Wataru Naito

Urban stormwater runoff introduces tire‑related contaminants such as 6PPD‑quinone (6PPD‑Q), which has been shown to exhibit acute toxicity in some salmonid species, although the underlying toxic mechanisms remain incompletely understood. To expand in vitro toxicological assessments, we evaluated the cytotoxicity of 3,4‑dichloroaniline and 6PPD‑quinone using six fish‑derived cell lines from salmonids and phylogenetically distant species, based on nominal concentrations at the time of addition. Among these cell lines, CSE‑119 cells, which are derived from coho salmon, exhibited the highest sensitivity to 6PPD‑quinone (EC ₅₀ = 8.54 µg/L), demonstrating pronounced species‑specific differences in sensitivity to quinone‑type toxicants and underscoring the value of comparative in vitro models. Using multiple fish cell lines enables the efficient, animal‑welfare–conscious screening of tire‑associated chemicals. However, further investigation is needed into evaluation methods that take into account factors such as the stability of chemicals in the culture medium.

Chemical pollution has emerged as a major threat to freshwater sustainability; however, its impacts and associated responsibilities across global supply chains remain insufficiently understood. This study developed an assessment framework to trace water pollution from chemical emissions embodied in global supply chains driven by final consumption. Pollutant Release and Transfer Register (PRTR) data, grey water footprint assessment, and a multi-regional input-output (MRIO) model were integrated. Annual emissions of 17 chemicals reported in PRTR systems from 39 countries were harmonized and linked to the GLORIA database. Freshwater emissions were estimated using substance-specific transfer fractions from the USEtox model, converted into assimilation water volumes using the grey water concept as a proxy for water pollution. Consumption-based water pollution was quantified using MRIO analysis. Although a limited number of chemicals dominated emissions, priority chemicals changed after accounting for environmental fate and substance-specific water quality thresholds. More than half of the impacts in some countries were induced by foreign final consumption, highlighting the importance of consumption-based responsibility. By identifying priority chemicals, sectors, and consuming countries, this framework provides a basis for understanding the responsibility for water pollution across global supply chains and supports supply chain-oriented chemical management strategies to mitigate chemical water pollution.

Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) are notable cyclic volatile methyl siloxane (cVMS) compounds primarily used in the production of silicone polymers, which are extensively utilized across a wide range of industries, including construction, automotive, electronics, and more. The widespread use of cVMS, particularly D4, D5, and D6, in industrial applications has raised environmental concerns due to their persistence, bioaccumulation potential, and possible toxicity. Despite their low water solubility, cVMS are found to adsorb strongly to organic matter in sediments and soil, raising questions about their long-term environmental effect. Given Tokyo Bay's strategic economic importance and the high level of industrial and urban activities in its catchment, this study aimed to provide a comprehensive understanding of cVMS contamination in the area. The spatial and temporal distribution of cVMS concentrations in the sediments and fish of Tokyo Bay were systematically examined from 2011 to 2021. The results showed that cVMS were widely distributed across the bay's sediments. Regarding the samples collected in 2021, the concentration of D4, D5 and D6 ranged from <1.04 to 14.5 ng/g- dry weight (dw), from 6.3 to 494 ng/g-dw, and from 2.3 to 89.5 ng/g-dw, respectively. A clear decline of observed concentrations in the seaward direction underscored the influence of riverine inputs. Temporal trends indicated a general decline in cVMS concentrations in sediments at multiple sites, while concentrations in fish varied by species and trophic level. Ecological risk assessment revealed that current cVMS concentrations in the sediments do not pose a threat to benthic organisms. This study is the first to provide a long-term analysis of cVMS in Tokyo Bay, contributing valuable data for environmental monitoring efforts. The findings underscore the importance of continuous assessment of these compounds to understand persistence, bioaccumulation and risks to ecosystems.

Microplastic particles (MPs) are ubiquitous in marine environments; however, comprehensive studies comparing their characteristics across environmental matrices and size ranges remain limited. This study investigated MPs in surface water and sediment samples collected from eight stations in Tokyo Bay during October–November 2023. The samples were analyzed for MP concentration, size distribution, morphology, and polymer composition. The results showed that small MPs (20–100 μm) numerically dominated in both surface water and sediment, accounting for more than 70% of total MP concentration. However, medium and large MPs (100–350 μm and ≥ 350 μm), although less abundant, contributed substantially to total estimated MP mass in both matrices. Morphologically, fragmentary MPs were dominant in most samples, particularly among small and medium MPs, whereas fibrous MPs became more prevalent with increasing particle size in both matrices. In terms of polymer types, PMMA, PE, and PP were abundant in surface water, while PE and PP predominated in sediments, with PMMA occurring at a smaller proportion. In the large-size fraction (≥ 350 μm) of both matrices, PET was the most abundant polymer in both number- and mass-based compositions. By targeting a broad size range of MPs (20–5000 μm), this study demonstrates that key MP characteristics, particularly morphology and polymer composition, vary systematically across size classes and environmental matrices. These findings suggest that both particle size and the selected quantification metric (number- or mass-based) play an important role in the assessment of MP pollution. The dataset provides valuable size-resolved baseline information for long-term monitoring and supports future ecological risk assessments in urbanized coastal environments.

The acute toxicities of para-phenylenediamine quinones (PPDQs) to salmonids at the whole-organism level remain unclear. Therefore, in this study, we investigated the acute toxicities of three PPDQs, 6PPDQ, 7PPDQ, and 8PPDQ in coho salmon ( Oncorhynchus kisutch ) juveniles and embryonic cells (CSE-119) using 24-h acute toxicity tests and cytotoxicity assays. The 24-h 50% effect concentration values of 6PPDQ and 7PPDQ for the juveniles were 41.5 ng/L (95% confidence interval: 21.6–61.4) and 5.39 μg/L (2.26–8.53), respectively, while 8PPDQ did not affect mortality at the tested concentration (60.6 μg/L). Further, the 24-h 50% effect concentration value of 6PPDQ for CSE-119 was 14.9 μg/L (13.0–16.7), but 7PPDQ and 8PPDQ did not sufficiently reduce CSE-119 cell viability even at concentration up to 500 μg/L, indicating marked differences in toxicity among the compounds. These findings indicate that, at both the whole-organism and cellular levels, 6PPDQ exhibited the highest toxicity among the three PPDQs, likely due to differences in chemical structure.

Given that mass gathering events involve heterogeneous and time-varying contact between residents and visitors, we sought to identify possible measures to prevent the potential acceleration of the outbreak of an emerging infectious disease induced by such events. An individual-based simulator was built based on a description of the reproduction rate among people infected with the infectious disease in a hypothetical city. Three different scenarios were assessed using our simulator, in which controls aimed at reduced contact were assumed to be carried out only in the main event venue or at subsequent additional events, or in which behavior restrictions were carried out among the visitors to the main event. The simulation results indicated that the increase in the number of patients with COVID-19 could possibly be suppressed to a level equivalent to that if the event were not being held so long as the prevalence among visitors was only slightly higher than that among domestic residents and strict requirements were applied to the activities of visitors.