## 2 Materials and Methods  
### Study Area and Sample Collection  
In this study, we used daily measurements of PM\({}_{10}\), PM\({}_{10}\)-bound PAHs, and PM\({}_{10}\)-bound heavy metals (Ni, Cd, Pb, and metalloid As) collected from 2018 to 2022. The data was retrieved from the Chief Inspectorate of Environmental Protection (CIEP) website [61] in Poland. The air monitoring data were officially validated data. We selected those monitoring stations located in urban agglomerations that met the criterion of simultaneous measurement of PM\({}_{10}\) and PAHs such as benzo[a]anthracene B[a]A, benzo[a]pyrene B[a]P, benzo[b]fluoranthene B[b]F, benzo[j]fluoranthene B[j]F, benzo[k]fluoranthene B[k]F, dibenzo[a,h]anthracene DBA, indeno[1,2,3-cd]pyrene IP, and Ni, Cd, Pb, and As concentrations in the period 2018-2022. The selected sites include Tricity (Gdansk, PL0052A); Upper Silesia (Katowice-PL0008A); Cracow (PL0501A); Lublin (PL0085A); Lodz (PL0100A); Szczecin (PL0248A); Warsaw (PL0214A); and Wroclaw (PL0194A). The locations of the individual stations represent diverse environmental settings, including highly urbanised and industrialised areas (Upper Silesia, Wroclaw, and Lodz); agglomerations with dominant administrative and service functions (Warsaw, Cracow); coastal regions (Tricity, Szczecin); and cities with moderate levels of traffic and industrial activity (Lublin). The individual agglomerations also differ in terms of meteorological conditions. Tricity and Szczecin have a maritime climate characterised by high air humidity, frequent winds, and relatively mild winters that favour the dispersal of pollutants. However, frequent temperature inversions, especially during the heating season, expose Upper Silesia and Cracow, fostering the accumulation of pollutants near the ground. In addition, due to its location in a valley with limited air ventilation, Cracow has unfavourable conditions for the dispersal of pollutants. Warsaw, Lodz, and Wroclaw have transitional conditions between maritime and continental climates, with periodic stagnant air and varying wind speeds. Lublin, which lies in the valley of the Bystrzyca River, has numerous depressions of land and hills and is characterised by moderate circulation conditions and relatively few inversions. This arrangement of stations allows us to evaluate how risk patterns change in different environmental and human-made situations (Figure 1). Figure 1: Sampling sites in the Polish Agglomerations. Detailed information on the location of air quality monitoring stations and the number of inhabitants in individual agglomerations is shown in Table S1. At the air quality monitoring stations, particulate matter was manually collected over continuous 24 h periods using high-volume samplers operating at an airflow rate of 30 m\({}^{3}\).h\({}^{-1}\). Each sampler was fitted with a PM\({}_{10}\) inlet and a system for automatically placing quartz fibre filters into the sampling channel. The amount of PM\({}_{10}\) was measured using a standard method that involves weighing, following the PN-EN 12341:2014\(-\)07 standard [62], and it can detect levels as low as 1 \(\mu\)g\(\cdot\)m\({}^{-3}\). The PM\({}_{10}\)-bound polycyclic aromatic hydrocarbons (PAHs) were isolated using an organic solvent extraction method. Analysis was then carried out using high-performance liquid chromatography (HPLC) with fluorescence detection, in accordance with the PN-EN 15549:2011 standard [63]. This analytical technique enables the quantification of benzo[a]pyrene and its congeners within a concentration range of approximately 0.05 to 20 ng\(\cdot\)m\({}^{-3}\). The PM\({}_{10}\)-bound heavy metals (Ni, Cd, Pb, and As) were mineralised using microwaves and analysed using a graphite furnace and atomic absorption spectrometry, according to PN-EN 14902:2010 [64]. We apply this method across the following concentration ranges: Ni: 2-100 ng\(\cdot\)m\({}^{-3}\), Cd: 0.1-50 ng\(\cdot\)m\({}^{-3}\), Pb: 1-4000 ng\(\cdot\)m\({}^{-3}\), As: 0.5-350 ng\(\cdot\)m\({}^{-3}\).