4,200 megawatts, respectively. (3) These deposited Hg2+ could cause the water in the bay increase in Hg2+) 0.2, 0.4, and 0.6 ng/L, or 0.44, 0.88, and 1.32 %, and methylmercury, from the methylation of the Hg2+, 0.04, 0.08, and 0.12 ng/L, or 0.74, 1.49, and 2.23 %, respectively. (4) The three scenarios of prospective contaminations in the study bay will possibly not cause the bay to be at risk (HQ = 0.140, 0.141, and 0.142, respectively). (5) Based on the mercury risks to the supposed seagrasses (the Hg2+&ndashbased HQ = 3x10-6), and dugongs (the methylmercury–based HQ = 0.006), the ‘dugongs return home’ in and near the MTP Bay is possible. (6) The prospective contaminations of methylmercury in fish and shellfish in the study bay possibly will not put the seafood consumed humans at risk from neurological adverse effects (HQs = 0.08 to 0.21 in youngs, 0.12 to 0.13 in adults). (7) However, the approximations of Hg2+) and methylmercury in the study bay were based on the modeling in which the advection and dispersion effects have been neglected; thus further studies on these issues and other site–specific data can improve the predictions.

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