PhD Oral Exam - Sarah Sajedi, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The global reliance on single-use plastic (SUP) water bottles has intensified environmental pollution and raised critical public health concerns due to widespread contamination by nano and microplastics (NMPs). This dissertation presents the first comprehensive, interdisciplinary assessment integrating environmental science, analytical chemistry, human exposure modeling, behavioral research, and regulatory analysis to evaluate the magnitude, mechanisms, and consequences of NMP exposure from SUP water bottles. Drawing upon an extensive review of over 141 scientific studies, advanced detection technologies, a global survey of 15,143 individuals across 81 countries, and quantitative toxicokinetic modeling using the HEASI Plastic Model, this research identifies SUP water bottles as a significant and underregulated source of chronic NMP ingestion.

Analytical evidence demonstrates that bottled water contains NMP concentrations ranging from hundreds of thousands to over 10¹¹ particles per liter, with nanoplastics (NPs) (<100 nm) presenting the greatest biological risk due to their enhanced ability to cross cellular, blood-brain, and placental barriers. Variability in reported concentrations is driven largely by inconsistencies in analytical methods such as fluorescence spectroscopy, Raman-based imaging, SEM-EDS, SRS, and mass spectrometry. This variability complicates exposure assessment and regulatory decision-making, highlighting the need for standardized detection protocols. Synthesized findings show that individuals ingest substantial quantities of NMPs annually, with bottled water consumption adding up to 90,000 additional particles compared to tap water. Chronic exposure is associated with respiratory, reproductive, neurological, endocrine, and carcinogenic outcomes, underscoring the potential long-term health risks linked to nanoscale plastic ingestion.

Complementing the environmental and analytical findings, the global behavioral survey reveals that SUP water bottle use is strongly shaped by demographic and contextual factors. Younger adults and highly educated individuals report higher consumption and recycling, while older adults more commonly rely on refillable bottles. Motivations for SUP use include convenience, travel, perceived water safety, and exposure to advertising. Despite widespread awareness of contamination, 75% of respondents acknowledged NMPs in bottled water, and 70% perceived a cancer risk behavioral inertia persists, particularly in regions with limited drinking-water infrastructure. Cluster analysis identifies distinct behavioral subgroups, each with unique motivations, vulnerabilities, and implications for targeted interventions.

To address methodological inconsistencies, this dissertation employs a multimodal analytical workflow including Nanosight pro NTA, SEM-EDS, pyrolysis-GC/MS, FTIR, and QSense to differentiate total nanoparticle counts from polymer-specific NPs. While total nanoparticle concentrations ranged from ~4.1 × 10⁸ to 8.8 × 10⁸ particles/L, polymer-verified NPs comprised only ~4.89 × 10⁷ to ~2.53 × 10⁸ particles L⁻¹, revealing that non-plastic particulates often inflate exposure estimates. QSense measurements indicated baseline polymer mass concentrations of ~5.42 to ~45.35 μg L⁻¹. Under simulated handling conditions involving 12 hours of shaking and UV exposure, Total particles (NTA) counts increased by ~4.10% to 243.30% depending on brand, while polymer mass (QSense × Py-GC/MS) increased by ~104% to 343%, reaching 150 μg L⁻¹ in the most responsive case, demonstrating that mechanical agitation and photochemical stress substantially accelerate polymer release. These findings show that real-world handling can significantly amplify both particle number and polymer mass exposure. This integrated approach yields more accurate and chemically defensible measures of true NP exposure.

To build a realistic NP exposure framework, this dissertation applies to the steady-state HEASI Plastic Model in two stages. First, the model was run using literature-reported NP concentrations from Qian (2024) and Zhang (2023), which span from lower-end values of approximately 10⁵ particles L⁻¹ to upper-bound estimates near 10¹¹ particles L⁻¹. These inputs produced steady-state tissue burdens ranging from only a few particles to more than 10¹⁰ particles per capita, confirming that high reported concentrations can generate meaningful long-term accumulation. The model was then independently populated with polymer-verified release data generated experimentally in this dissertation for four commercial SUP water bottle brands. Using these measured particle counts and polymer-specific masses, baseline daily tissue burdens ranged from ~ 0.005 mg day-1 for lower-release products to 0.045 mg day-1 for the highest-release brand equivalent to ~2-17 mg year-1. When subjected to simulated real-world handling involving 12 hours of shaking and UV exposure, release rates and tissue burdens increased substantially, with the highest-release brand reaching ~0.15 mg day-1 (≈ 55 mg year-1) and others rising to ~0.033 mg day-1 (≈ 12 mg year-1). Projected over 10- and 20-year exposure horizons, these values scale proportionally and reveal pronounced divergence among brands, with high-release products producing consistently greater long-term accumulation. Together, the two modeling stages demonstrate that realistic consumer use, combined with experimentally measured polymer-specific release rates, can generate sustained and clinically relevant long-term NP accumulation. The HEASI modeling built in this dissertation shows that polymer-verified exposures under baseline and shaking + UV (12h) stress produce meaningful, time-dependent tissue burdens that differ systematically across brands and handling conditions. These findings provide the first integrated, polymer-resolved estimate of steady-state NP accumulation from bottled water consumption and underscore the need for validated toxicokinetic parameters and brand-specific regulatory oversight.

Finally, a critical review of international regulatory frameworks identifies substantial deficiencies in bottle-specific oversight. While many jurisdictions regulate other SUP items such as bags and straws, water bottles remain largely unregulated regarding NP contamination. Evidence-based policy recommendations emerging from this work include standardized analytical testing requirements, mandatory labeling, extended producer responsibility, improved drinking-water infrastructure, and demographic-tailored behavioral interventions.

Collectively, this dissertation offers an integrated and actionable framework for understanding, measuring, and mitigating NP exposure from SUP water bottles. It advances scientific knowledge, informs policy development, and supports the transition toward safer and more sustainable hydration systems.