Particulate matter (PM) pollution is a significant issue in atmospheric environmental governance, negatively impacting air quality, visibility, climate change, atmospheric radiation, and human health (1). In recent years, it has garnered widespread attention in the field of atmospheric science (2–4). PM_2.5, defined as particulate matter with an aerodynamic equivalent diameter of 2.5 μm or less, has a larger specific surface area, allowing it to effectively accumulate organic compounds, viruses, bacteria, metals and metalloids (5). Over the past decades, a significant amount of epidemiological evidence has indicated PM_2.5 is associated with diseases. There is a positive correlation between short-term exposure to PM_2.5 and cardiovascular and respiratory diseases (6), while long-term exposure is significantly correlated with increased mortality from diabetes, cardiovascular disease, lung cancer, and chronic obstructive pulmonary disease (7–10). A study from Thailand found that if PM_2.5 concentrations reached the World Health Organization’s (WHO) short-term gold standard of 15 μg/m³, approximately 8 premature deaths per 100,000 people could be prevented. Moreover, if PM_2.5 reached the WHO’s long-term gold standard of 5 μg/m³, an estimated 159 premature deaths per 100,000 population could be avoided (11). Furthermore, an in vivo experiment in mice confirmed that lung metabolite levels were perturbed after PM_2.5 intratracheal instillation, mainly affecting metabolic pathways such as citric the acid cycle, pyruvate metabolism, purine and pyrimidine metabolism, as well as valine, leucine, and isoleucine metabolism.

In addition, PM_2.5 exposure affected the relative abundance of Ruminococcaceae, Enterobacteriaceae, and Pseudomonas (12). Moreover, Tong-Hong Wang et al. (13) found that short-term (24-h) exposure to PM_2.5 could activate the EGFR pathway in lung cancer cells, while long-term (90-day) exposure promoted tumor progression through the activation of EGFR and AhR, by enhancing the TMPRSS2-IL18 pathway. Metals and metalloids, as the important components of PM_2.5, through inhalation, skin exposure, and oral exposure. They are widespread public concern due to their high toxicity, persistence, bioaccumulation and potential health risks to ecosystems and humans (14, 15). Research has shown that Long-term exposure to high concentrations of metals and metalloids may lead to cardiotoxicity, neurotoxicity, immunotoxicity, and cancer, thereby increasing mortality rates (16, 17).

There is evidence suggesting a close relationship between metal accumulation (such as Zn, Mn, Cu and Ni), abnormal protein expression, and the pathogenesis of neurodegenerative diseases (18). Additionally, Baoman Li et al. (19) found that excessive intake of metals and metalloids (Such as Hg, Pb, As, Al, Bi, Zn, Cu, Cd, Ni, Mn, and Fe) could disrupt the homeostasis and neuroprotective functions of astrocytes, including the glutamate/GABA-glutamine shuttle, antioxidant mechanisms, and energy metabolism, ultimately leading to neuronal damage and initiating neurodegenerative diseases. The skeletons serve as long-term storage site for lead and other metals, accounting for approximately 90% of the total body burden in mammals. Studies have shown that metal exposure (especially as Hg, Pb, Cd, Cr, Al, and Ni) increases the risk of osteoporosis and fractures (20). Moreover, toxic metal ions could exert harmful effects by strongly interacting with essential biomolecules, replacing vital metal ions in proteins, enzymes, or hard structures such as bones and teeth. Additionally, metals with redox properties are key inducers of reactive oxygen species, which may lead to oxidative stress and cellular damage (21).

Although metals and metalloids account for less than 10% of PM_2.5, it is crucial to analyze their pollution characteristics and the impacts on human health due to their toxicity and enrichment effects. In recent years, many scholars have focused on assessing the health risks of metals in PM_2.5. Data from Kolkata illustrated that concentrations of carcinogenic metals, such as Ni, Pb, Cd, and Cr (VI), exceeded the guideline limits set by U. S. Environmental Protection Agency (USEPA) (22). A study conducted in a smelting district in Northeast China found that Cd and Pb in PM_2.5 posed non-carcinogenic health risks, while Cd also presented a potential carcinogenic risk to human health (23).

