Cadmium exposure in marine crabs from Jiaxing City, China: Insights into health risk assessment

2.1 Instruments and reagents

An inductively coupled plasma mass spectrometer (NEXION 350D, PerkinElmer, USA), atomic absorption spectrophotometer (ZEEnit650P, Jena, Germany), microwave digestion system (MARS6, CEM, USA), electronic balance (AL204, METTLER TOLEDO), and ultra-pure water system (Milli-Q, Millipore Corporation, USA) were used. Cadmium standard: 1,000 mg/L, GBW08666; GSB-15 Scallop Quality Control Material: Cd 1.06 ± 0.10 mg/kg GBW10024 were also used.

2.2 Sample collection and pretreatment

In accordance with the requirements of the National Food Safety Standard for the Determination of Cadmium in Food (GB 5009.15-2014) [25], samples of seawater and freshwater crabs were collected and monitored for cadmium content from 2015 to 2023. A total of 202 seawater crab samples (including green crabs, flower crabs, bread crabs, and swimming crabs) and 32 freshwater crab samples (including river crabs and Chinese mitten crabs) were collected. Additionally, 27 seawater crab samples were analyzed for cadmium content in different edible parts (crab paste, crab roe, and crab leg muscle). Each sample collected was higher than 1 kg. After weighing all seawater crab samples, the edible portions were homogenized and stored in a freezer (−20°C) until analysis.

2.3 Sample detection method

Referring to the National Food Safety Standard for the Determination of Cadmium in Food (GB 5009.15-2014) and the National Food Safety Standard for the Determination of Multielements in Food (GB 5009.268-2016) [26], accurately weighed and homogenized samples of 0.300 0 g (precise to 0.000 1 g) were taken. Then, 5 mL of nitric acid and 3 mL of hydrogen peroxide were added to each sample, and the mixture was covered overnight. The following day, microwave digestion was carried out at 190°C for 25 min. After digestion, the solution was heated to near dryness and brought to a volume of 20 mL with 0.15 mol/L nitric acid. For high-concentration samples, dilution with 0.15 mol/L nitric acid was performed. Samples from 2015 to 2017 were analyzed using AAS, while samples from 2018 to 2023 were analyzed using ICP-MS.

2.4 Quality control

To ensure the accuracy of the detected data for cadmium content in the samples, a standard curve with a correlation coefficient greater than 0.999 was plotted before each analysis. Quality control for the detection method is conducted through precision experiments and testing with scallop quality control materials. The cadmium content in the scallop quality control samples was determined to range from 1.01 to 1.13 mg/kg, with all results falling within 1.06 ± 0.10 mg/kg. The relative standard deviations (RSDs) were all below 10%. The limit of detection (LOD) of the method was calculated as three times the standard deviation of 20 blank sample solutions, resulting in a standard deviation of 0.0052. Based on a sample weight of 0.3 g and a dilution volume of 20 ml, the LOD for this method was determined to be 0.001 mg/kg.

2.5 Pollution analysis

2.6 Statistical description

In Microsoft Office 2016, a database was established, and for samples with detection results below the LOD, the recommended substitution method by the WHO was applied, incorporating results at ½ LOD into the statistical analysis [32]. Data statistical analysis was conducted using SPSS 22.0 software, charts were created using Origin 2021 software, and Oracle Crystal Ball 11.0 software was employed for distribution goodness-of-fit tests to determine the most suitable distribution model and its parameters, along with Monte Carlo simulations. The detection results in this study were confirmed as non-normally distributed through the Kolmogorov–Smirnov (K-S) test (P < 0.05); thus, it is appropriate to use the median and interquartile range to describe the central tendency and dispersion of the data.

3.1 Overall distribution of cadmium content in marine and freshwater crabs

Among the 202 marine crab samples, the Cd content ranged from 0.091 to 21.136 mg/kg, with a median value of 1.360 mg/kg, primarily concentrated between 0.091 and 5.000 mg/kg. In contrast, the 32 freshwater crab samples exhibited a cadmium content range of 0.045 to 2.151 mg/kg, with a median value of 0.249 mg/kg, predominantly concentrated between 0.045 and 1.000 mg/kg. Comparatively, the cadmium content in freshwater crab samples was significantly lower than that in marine crabs. The distribution of cadmium content is illustrated in Figure 1. Existing studies indicate that the level of heavy metal contamination in marine crabs is significantly higher than that in freshwater crabs, which is consistent with the findings of this study [33]. This phenomenon may be related to the richer sources of cadmium in marine environments. Additionally, studies have pointed out that cadmium is one of the major heavy metal pollutants in marine areas, and marine crabs exhibit more pronounced bioaccumulation effects due to their position higher up the food chain [34].

Figure 1

Distribution of cadmium content in freshwater crab (a) and marine crab (b).

