Tea, an esteemed beverage recognized for its health properties, enjoys widespread consumption globally, including in Guangzhou, China[25]. Tea can be classified into various types based on its production process: non-fermented teas like green tea, semi-fermented teas such as oolong tea, fully fermented teas like black tea.[26, 27]. Certain research findings indicate that regular tea drinking can provide diverse health advantages, encompassing antioxidant attributes and anti-inflammatory effects., anti-cancer properties and cardiovascular protection[28–33].
With the rapid development of China's industry and the wide application of agricultural chemicals, heavy metals that penetrate into the soil can be absorbed by the roots of tea plants, resulting in heavy metal content in tea exceeding the safety standards[34, 35] and endangering human life and health[36]. For the six heavy metals of in this study, it has been shown that As has the effects of inflammation, carcinogenicity, oxidative stress and endocrine system disruption[37]. Pb can reduce bone mineral density and cause mineralization of teeth[38] while Hg can cause kidney damage[39] and both can potentially exert negative effects on the nervous, cardiovascular, and reproductive systems. [40–44] Cd can induce gene mutation, which contributes to cancer[45]. Cr can cause genotoxicity through generating metabolites and reactive oxygen species[46]. It has been confirmed in animal studies that Cu can induce haemolytic crises, severe gastroenteritis, and hepatocyte necrosis[47]. Therefore, the assessment of heavy metal content in tea is imperative for ensuring its quality and safety.
Our study collected and analyzed 50 samples of tea leaves from three districts in Guangzhou (Baiyun, Tianhe, and Haizhu). The findings suggest that the maximum and average concentrations of six heavy metals remained within the limits outlined in GB2762-2022 "National Food Safety Standard Maximum Levels of Contaminants in Foods," NY659-2003 "Limits of Chromium, Cadmium, Mercury, Arsenic, and Fluoride in Tea," and NYT288-2018 "Green Food - Tea". However, the detection rate of Cu is highest.
We have further analyzed EDI, HI, THQ, and ILCR of six heavy metals which based on the types of tea and regions as the classification standard. In our study, the EDI showed the highest daily intake of heavy metals from tea is Cu while the lowest is Hg. Classified by tea varieties, the HI degree of tea was dark tea > green tea > black tea > oolong tea. The heavy metal pollution degree was As > Cd > Hg > Pb > Cr. The THQ showed that As polluted the four kinds of tea, and the dark tea was the most seriously polluted. Classified by three districts investigated, the THQ showed that Cu polluted all regions, and Baiyun was the most seriously polluted. The HI arrange is Baiyun > Haizhu > Tianhe, and the heavy metal pollution degree was Cu > Cr > Pb > As > Cd > Hg, which have shown minimal health risks in terms of non-carcinogenic and carcinogenic effects. This notwithstanding, we found the values of all these indices were below the hazardous threshold. These results suggest that tea consumption in the surveyed areas is a relatively healthy dietary habit due to the low cancer risk that all kinds of tea have shown. Therefore, the region could further promote tea production and consumption through relevant policies and social campaigns.
However, several findings reveal that the tea leaves’ heavy metal levels exceeds the standards in other regions of China. Zhan[48] measured the average content of Pb, Cd, Cr, and As in tea leaves produced in Zhejiang to be 0.510–2.763, 0.033–0.099, 0.553–2.079, and 0.053–0.121(Unit: mg/kg) separately. Ma[49] analyzed 175 samples of green tea from 16 provinces in China, with average Pb content of 0.945 mg/kg, Cd content of 0.052 mg/kg, and As content of 0.088 mg/kg, all exceeding the standards. Jiang[50]discovered samples of tea leaves sold in Shenzhen with excessive Pb levels. Additionally, Jiang Yuehua et al. [51]detected Cd exceeding the standards in honeysuckle tea sold in Nanning City. This indicates that there are significant differences in the soil and heavy metal pollution of tea plantations in various regions of China. The main reason for these differences is the variation in the soil parent materials of different tea plantations. Additionally, the processing of tea leaves circulating in the Chinese market is primarily composed of traditional manual picking and mechanized processing, which has relatively little impact on heavy metal pollution. However, storage and preservation have a direct influence on the quality of tea[52]. A horizontal comparison of the risk assessment of tea drinking health among local populations in China or worldwide awaits further data collection and analysis.
