Exposure to excess Cd may pose health risks [31, 32]. Clinically ascertaining threshold concentrations of UCd is greatly important and aids understanding of the health risks. Sensitivity indexes for the UCd burden have been a topic of wide concerned [13, 19]. In response to our questionnaire, more than 50% of respondents indicated their awareness of suffering high Cd exposure for a certain time. The results of a preliminary investigation of UCd levels and sensitivity biomarkers are discussed here.
In our study, UCd values for females varied from 0.030 µg/g cr to 20.279 µg/g cr, higher than those found in past research, which has suggested the UCd levels in females ranged from 0.49 to 0.69 µg/g cr, as deduced from individuals aged 50–79 in the US [33]. It has been speculated that age is a factor that determines UCd levels [34], and this is validated here. We found that UCd levels in the population increased with age (Table 1).
Additionally, UCd levels were affected by gender. The mean UCd among females differed statistically significantly from that of males (Table 2). The odd number of female UCd was 1.62 times higher than that of male (p < 0.05) (Table 3). This indicates that the females had accumulated more UCd, perhaps due to increased mineral retention and increased consumption of Cd-contaminating food during pregnancy [35]. Recent work has shown that lower iron content in females than in males may be due to their different physiologies, which may allow for greater absorption of Cd by the female gastrointestinal tract [36, 37]. The suggestion in this study that females may be more sensitive to Cd-induced nephrotoxicity than males supports an earlier study [38].
Although mean levels of UCd and renal dysfunction were different for all age groups, an obvious dose–response relationship existed between UCd and the odds of renal dysfunction. This indicates that Cd exposure can cause irreversible damage to the kidney [19, 39]. For average UCd level, the odds of female were lower than that of males (Table 2); the reason might come from the sample sizes. Moreover, it has been found that endogenous hormones contribute to the UCd burden [40, 41]. The participants in our survey were all native residents, and most had been living in the study site more than 25 years. Long-term Cd exposure has been reported, and renal dysfunction has already been proven [42, 43]. Uβ2-MG and UNAG are in use as biomarkers for clinical diagnosis [13, 19] and animal experiments [44, 45]. In the later stages of Cd-induced renal injury, the tubular epithelial cells may shrink, fall off, and even disappear, and the tissue source of UNAG is lost [46]. The reabsorption function of the kidney is impaired when UNAG was released, and the excretion of Uβ2-MG may be greater than that of UNAG [47, 48], which may be why Uβ2-MG produces higher values than UNAG in biological monitoring. In addition, the correlation coefficients for Uβ2-MG affecting UCd were higher than those for UNAG (Table 4). Traditionally, Uβ2-MG could be more sensitive as a renal health effect indicator than UNAG.
A comparison of the threshold concentration of UNAG and Uβ2-MG is essential for assessing the BMD of UCd using index total protein, UNAG, and Uβ2-MG in non-occupationally endangered people [19, 49], and the relationship between the UCd burden and renal indexes was found to be statistically significant (p < 0.01), as cross-checked by different models.
It has been suggested that the accumulation and excretion of UNAG and Uβ2-MG increases significantly with the increase of Cd burden in the organism, and irreversible kidney damage becomes more serious as Cd exposure grows [48, 50].
In this study, subjects of different genders were divided into four groups according to UCd levels (0 to 2, 2 to 5, 5 to 10, and > 10 µg/g cr). The results for BMDs showed that odd had a significant dose–effect trend with the selected effects endpoint. UNAG and Uβ2-MG were found to have good fit to the logistic model through BMD calculation.
The BMDLs of UCd for UNAG and Uβ2-MG in females were lower than those for males according to the BMDs results (Table 6), which also indicated that females were more sensitive to Cd exposure than males.
To protect human health from the high Cd burden, greater restrictions on sensitivity biomarkers and the BMD should be considered. BMD values of UCd for different renal indexes were lower than the standard threshold (5 µg/g cr) (Table 6). More than 20% of people might be at risk for Cd exposure where the UCd level was around the national standard threshold value (5 µg/g cr) for BMR was 20% (Table 7). Following our results, we recommend a value of 2.2 µg/g cr.