3.2 Cadmium content in different crab species

The Kruskal–Wallis non-parametric test revealed significant differences in cadmium content among different crab species (P < 0.05). The cadmium exceedance rate in marine crabs was 15.8%, while no exceedance was observed in freshwater crabs. The median cadmium content in marine crabs (1.360 mg/kg) was 5.6 times higher than that in freshwater crabs (0.249 mg/kg). Among different marine crab species, the median cadmium content was ranked as follows: freshwater crabs (0.249 mg/kg) < green crabs (1.070 mg/kg) < flower crabs (1.226 mg/kg) < swimming crabs (1.422 mg/kg) < bread crabs (1.896 mg/kg). This is consistent with the findings of the existing literature [35], as shown in Table 2.

The observed differences may be related to their habitats. Green crabs and flower crabs typically inhabit coastal areas, estuaries, and mangrove zones where freshwater and seawater mix. These regions are closer to land, which may explain their relatively lower cadmium levels [35]. In contrast, swimming crabs and bread crabs usually reside in offshore or deep-sea areas, which are less affected by terrestrial pollution but may be influenced by ocean currents and deep-sea sediments. The longer accumulation time in these environments could be a reason for their higher cadmium content. Additionally, body size also has some influence on cadmium accumulation. The bread crab, with the largest body size, exhibits the most severe cadmium contamination, while the freshwater crab (Chinese mitten crab), with the smallest body size, shows the lowest cadmium contamination. The relationship between body size in different marine crab species is shown in Figure 2.

Figure 2

Size comparison chart of adult bread crab, green crab, flower crab, swimming crab, and Chinese mitten crab (freshwater crab).

3.3 Detection of cadmium elements in different parts of marine crabs

The median cadmium content in marine crabs (1.360 mg/kg) was 5.6 times higher than that in freshwater crabs (0.249 mg/kg). Cadmium concentrations in different parts of 27 marine crab samples were measured, and the results indicated that the median cadmium content in crab roe and hepatopancreas was 2.730 mg/kg, with 11 samples exceeding the standard limit, resulting in an exceedance rate of 40.7%. The median cadmium content in the crab breast muscle was 0.138 mg/kg, and in the crab leg muscle, it was 0.079 mg/kg. Neither the breast muscle nor the leg muscle exceeded the national limit of 3.0 mg/kg. The Kruskal–Wallis test indicated significant differences (P < 0.05) between the cadmium concentrations in the breast muscle, leg muscle, and crab roe/hepatopancreas. The cadmium levels ranked from low to high as follows: crab leg muscle < crab breast muscle < crab roe/hepatopancreas, with more than 88% of the cadmium accumulation occurring in the crab roe/hepatopancreas, as shown in Figure 3. According to relevant studies, crab roe primarily consists of the ovaries and digestive glands of female crabs, while the hepatopancreas mainly consists of the accessory glands and secretions of male crabs. Both include the liver and gonads of crabs, which are known to be primary sites of heavy metal cadmium accumulation [36]. Therefore, the high cadmium content in crab roe and hepatopancreas, along with the severe exceedance of standards, is consistent with previous research findings. Consumers should minimize their intake of crab roe and crab fat to reduce health risks (Figure 3).

Figure 3

Percentage of cadmium content in crab legs, thorax, and roe/paste (a) and anatomical diagram of crab parts (b).

3.4 Detection results of samples from different years

In a total of 202 marine crab samples collected from 2015 to 2023, the median cadmium content was highest in samples from 2021, at 1.647 mg/kg, and lowest in samples from 2017, at 1.081 mg/kg. The Kruskal–Wallis test revealed no statistically significant differences in cadmium content among different years (P = 0.101). However, the trend chart of median content showed an increasing trend in cadmium content. For specific results, refer to Figure 4. In recent years, the Jiaxing Port Area has focused on implementing industrial transformation and upgrading while continuously promoting the high-quality development of its chemical industrial park. The industrial output growth rate has exceeded 7.4%. The growth of chemical enterprises may be one of the factors contributing to the increased cadmium accumulation. However, the annual sample size may introduce bias into the results, potentially affecting the reliability of the conclusions. Therefore, future research should aim to expand the sample size and optimize sampling design to enhance the accuracy and representativeness of the findings. This will provide more reliable evidence for the prevention and control of cadmium pollution.

Figure 4

Trend of median cadmium concentrations in marine crabs from 2015 to 2023.

3.5 Cadmium content in female and male marine crabs

Out of all 106 swimming crab samples, there were 50 male samples and 56 female samples. The median cadmium content in male swimming crabs was 1.382 mg/kg, with a detection range of 0.138–9.820 mg/kg. Six samples were found to exceed the standard, resulting in an exceedance rate of 12.0%. The median cadmium content in female swimming crabs was also 1.500 mg/kg, with a detection range of 0.091– 8.587 mg/kg. Eight samples exceeded the standard, leading to an exceedance rate of 14.3%. The Kruskal–Wallis test showed no statistically significant difference in cadmium content between male and female swimming crabs (P = 0.870). As shown in Table 3, the range of cadmium detection values in male swimming crab samples was slightly higher than that in female swimming crabs, which is consistent with previous research findings [37].