Literature showed that PM₁₀ may originate from resuspended soil and road dust, construction, crystalline sea salt, etc. PM_0.1 and PM_2.5 are direct emissions from human activities such as open burning, fuel burning, wildfire smoke, as well as secondary formation of volatile organic compounds, nitrogen oxides, and sulfur oxide pollutants (24). Phairuang et al. (25) found that the primary sources of PM_0.1 were derived from road traffic, industrial sector, and biomass combustion, in Bangkok, Thailand. The chemical composition and risk assessment of PM_2.5 were investigated in the brick kiln site and roadside by Ahmad et al. results indicated the combustion sources were the main source of PM_2.5 (26). In order to reduce the health risks caused by PM2.5, it is necessary to monitor the pollution situation, assess potential release sources and risks, as to take measures to control pollution emissions and protect the health of the population.

Wuxi, a water-bound city located in the lower reaches of the Yangtze River in China, had a permanent residential population of 7.49 million and a Gross Domestic Product (GDP) of 1.55 trillion RMB by the end of 2023. In terms of per capita GDP, Wuxi ranked third in China in 2023. As an economically and socially developed city, Wuxi faces environmental challenges that have triggered a series of health impacts (27–31). A study demonstrated that As and Sb in the drinking water represented an increasing risk to human health (27). Zhu et al. (28) found that cancer risk from the cumulative cancer risk trihalomethanes (THMs) exposure in the drinking water was 1.26 × 10⁻⁴ while the non-cancer risk was 2.02 × 10⁻¹. Additionally, evidence showed that prenatal exposure increased the risk of preterm birth and reduced gestational age in Wuxi (29). As is well known, differences in climate, geographical conditions, socio-economic structure, and lifestyle can result in varying degrees of environmental pollution. Therefore, long-term monitoring and analysis of PM_2.5 in Wuxi are of great significance.

In Wuxi, there have been few studies on the characteristics, sources, and health risk assessments of PM_2.5, particularly concerning metals and metalloids in the atmosphere. Wu Keqin et al. (32) studied metals in PM_2.5 from 2016 to 2020, but their study had limited elements and neglected the sources of metal elements. This article comprehensively monitors the pollution status of PM_2.5, analyzing the sources and potential health risks of 26 metals and metalloids in PM_2.5 from 2020 to 2023, including Sb, Al, As, Be, Cd, Cr, Hg, Pb, Mn, Ni, Se, Tl, Ba, Co, Cu, Fe, Mo, Ag, Th, V, Zn, Sr., Sn, Li, U, and Rb. The aim is to provides strategies for air pollution prevention and the protection of public health.

Air pollution has become a major public concern due to its association with various diseases, including lung cancer, cardiovascular disease, bladder cancer, childhood leukemia, dementia, and immune system disorders, all of which can contribute to abnormal death (48–51). According to WHO, approximately 2.4 million people die each year due to the health effects of air pollution (52). Among various pollutants, PM_2.5 is considered one of the most important environmental risk factors, contributing to cardiovascular disease (53), reduced childhood intelligence (50), asthma (54), chronic obstructive pulmonary disease (55), and allergic disease (56).

During the monitoring period, the concentration of PM_2.5 in Wuxi showed a significant downward trend, indicating that air pollution control measures have had a certain effect. Additionally, the proportion of clean days relative to the total number of monitoring days increased compared to the standard, suggesting that the emission reduction efforts in Wuxi have been effective over the past 4 years. This enhancement can be attributed to the implementation of “Action Plan for Air Pollution Prevention and Control” and the “Three Year Action Plan for Winning the Blue Sky Defense War.” In recent years, numerous policies have been continuously introduced across China to improve air quality and protect public health, with considerable results. For instance, the annual average concentration of PM_2.5 in China decreased from 61.8 μg/m³ to 42.0 μg/m³ in 2017 (57). Similarly, in Suzhou, located southeast China, the average PM_2.5 concentration declined from 51.2 ± 30.1 μg/m³ (2019) to 43.9 ± 25.0 μg/m³ (2021) (34). In the Beijing-Tianjin-Hebei (BTH) Region in northern China, the annual average PM_2.5 concentration decreased from 98.9 μg/m³ in 2013 to 64.9 μg/m³ in 2017 (58).

