The study aimed to determine the association between lung function and 10 heavy metals exposure in a typical rural population of northwest China, and to explore the possible effect of solid fuels use. There are many studies focusing on the effect of heavy metals on lung function, while most of their studying subjects are occupational population and very few studies have examined in rural populations of northwest China. This study provided evidence on the association between metal mixtures exposure with a higher risk of lung function decline in this population. Our results consistently showed that the increase of heavy metals was significantly associated with lung function decline and increased risk of PVD.
Several cohort studies have shown that some heavy metals were significantly associated with lung function decline and the morbidity of COPD(Dai et al. 2019; Heo et al. 2017; Leem et al. 2014; Zeng et al. 2016). A retrospective study of ferromanganese refinery workers showed that manganese dust could impair pulmonary ventilation function in male workers(Wang et al. 2015). There is also evidence that Mn overexposure may be associated with reduced FEV1% and FVC%(Madrigal et al. 2018). In a Korean cohort study of COPD, they found serum Hg level may be a risk factor for reduced lung function and obstructive lung disease(Heo et al. 2017). Our study presents similar results, in which we found Hg and Mn showed hazardous effects on lung function and the solid fuels use increased the risk of PVD. It is well-known that elemental Hg is liquid even at ordinary temperature and can easily evaporate to produce steam so that it can enter the body more easily through the respiratory tract(Balali-Mood et al. 2021). Some animal experiments have shown that epithelial cells and cell fragments were observed in the lungs of Hg exposed rats, leading to occlusion of the bronchiole lumen(Koopsamy Naidoo et al. 2019). At the same time, other studies have shown that type II alveolar cells are susceptible to oxidative stress, and Hg and Mn can induce type II epithelial cell damage through oxidative stress-related mitochondrial cell death pathway(Lu et al. 2010).
Mn and Hg were the main metals affecting FEV1% and FVC% in lung of rural population in northwest China, which may be related to the local heavy metal background value and the use of coals heating in winter(Zhang & Wang 2020). It is well-known that the particles from coals burning contain trace amounts of heavy metals such as Pb, Hg, As, Cd, Cr, Mn, Sb and so on, which can accumulate in their bodies(Munawer 2018). Moreover, studies have demonstrated the ability of the PM2.5 surface in absorbing a variety of toxic entities, such as organic compounds and toxic metals(Duan et al. 2012; Li et al. 2021). The indoor level of PM is relatively high in winter of northwestern rural house due to heating or cooking with solid fuel(He et al. 2021; Zhou et al. 2016). Studies have reported that Sb has a relatively high concentration in coal ash(Fernandez-Turiel et al. 1994). Therefore, Sb is more likely to be inhaled in daily life for cooking or heating. Previous studies have shown that Zn in kitchen dust comes from the combustion of solid fuels(Wang et al. 2020b). Our results of subgroup analysis also showed a higher risk of Sb-associated and Zn-associated PVD in people using solid fuels.
Interestingly, lower dose of heavy metals is common in our environment and diet, and even some of them are essential for health benefit. For example, organisms require different amounts of heavy metals, some trace elements such as Mo, Co, Cu, Zn, all of which are essentially required by humans(Fernandez-Luqueno et al. 2013). Generally speaking, Cu and Zn at low concentrations have been confirmed to be components of various proteins and enzymes and participate in redox reactions(Jomova & Valko 2011). However, the subjects in this study lived near a mining area and non-ferrous metals smelting area. Although production has been stopped at present, heavy metals like Co, Cu and Zn residues in the soil were still high(Liu et al. 2017; Zhang et al. 2022). In general, heavy metals would produce toxic effects when their concentration exceeds a certain threshold(Rahman & Singh 2019). Thus, it is reasonable that Co and Cu were negatively correlated with FEV1/FVC in our study. Previous studies have shown that heavy metals are mostly harmful to human body through the soil-grain-human system, and grain consumption is one of the main ways of human exposure to toxic elements(Liu et al. 2017; Liu et al. 2013). Most of these local planted grains were consumed by local residents, so it was likely that they might have a negative impact on the health of our study population. Besides, Mo and Pb showed a potential protective effect. This may be related to the fact that Mo are essential trace elements in human body(Fernandez-Luqueno et al. 2013). And Low level of Pb exposure has been reported to have immune-stimulating effects compared to higher exposures that normally lead to immunosuppression(Mishra 2009).
In further subgroup analysis, we found that males had a greater risk of PVD than females among Zn, Hg and Mn. This may be related to the higher smoking rate in males than females, indicating the possible synergistic effect between nicotine and heavy metals(Perez et al. 2020; Wei et al. 2021). In addition, the participants using solid fuels and BMI ≥ 24kg/m2 were more likely to suffer from PVD. Some studies have shown that obesity significantly interferes with respiratory function by reducing lung volume, especially expiratory reserve and functional residual volume(Mafort et al. 2016). There is evidence that restrictive PVD usually shows a decrease in FEV1 and a constant or slight increase in FEV1/FVC(Sobel et al. 2022). In this study, we mainly showed that heavy metals had adverse effects on restrictive PVD, but the relation of heavy metals exposure with lung function indexes was not obvious. This may be limited by the small sample size of our study population, which needs further study.
In this study, we found a relationship between metal mixtures and lung function using several statistical methods, especially among people who used solid fuels. We obtained the same results from various analysis, which proved the validity of the conclusion. However, many limitations should not be ignored. Firstly, this study was a cross-sectional study, so we could not determine the causal relationship between heavy metals exposure and lung function. In the next place, urinary heavy metals sometimes do not reflect individual exposure, and blood samples may be more representative. Thirdly, recall bias may exist because questionnaire data were self-reported. However, face-to-face surveys were conducted by trained investigators to a uniform standard, as well as each completed questionnaire was logically checked. Fourthly, this study was only targeted at the population in the rural areas of northwest China, so the sample size may be small and may be limited in applying to other population. Then, since the source of food is not very clear, we didn’t include that for analysis in this study. In addition, this study did not analyze the specific sources of heavy metals, which should be strengthened in future studies so as to carry out targeted prevention and control of heavy metal pollution.