## 4 Discussion
### Analysis of Population Health Risks and Environmental Factors  
In recent years, there has been an increasing interest in the assessment of human health risks [76]. In this study, we considered two exposure pathways: hand-to-mouth and dermal contact. Hand-to-mouth ingestion is one of the important pathways for HM exposure from sediments, especially for children and workers who frequently come into contact with sediments [77]. Children, due to their frequent finger-sucking behavior, are prone to ingesting contaminated sediments, and workers in the construction, mining, and agricultural sectors may also ingest HMs through sediment particles on their hands and clothing [78; 79]. Dermal contact is also another important pathway for HM exposure from sediments. Direct contact with contaminated sediments may occur during recreational activities, occupational exposure, and daily activities [80; 81]. This study's results show that the non-carcinogenic and carcinogenic risks of hand-to-mouth ingestion are both higher than those of dermal contact, which is consistent with the results of [PERSON]'s study, indicatingthat the exposure pathway through hand-to-mouth ingestion is much higher than that through dermal contact [1]. The risk assessment of HMs in the sediments of the YRB indicates that the non-carcinogenic risk of HMs in the sediments to adults is negligible, while the non-carcinogenic risk to children is significant. In terms of carcinogenic risk, the main carcinogen is As, which is consistent with the conclusions of previous studies [39]. This is due to the relatively high content of the HM As in the sediments (Table S3) and the large carcinogenic slope factor (SF) of As in the model [82] (Table S1). For example, in the downstream area D2-D9, the carcinogenic risk to children exceeds the carcinogenic risk threshold (TCR \(>10^{-4}\)), indicating that the local child population may be at health risk. The downstream area of the YR is economically developed, with a focus on manufacturing, chemical, and service industries [83]. Despite the dense economic activities, the level of HM pollution is relatively low, which may be related to stricter environmental regulation and pollution control measures [84]. The distribution of HMs in river sediments is controlled by natural factors and human activities, resulting in significant spatial variations. The degree of HM pollution in the YR sediments is \"low to moderate\", and it decreases gradually from the upstream to the downstream, which is similar to previous studies that found higher pollution levels in the upper and middle reaches than in the lower reaches [30]. Areas with higher pollution levels have specific environmental and socio-economic characteristics. The concentrations of Cr, Ni, Cu, Zn, and Cd are higher in the upstream, mainly due to the abundant mineral resources in the upper reaches of the YR, especially nonferrous metals and rare earths [85]. For example, the reserves of nonferrous metals and rare earths in Qinghai and Gansu account for a relatively high proportion of the national total. The economic output of the upper reaches is mainly concentrated in the mining and processing of nonferrous metals and rare earths [32]. In the middle reaches, the sampling sites M12, M13, and M14 are located in the Fen River (Shanxi), with an increased HM content in the sediments caused by mining and frequent industrial activities in Shanxi Province [86]. In the downstream areas, the HM content in the sediments is relatively low, but there are still some moderately polluted areas. For example, As and Cd pollution is relatively severe in Luoyang, Henan. To mitigate the risks associated with HM pollution in the YRB, it is essential to implement a comprehensive set of measures. This includes strengthening industrial pollution control, promoting sustainable agricultural practices, and improving water quality management.