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Sikkim, located in the Eastern Himalayas, is a prominent tourist destination. The state exhibits a diverse climatic range spanning from subtropical to alpine zones. Springs and glacier-fed rivers serve as the primary water sources for both residents and tourists. Furthermore, rapid urbanization, climate change, altered precipitation patterns and frequent landslides have significantly stressed these water sources, especially in the lower subtropical regions, resulting in declining water quality. This study aimed to assess heavy metal contamination in surface water, considering both natural and anthropogenic sources, and its impact on different population groups. A total of 155 water samples were collected from households, rivers, springs and hot springs across pre-monsoon, monsoon and post-monsoon seasons. Heavy metals (Al, As, Cd, Co, Cr, Cu, Hg, Fe, Mn, Ni, Pb and Zn) were analyzed, and water quality was assessed using various water pollution indices. The findings revealed that the water is unsuitable for consumption without treatment. A novel approach, Monte Carlo simulation, was employed in health risk assessment, incorporating sensitivity and uncertainty analyses. This method provided greater accuracy in evaluating both carcinogenic and non-carcinogenic risks, with the results indicating moderate to high cancer risks through ingestion and dermal absorption. It is strongly recommended that treated water be used for drinking to mitigate exposure to heavy metal contamination in the region., Competing Interests: Declarations. Ethical approval: Not applicable. Consent to participate: Not applicable. Consent for publication: All authors have read and agreed to the published version of the manuscript. Conflict of interest: The authors declare no competing interests., (© 2025. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

Polycyclic Aromatic Hydrocarbons (PAHs) compounds are ubiquitous in ambient air due to their persistence, carcinogenicity, and mutagenicity. Gangtok being one of the cleanest cities in India located in Eastern Himalayan region, witnesses high developmental activities with enhanced urbanization affecting the ambient air quality. The present study aims to measure PM 2.5 and PAHs in the ambient atmosphere of the Sikkim Himalaya to understand the influence of natural and anthropogenic activities on aerosol loading and their chemical characteristics. The PM 2.5 samples were collected and analysed for the duration from Jan 2020 to Feb 2021.The seasonal mean concentrations of PM 2.5 and PAHs were observed to be high during autumn and low during summer season. Overall, the annual mean concentration of PM 2.5 was found higher than the prescribed limit of World Health Organization and National Ambient Air Quality Standards. The concentration of the 16 individual PAHs were found to be highest during autumn season (55.26 ± 37.15 ng/m 3 ). Among the different PAHs, the annual mean concentration of fluorene (3.29 ± 4.07 ng/m 3 ) and naphthalene (1.15 ± 3.76 ng/m 3 ) were found to be the highest and lowest, respectively. The Molecular Diagnostic Ratio (MDR) test reveals higher contribution from heavy traffic activities throughout the winter and autumn seasons. The other possible sources identified over the region are fossil fuel combustion, and biomass burning. The multivariate statistical analysis (Multifactor Principal Component Analysis) also indicates a strong association between PM 2.5 /PAHs and meteorological variables across the region in different seasons. The precipitation and wind pattern during the study period suggests that major contribution of the PM 2.5 and PAHs were from local sources, with minimal contribution from long-range transport. The findings are important for comprehending the trends of PAH accumulation over a high-altitude urban area, and for developing sustainable air quality control methods in the Himalayan region., Competing Interests: Declaration of competing interest This work is original work and there is no conflict of interest., (Copyright © 2023 Elsevier B.V. All rights reserved.)