大氣塑膠微粒動態模擬與分析及人體暴露相關肺臟毒性風險評估

Abstract

鑑於公眾對普遍存在室內外環境之大氣塑膠微粒意識之增長，其因呼吸暴露造成之潛在衝擊備受高度關注。大氣塑膠微粒主要來源為輪胎磨損顆粒，對人體健康易產生不利影響。因此，本論文目的為(1)預測自輪胎磨損所釋放之大氣塑膠微粒之全球分佈型態，(2)了解人體呼吸系統中可吸入性塑膠微粒之沉降與停留動態，(3)建立可吸入性塑膠微粒對肺臟毒性之濃度－反應關係並推估濃度暴露閾值，及(4)建構一結構化風險評估架構以研析大氣塑膠微粒暴露所導致之人體健康風險。 本論文建立以數據驅動之模式方法推估交通運輸中輪胎磨損顆粒釋放量。藉由整合自多元線性回歸模式推估之各國車行里程與不同粒徑分布之釋放因子，本研究呈現全球輪胎釋放量現況並延伸探討新冠肺炎疫情期間與未來汽車電動化造成之影響。本研究量化大氣塑膠微粒之環境暴露濃度與依微粒物理形態相關之氣動直徑。在一般設定與實際場景下，被吸入塑膠微粒之沉降與清除動態以建構之團塊人體呼吸道模式模擬。貝氏濃度反應模式用於推估塑膠微粒暴露之閾值。最後，進行機率風險評估以描述吸入暴露於空氣中塑膠微粒對人類肺部健康之潛在風險。 研究結果顯示，所有車種與特定車型(自小客車、輕型商用車、重型車輛、摩托車與輕型機車及公車)之最佳車行里程模式包含2至5個預測變數，可解釋來自不同國家之過往資料總變異量達96%。車輛使用為模式中之主要指標，與車行里程呈顯著正相關。本研究顯示每年自2015至2019年，十億至兆之各國車行里程已造成103–105噸空氣可傳播之輪胎磨損顆粒，小客車與重型卡車之輪胎磨損釋放量貢獻最高可達噸。美國、印度及中國之輪胎磨損顆粒釋放量冠居全球，顯示實施有效且全面之控制策略有其必要性以防高暴露量之輪胎磨損氣懸顆粒。本研究發展之系統性方法更加有助於了解全球輪胎耗損顆粒釋放量之增加趨勢。此外，於2020年新冠肺炎疫情期間，空氣中輪胎磨損顆粒釋放自小客車大幅減少；然未來30年，汽車電動化將造成輪胎磨損顆粒釋放量明顯上升。 空氣中懸浮塑膠微粒之主要形狀中，平均氣動直徑（均<70 μm）為碎片>纖維>球體。本研究發現室外空氣中懸浮塑膠微粒濃度與採樣高度相關，其濃度主要介於 1–15 顆 m−3。各累積塑膠微粒濃度於12個團塊中差距甚廣，橫跨十個數量級。其中因清除機制造成淋巴結區域中濃度為最高。本研究結果顯示大粒徑塑膠微粒(氣動直徑>40 μm)僅能累積於胸外與支氣管區域之呼吸道。靈敏度分析顯示無論粒徑大小，環境濃度參數始終對體內塑膠微粒濃度造成顯著且正向之影響。暴露於大粒徑塑膠微粒之情境下，鼻腔氣流占比與呼吸率為最靈敏之輸入參數。此外，以暴露情境為例之研究結果顯示，塑膠微粒僅累積於上呼吸道，絕大部分則沉積於人體呼吸道組織。 按基準濃度數值排序，結果顯示列首要關注為由塑膠微粒與小粒徑輪胎磨損顆粒（< 2.5 µm）引致毒性效應。於塑膠微粒毒性方面，A549細胞內之抗炎性細胞激素(IL-8與IL-6)與呼吸道收縮反應為最敏感之觀測終點，其BMC10推估值均小於10 μg mL−1。藉由劑量調校與殘餘不確定性因子之轉換，推估得急性與慢性暴露下之閾值分別為0.09 and 3.11×10–3 μg mL–1。 超越風險結果指出所有暴露情境包含中國巨型都市、加州不同年齡層及溫州都市/鄉村地區中有一半以上之人口具極微小之肺臟毒性反應。本研究延續暴露情境設定下之暴露評估結果，風險評估結果顯示急性暴露下之肺臟毒性風險水平為可接受，然慢性暴露一年後則具顯著風險。當暴露時間從一年延長至十年，平均風險特性化比相對1高兩個數量級。都市人口之風險特性化比>1之結果可供作一指標特點，指出儘管外科口罩之過濾效率> 80%，但僅使用口罩作為防護之成效不足，將需要額外之控制措施和管理。 總結本研究發現，龐大的輪胎磨損氣懸顆粒釋放量被證實可視為大氣塑膠微粒之主要來源。本研究建立之模式方法提供之注釋和影響有助於暴露評估人員推估空氣可傳播之輪胎磨損顆粒與塑膠微粒暴露風險。因此，將削減空氣可傳播之塑膠微粒與輪胎磨損顆粒排放量納入空氣污染控制考量至關重要，可供未來監管徹底檢視塑料廢物管理措施並減少單一國家甚至全球尺度下塑膠微粒排放量。