3.6 Cadmium concentrations in marine crabs from different counties and districts in Jiaxing City

According to the Kruskal–Wallis test (P = 0.888), there were no statistically significant differences in cadmium levels among seawater crabs from various counties and districts of Jiaxing City (Nanhu, Xiuzhou, Haining, Haiyan, Jiashan, Tongxiang, and Pinghu). This may be attributed to several factors. First, crabs from different regions might share similar habitats and environmental conditions, leading to comparable levels of cadmium accumulation. Second, the biological characteristics of the crabs, such as their feeding habits and physiological processes, may be consistent across regions, resulting in similar patterns of cadmium uptake and distribution. Finally, the sample size or variability within each region might not have been sufficient to detect significant differences, suggesting that cadmium exposure and accumulation are relatively homogeneous across the studied areas, as shown in Table 4.

3.7 Pollution level rating in marine crabs

The single-factor pollution index (P _i ) method was used to assess cadmium pollution levels in collected seawater and freshwater crabs, with the median (P₅₀) representing general intake levels and the 95th percentile (P₉₅) representing high intake levels.

At the general intake level (P ₅₀), freshwater crabs are classified as clean (P _i = 0.083), while marine crabs fall under light pollution (P _i = 0.453), with bread crabs having the highest pollution level (P _i = 0.631), classified as moderate pollution. At the high intake level (P ₉₅), freshwater crabs are categorized as light pollution (P _i = 0.558), while all four marine crab types are classified as severe pollution (P _i > 1). The pollution index ranks from low to high as follows: freshwater crab < green crab < flower crab < swimming crab < bread crab. Regarding the cadmium contamination ratings of different parts, crab roe, and hepatopancreas are classified as moderate pollution (P _i = 0.910) at the general intake level (P ₅₀). At the high intake level (P ₉₅), crab roe and hepatopancreas are categorized as severe pollution (P _i = 3.370). The pollution index ranks from low to high as follows: crab leg muscle < crab breast muscle < crab roe, and hepatopancreas.

These results indicate significant differences in cadmium contamination levels among different crab species and their parts, with the highest contamination found in crab roe and hepatopancreas, particularly reaching severe pollution at high intake levels. It suggests that consumers should be especially cautious about reducing the consumption of crab roe and hepatopancreas to lower cadmium exposure risks, as shown in Table 5.

3.8 Cadmium intake and health risk analysis

The simulation results show that the median MOS value for marine crabs is 3.47, while for freshwater crabs, it is 17.14, indicating that at general dietary levels (P₅₀), the cadmium exposure risk from consuming these two types of crabs is low and within a safe range. However, at high intake levels (P₉₅), the MOS values for both swimming crabs and bread crabs are less than 1, suggesting a potential risk of cadmium exposure. In contrast, the MOS values for freshwater crabs, blue crabs, and flower crabs remain greater than 1, indicating that their health risks at high intake levels are still low. The overall risk ranking is as follows: freshwater crab < green crab < flower crab < swimming crab < bread crab. The distribution of MOS values for different types of crabs is shown in Figure 5.

Figure 5

Distribution of MOS values in marine crab (a), freshwater crab (b), green crab (c), flower crab (d), bread crab (e), and swimming crab (f).

Although the cadmium exposure risk for marine and freshwater crabs is low at general dietary levels, high intake of swimming crabs and bread crabs could pose health risks. Consumers should pay special attention to controlling their consumption of these crabs. Freshwater crabs, green crabs, and flower crabs still demonstrate low risk at high intake levels and can be considered relatively safer choices.

The simulation risk analysis results for cadmium intake from marine crabs in different populations indicate that at the general intake level (P₅₀), the MOS values for adult males, adult females, as well as adolescent males and females, are all greater than 1, indicating a low health risk from cadmium intake. However, at the high intake level (P₉₅), the MOS values for all populations are less than 1, suggesting a certain risk of cadmium exposure. The risk ranking is as follows: adult males (MOS 0.88) < adult females (MOS 0.76) < adolescent males (MOS 0.56) < adolescent females (MOS 0.50). The distribution of MOS values for different populations is shown in Figure 6.

Figure 6

Distribution of MOS values in adult males (a), adult females (b), adolescent males (c), and adolescent females (d).

Although the cadmium exposure risk from marine crabs is low at general intake levels, all populations face potential health risks at high intake levels, with adolescent females at the highest risk. Therefore, it is recommended that consumers, particularly adolescents and females, control their intake of marine crabs to reduce the potential health impacts of cadmium exposure.

3.9 Monthly intake estimations

According to the Provisional Tolerable Monthly Intake (PTMI) for cadmium established by JECFA, which is 0.025 mg/kg bw, monthly intake estimations were made. For the 202 samples of marine crabs measured, with an average weight of 432 g, the edible portion was calculated as 1/3 [38], which is 144 g. The cadmium content in marine crabs, calculated using the median value, is 1.360 mg/kg. Using Formula (4), the results show that adults and adolescents should not consume more than 8 and 5 marine crabs per month, respectively, to ensure that cadmium intake remains within the safe range.

These results indicate that although the cadmium content in seawater crabs is relatively high, the intake risk can still be reduced to a safe level by controlling consumption. However, considering factors such as individual body weight, dietary habits, and the potential accumulation of cadmium from other foods, it is recommended that consumers further reduce their intake of seawater crabs, especially for sensitive groups, such as adolescents, to minimize health risks to the greatest extent.