However, the annual average concentration of PM_2.5 in Wuxi was 40.4 ± 26.1 μg/m³, which still exceeded both the CNAAQS IT-1 (35 μg/m³) and the WHO guideline of 5 μg/m³. This indicates that the air pollution remains concerning and requires serious attention.

It was showed that PM_2.5 concentrations in winter and spring were significantly higher than those in summer and autumn (p < 0.05). This seasonal trend is consistent with findings from Suzhou (34) and Hangzhou (59), and may be attributed to lower temperature, reduced precipitation, lower wind speeds and lower relative humidity during winter and spring, resulting in difficult for pollutants to disperse. Therefore, environmental protection departments should focus on atmospheric conditions during these patterns and increase monitoring frequency accordingly.

During the monitoring period, Fe, Al and Zn were identified as the main components of PM2.5-bound metals and metalloids in Wuxi. It is well known that Al and Fe are the third and fourth most common elements in the nature, accounting for approximately 7.73 and 4.75% of its total weight, respectively. In contrast, Zn is mainly associated with tire wear, vehicle exhaust emissions, and the use of Zn in rubber products. This is particularly related in Wuxi, where, as of 2023, there were almost 2.72 million registered vehicles and approximately 105 rubber factories, contributing increasingly to Zn emissions.

The annual mean concentrations of As, Cd, Hg and Pb in Wuxi were significantly lower than the limits set by the CNAAQS. However, it is noteworthy that the annual mean concentrations of total Cr were 3.75 ± 1.78 ng/m³. Although previous studies showed that Cr (VI) accounts for about 1/7 of total Cr (60), the estimated concentration of Cr (VI) still exceeded the CNAAQS (0.025 ng/m³) by a factor of 20. Cr (VI) is classified as a Class I carcinogen by the International Agency for Research on Cancer (IARC) and enter the human body through ingestion, inhalation or touch with skin and mucous membranes. Therefore, it is necessary to pay special attention to Cr (VI) exposure. In this study Cr (VI) concentration were defined based on literature. Future research should focus on speciation analysis of chromium to provide a more accurate risk assessment associated with Cr (VI).

The concentrations of Co, Sr., Cu, Fe, Rb, Tl, Zn, Cd, Al, Ba, V, Se, and Mn in PM_2.5 showed a decreasing trend to varying degrees, as analyzed by Scheffe’s test or the Kruskal Wallis H test, as Co showed the largest reduction in concentration, followed by Sr. and Cu This trend was consistent with the decline in PM_2.5, as positive correlations were observed between PM_2.5-bound metals and metalloids and PM_2.5 in Wuxi. This may be due to the implementation of the Action Plan for Air Pollution Prevention and Control and the Three Year Action Plan for Winning the Blue Sky Defense War. Among them, measures such as reducing coal combustion, promoting green travel, and using clean energy have led to a decrease in PM_2.5 related metal concentrations. The seasonal patterns of metals and metalloids in PM_2.5 can be attributable to source emissions, meteorological factors, anthropogenic activities, and environmental transport. As shown, the concentrations of PM_2.5-bound metals and metalloids in Wuxi exhibited obvious seasonality. In addition to meteorological factors, this may be due to the increased use of coal-fired heating in spring and winter, which leads to significant emissions of sulfates and nitrates. These emissions improve the acidity of droplets, increasing the solubility of metal elements and making them more readily captured by particulate matter (61–63).