Rising public awareness of the ubiquity of atmospheric microplastics (MPs) in outdoor and indoor environments has highlighted the concern on the potential impacts from inhalation exposure. Moreover, tire wear particle (TWP) as a significant contributor of MPs is highly concerned due to its detrimental effect on human body health. Therefore, the purposes of this dissertation were: (i) to predict global patterns of TWP emissions via a probabilistic-based algorithm, (ii) to understand the dynamics of particle deposition and retention of airborne MPs in the human respiratory system, (iii) to establish the concentration-response relationships of inhalable MPs causing pulmonary toxicities to estimate the threshold exposure level, and (iv) to construct a structured risk assessment framework for appraising human health risk posed by airborne MPs. This dissertation established the data-driven modeling approaches for TWP emissions from on-road vehicles. By integrating country-specific vehicle-kilometers traveled (VKT) estimated from the multiple linear regression models with size-fractionated emission factors, a global pattern of TWP emissions associated with atmospheric MPs was performed upon current situation and further implications on COVID-19 pandemic and future vehicle electrification. In the context of atmospheric MPs, exposure concentrations and aerodynamic diameters (ADs) depending on physical morphology were characterized. A compartmental human respiratory tract (HRT) model was constructed to simulate the dynamics of deposition and clearance of inhaled MPs under general settings and practical scenarios. A Bayesian concentration-response model was used to estimate threshold levels of MPs exposure. Finally, the probabilistic risk assessment was performed to characterize potential risks of human pulmonary health from inhalation exposure to airborne MPs. The results showed that the optimal vehicle-specific VKT models comprised of 2–5 predictors enabled to explain > 96% variance of the historical data from 11 countries. Vehicle-in-use was the principal indicator showing a significantly positive relationship with VKT. This study showed that country-specific VKT from billion to trillion vehicle-kilometer resulted in 103–105 metric tons of airborne TWP annually in the period 2015–2019, with the highest emissions from passenger cars and heavy-duty vehicles. The highest TWP emissions were found in the United States, India, and China, indicating the necessity of implementing efficient and comprehensive control measures to prevent high-level exposure to airborne TWPs. This systematic approach has achieved a better understanding with respect to an increasing trend in higher TWPs emissions worldwide. In addition, airborne TWP emissions from passenger cars by country had a substantial decrease (up to ~33%) during COVID-19 lockdowns in 2020 and a pronounced increase (by a factor ~1.9) from vehicle electrification by the next three decades. Among the predominant shapes of suspended MPs in air, the averaged AD (all < 70 μm) was fragment > fiber > sphere. This study found a concentration-sampling height relation for suspended MPs in outdoor air where airborne concentration of MPs mainly ranged between 1–15 particles m−3. Cumulative MPs concentrations in each 12 compartments exhibited a wide range of variation spanning over ten orders of magnitude with the highest concentrations in lymph nodes region due to clearance mechanisms. Large-sized MPs (AD > 40 μm) could only be accumulated in the airways of extrathoracic and bronchi regions. The sensitivity results demonstrated that environmental concentration consistently showed significant and positive effects on internal MPs concentration regardless of the particle size. Nasal portion of respiratory airflow and inhalation rate were the most sensitive input parameters under large-sized MPs exposure. Moreover, scenario-based exposure results showed that MPs were only accumulated in the upper airways and mostly deposited in the respiratory tissues. Ranking by the potencies of the benchmark concentration (BMC10) values, the effects induced by MPs and small-sized TWPs (< 2.5 µm) were at top-priority concern. In terms of adverse effects of MPs, IL-8 and IL-6 cytokines from A549 cells as well as contractile responses of airways were the most sensitive endpoints where BMC10 estimates were all less than 10 μg mL–1. By applying the dosimetric adjustment and residual uncertainty factors, threshold estimates for acute and chronic exposures were 0.09 and 3.11×10–3 μg mL–1, respectively. Results of exceedance risk indicated minimal pulmonary effects for 50% of human populations in all exposure settings including Chinese megacities, age groups in California, and urban/rural areas in Wenzhou. From the extension of the findings of scenario-based exposure assessment, probabilistic risk assessments revealed an acceptable risk level for acute exposure, whereas significant risks were found over 1-year chronic exposure. When the exposure period expanded from 1 year to 10 years, the mean risk characterization ratio (RCR) values were 2 magnitudes of order higher above 1. Values of RCR > 1 for urban people may provide an indicator that using a face mask alone is insufficient despite the filtering efficiency of surgical mask > 80%. Additional control measures and management will be needed. In summary, this dissertation evidenced that the stunning mass of airborne TWP is a predominant source of atmospheric MPs. The modeling approach for interpretation and implications aids exposure assessors in estimating airborne TWPs and MPs exposure risk. Taking reduction of airborne MPs and TWP emissions into consideration for air pollution control is thus critical, presenting the opportunity for future regulation to overhaul plastic waste-management practices and to mitigate MPs emissions at national to global scales.