Meanwhile, this study further observed a correlation between certain elements and amassing of heavy metal contamination in the corresponding cultivated soils. Peng[53] conducted an investigation into the current situation and origins of heavy metal pollution within a major tea-producing region located in Guangxi. The proportions of excessive As and Cd in tea garden soil were 6.67% and 2.22%, respectively. Although no elevated levels of heavy metals were detected in tea leaves, the relatively high bioconcentration factor of Cd suggests that the tea leaves’ ability to absorb Cd from the soil remains significant. Yao[54] surveyed 135 samples of soil and tea leaves from a typical tea-producing region in Hubei, revealing varying degrees of excessive Cd, Pb, Cu, Cr, and As in the surface soils within this area. The tea plants exhibited significant differences in their ability to accumulate different heavy metals, with much higher accumulation abilities for Cu and Pb compared to other elements. In addition, although the quantities of heavy metals in soils and tea leaves exhibit a weak connectivity, a noteworthy positive correlation was observed between the tea leaves’ concentration of heavy metals and abilities for accumulation. From this, we can tentatively conclude that the properties of tea garden soil influence the types of enriched elements in tea leaves. Different types of tea leaves exhibit significant differences in their ability to accumulate the same element, and the differences in accumulation abilities within the same type of tea may be related to genetic variations in the tea leaves[49–51].
A study on heavy metal pollution in urban soils in Guangzhou[55] revealed that in 133 samples of soil from park green areas in the main urban area of Guangzhou, the levels of Hg, Cd, Zn, As, Pb, and Cu all exceeded background values. Simultaneously, researchers assessed the health risk exposing to heavy metal and found that the main carcinogenic risk from metals in urban park soil was attributed to As and Cr. In another analysis assessing contamination in soils in the of Guangzhou suburbs[56], the researchers analyzed the single-factor contamination index frequency distribution and found that among the eight heavy metals investigated, Hg had the highest contamination frequency, with frequencies of slight, moderate and severe contamination being 25.4%, 9.1% and 16.3% respectively, followed by Cd which also showed high contamination frequencies with rates of slight, moderate and severe contamination at 23.5%, 5.0%, and 3.4% respectively. These findings indicate the presence of varying degrees of heavy metal pollution in both urban and suburban areas of Guangzhou, highlighting the necessity for stringent control measures for heavy metals.
In conclusion, utilizing health risk assessment models (including HI, THQ, ILCR, etc) to measure the health risks of consuming tea can help evaluate the heavy metal pollution, providing valuable insights for policy formulation and implementation[57]. In terms of specific policy implementation, measures such as applying organic fertilizers, ground cover grassing, and utilizing the biological nitrogen fixation characteristics of green manure crops can control the accumulation and migration of heavy metals, effectively enhancing the organic matter and available potassium fertility levels in surface soil[54]. Regarding heavy metal control, further research is needed to delve into the morphological characteristics and migration transformation laws of heavy metals of soils so as to clarify their potential risks, particularly concerning common heavy metals like Pb and Cu in tea leaves. Additionally, other elements such as Hg, Cd, and Zn should also be assessed and controlled in the Guangzhou area to avoid significant foodborne hazards. In this study, we somewhat overlooked further exploration of the correlation between heavy metal contamination of soil in Guangzhou and the collected tea leaves samples. Besides, there is a lack of discussion on variations in tea consumption of heavy metals related to differentiated urban and demographic structures in Guangzhou. In addition, there may also be errors caused by the leaching rate and bias in statistical methods.
Therefore, considering the gradual gathering of heavy metals in tea leaves, it is paramount to modify the tea cultivation structure and select tea cultivars with minimal heavy metal accumulation to mitigate health risks. Apart from soil as a vital factor affecting tea tree growth, the heavy metal contamination brought about during the processing and transportation of tea products also deserves attention and further exploration.