In this study, the EFs of Se, Mo, Cd, Ag, Sb, Zn, Sn, Pb, Cu, As, and Tl were above 100, indicating that these elements were mainly influenced by anthropogenic sources, exhibiting significant high enrichment and severe pollution, which needs attention. On the other hand, Ni, Mn, Cr, Sr., Co, Ba, Li, and V showed moderate enrichment. Spearman correlation analysis revealed strong correlations between various pairs of metals, such as Mn and Ni, Zn, Fe; Sb and As, Se, Pb; Tl and Cd, Pb, Rb and Sb; Pb and Rb; Zn and Fe; Sr. and Ba; Zn and Cu; Cr and Mn, Ni; Cd and Pb. Based on the EFs and these correlation results, the particular sources of Se, Mo, Cd, Sb, Zn, Pb, Cu, As, Tl, Ni, Mn, Cr, Sr., Co, Ba, and Li needed special attention.

To accurately identify the sources of metal pollutants, the PMF5.0 was utilized in this study. As illustrated in Figure 5, the contribution rates of industrial emissions, automotive emissions, fuel emissions, and dust during the monitoring period in Wuxi were 32.4, 27.9, 26.2, and 13.5%, respectively. According to the Statistical Bulletin on National Economic and Social Development of Wuxi City, from 2020 to 2023, the added value of industrial enterprises above designated size reached 396.9, 492.6, 558.6 and 600.5 billion RMB, increasing by 6.60, 12.9, 5.40 and 7.80% year-on-year, respectively. Industry plays an essential role in Wuxi’s economic and social development. However, as a major source of PM_2.5-bound metal pollution, it is necessary to optimize the industrial structure and accelerate industrial transformation while maintaining development. The contributions from automobile emissions and combustion suggest the importance of promoting green transportation, clean energy, and transitioning from thermal power to wind and hydropower. Notably, a waste incineration power plant has been built in the east area of Wuxi, which helps control city’s living garbage and provides a cleaner, recyclable energy source compared to conventional electric power generation. Additionally, to control soil dust pollution, attention should be made to enforce civilized construction practices, promote enclosure engineering, harden exposed roads, and wet operations measures.

Regarding non-carcinogenic risks, the HQs of Sb, Al, As, Be, Cd, Cr (VI), Hg, Pb, Mn, Ni, Se, Co, Cu, Mo, V and Zn were all below 1 in Wuxi, indicating that long-term exposure to these metals and metalloids is unlikely to cause adverse health effects. Moreover, the non-carcinogenic risks for residents in Wuxi were lower than other cities in southeastern China, such as Hangzhou and Ningbo (59).

However, it is noteworthy that the risk from inhalation accounts for only a small portion compared to oral and skin exposure routes (64). Therefore, although the non-carcinogenic risk from inhalation remains within an acceptable range, the overall risk of human exposure to various environmental elements should not be overlooked.

As Mn has the highest HQ among all elements in this study, its excessive accumulation in the central nervous system may lead to neurotoxicity, cussing brain disease (65–67). And the disturbance of manganese homeostasis caused by excessive intake of manganese is related to the occurrence of osteoporosis, obesity, type 2 diabetes/insulin resistance, non-alcoholic fatty liver, atherosclerosis and other diseases (68). Co-exposure to manganese, lead, and chromium may exacerbate oxidative stress (69). Al, which had the second highest annual average concentration during monitoring period, has been associated with Alzheimer’s disease, epilepsy and autism (70). As a result, the potential health risks of Mn and Al should not be ignored, and more studies are needed to explore their non-carcinogenic influences on local residents. Notably, the HQs value of Mn in spring and winter compared to summer and autumn, indicating that the non-carcinogenic risks to Wuxi residents posed by Mn in colder seasons need particular attention.

For carcinogenic risks, the RISK values of Cd, Pb, and Ni were below 1 × 10⁻⁶ for all seasons during the monitoring duration, while the RISK values of Cr (VI) and As were between 1 × 10⁻⁶ and 1 × 10⁻⁴. This illustrates that the RISK of Cd, Pb, and Ni via inhalation was negligible.

As the RISK value of Cr (VI) and As were higher than 1 × 10⁻⁶ for all the seasons. Excessive intake of As will lead to skin cancer, lung cancer, bladder cancer, liver cancer, cardiovascular disease and nervous system damage (71, 72). It was indicated that combined effect of As and Cd were synergistic effect, and Cd may potentiate As nephrotoxicity during the long-term (73). Cr (VI) is related with carcinogenic, genotoxic, and mutagenic effects (74, 75). Chronic exposure to Ni, Cr (VI) or As has long been known to increase cancer incidence among affected individuals (76). Therefore potential risks to human body after long-term exposure should be taken seriously.

Moreover, the combined effects of exposure multi-metal cannot be ignored. In summary, Cr (VI) and As could be considered key carcinogenic risk factors affecting the health of local residents, which was consistent with previous observations in Suzhou (34) and Zhejiang Province (59). Hence, it is necessary for the local government to take pertinent actions to control the emission sources of PM_2.5-bound metals and metalloids.

This study provides a four-year reference for experimental and field studies on PM_2.5. The annual average concentration of PM_2.5 in Wuxi from 2020 to 2023 was 40.42 ± 26.11 μg/m³, showing a downward trend over the years. This shows that Wuxi has achieved certain results in environmental protection and energy conservation over the past 4 years. However, although the annual mean concentration of PM_2.5 in Wuxi met the CNAAQS IT-2 standard, there was still a gap compared to CNAAQS IT-1 and WHO standards, suggesting that the energy conservation and emission reduction should not be overlooked. During the monitoring period, the concentrations of PM_2.5 indicated clear seasonal distribution characteristics, with levels in winter and spring significantly higher than those in summer and autumn.

Fe, Al, Zn, Mn, Pb, Cu, and Ba were seven dominant metals in PM_2.5 accounted for 95.7% of TMs. During the monitoring period over the past 4 years, the concentrations of most metals in PM_2.5 showed a remarkable decline, as Co showed the largest reduction in concentration, dropping by 55.1%, followed by Sr. and Cu. Seasonal variation could observed in Pb, Zn, Ba, Ag, Rb, Al, Cd, Sb, Tl, Sr., Mn, Ni, and As, with higher concentration in winter.

PMF analysis revealed that metals in PM_2.5 in Wuxi City mainly generated from fossil fuels combustion, industrial pollution, vehicle emissions, and construction dust pollution. The sequence of non-carcinogenic metals by their mean HQ values was found to be Mn > As > Pb > Cd > Co > Ni > Al > V > Mo > Sb > Cr (VI) > Zn > Cu > Se. And the sequence of carcinogenic metals by their mean RISK values was found to be As > Cr (VI) > Cd > Ni > Pb, During the monitoring period.

Considering both non-carcinogenic risk and carcinogenic factors, the risk levels of individual elements monitored during the study were within acceptable range according to EPA. However, it is worth noting that PM_2.5 in the air can enter the human body not only through inhalation but also through both oral and skin exposure. Hence, long-term exposure risks multiple pathways should not be ignored, especially for Mn, which had the highest HQ in the monitored metals.

Moreover, the RISK values of As and Cr (VI) were above 1 × 10⁻⁶, indicating that the potential health risks from long-term exposure cannot be ignored. It is also noteworthy that even after conversion by 1/7, the concentration of Cr (VI) still exceeded the CNAAQS standard. This highlights the need for the accurate detection of Cr (VI) concentrations to precisely estimate its risk, as it remained the most hazardous metal during the monitoring period.

This study suggests that future efforts should not only continue promoting clean energy, green transportation, civilized construction and decreasing industrial pollution emissions to reduce the PM_2.5 concentration, but also increase the frequency of air monitoring in winter and spring. Additionally, targeted measures should be taken to reduce the concentrations of Cr (VI), As and Mn, to better protect the health of local residents in Wuxi.