Introduction

Traditional Chinese medicine is able to reduce the symptoms and complications of certain diseases and, thus, has played a unique role in the history of human resistance to diseases, including the three major respiratory infections of this century: severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and the new coronavirus disease 2019 (COVID-19) (Leung 2007; Luo et al. 2020; Xu and Zhang 2020). According to the traditional Chinese medicine philosophy, medicine food homology products refer to materials derived from food and medicine as therapeutics without side effects (Jeong et al. 2012; Hou and Jiang 2013; Gong et al. 2019). With the aging of the global population and more people suffering from noncommunicable diseases or poor health, some medicine food homology products are brewed as tea and consumed as a low-cost and effective complementary and alternative medicine therapy (Shahrajabian 2019; Jiang et al. 2020; Zhang et al. 2021). For example, chrysanthemum tea and honeysuckle tea relieve heat, toxicity, fever, and cold symptoms; Chinese red date tea and matrimony vine tea are useful for treating anemia and insomnia; Scaphium scaphigerum and Momordica grosvenori teas can treat cough and chronic pharyngitis. Therefore, medicine food homology tea (MFHT) infusions are the most commonly consumed beverages aside from traditional teas (e.g., black and green teas) (Karak and Bhagat 2010).

Medicine food homology teas may contain a substantial amount of essential trace metals for the human body such as Cu, Fe, Mn, Zn, Cr, and Ni (Soman et al. 1970). Trace elements in MFHTs play an important role in supplementing and regulating various trace elements in the human body and thus in curing diseases (Tian et al. 2005; Rasdi et al. 2013; Horning et al. 2015). For example, Fe is essential for the production of erythrocyte cells; Mn, Cu, and Zn are important for the immune system and the nervous system; Cr and Ni can prevent high blood pressure, diabetes, and other diseases. However, MFHTs absorb various trace elements from the soil and the surrounding environment during growth, which may result in some of these trace elements being in excess of the standards (Brzezicha-Cirocka et al. 2016; Yu et al. 2017; Wang et al. 2019). The trace elements that enter the body via MFHT consumption can be harmful to human health. When essential trace metals accumulate in excessive concentrations in specific tissues or organs of the body, they can be toxic (Prasad 1988; Xu et al. 2004; Horning et al. 2015; Mohammed Abdul et al. 2015; Xiao et al. 2019; Chen et al. 2020). For example, excessive intake of Mn and Zn can cause sluggish movements, dull expressions, dizziness, vomiting, and diarrhea. Excessive amounts of non-essential trace elements such as As, Cd, Pb detected in both dry MFHT materials and the infusion can cause irreversible damage to the human body and multi-organ failure (Singh et al. 2011). Chronic As exposure in Bangladeshi adults has been reported to cause dose-dependent hypermethylation of whole blood DNA in vivo study (Keshvari et al. 2021). Exposure to common levels of Mn in groundwater exposures is associated with intellectual disability in children (Bouchard et al. 2011).

Considering the potential hazards of trace elements, strict limits on the content of some heavy metal elements (Cd, As, Pb) have been set in the quality standards for traditional Chinese medicine (Liu et al. 2013; Li et al. 2020). Studies on the contamination of traditional Chinese medicines have mainly focused on heavy metal content and potential health risk assessment, with lesser focus on other trace element contents (Locatelli et al. 2014; Rubio et al. 2012; Liu et al. 2018; de Oliveira et al. 2018; Ghuniem 2019). However, systematic studies on trace elements in MFHTs have been scarce. The use cycle of traditional Chinese herbs as medicine is usually 1 to 4 weeks, but MFHTs are consumed as daily drinks for a long time. Trace elements enriched in MFHTs are more likely to accumulate in the human body and are not easily dissolved (Ting et al. 2013). After the accumulation reaches a certain amount, it disrupts the original balance in the human body, which may lead to chronic poisoning. Studies on trace metal exposure from drinking tea often focus on a specific component, such as one variety of tea, a few target elements, or whether the risk is non-carcinogenic or carcinogenic (Kara 2009; Martín-Domingo et al. 2017). As a result, health risk assessments in those studies are specific to the location, and the small number of target factors may lead to an underestimation of the risks. Therefore, it is essential to systematically study the types and contents of trace elements in MFHTs consumed by the general public and to assess the effects of long-term exposure to these trace elements on human health.

The differences in geological environments cause variations in the active ingredients of the same traditional Chinese medicine produced in different producing areas (Huang et al. 1996; Shahrajabian 2019; Chen et al. 2021), but there is limited research on the relationship between the geochemical environment and active ingredients in traditional Chinese medicine. The content of trace elements varies greatly among different producing areas owing to the geochemical environment of those areas (Zhou et al. 2014). Previous studies have determined and compared the elemental content of traditional tea leaves, without considering the influence of the geological environment on the enrichment of trace elements. The differences between producing areas may control one or more environmental/geological factors, including temperature, light intensity, air humidity, rainfall, soil element content, plant community, and cultivation measures. Thus, studying the relationship between trace elements in traditional MFHTs and environmental/geological factors can help reveal the causes of trace element enrichment in Chinese herbal medicine.

In this study, 12 types of MFHTs, including honeysuckle and chrysanthemum teas, were used to determine the content of nine trace elements (Fe, Mn, Zn, Cu, Cr, Cd, As, Pb, Ni) in herbs and their infusions. The exceedance of each trace element, contamination level, and the consumability of MFHTs were analyzed in conjunction with traditional Chinese medicine and food quality standards to assess the risks to the human health upon daily consumption of these MFHTs over a long period of time. The factors affecting the trace element enrichment in MFHTs were also explored.

Material and methods

Sample collection

In this study, 10 samples of each of the 12 types of MFHTs were collected: Scaphium scaphigerum, Momordica grosvenori, honeysuckle, matrimony vine, Rosa rugosa, dandelion, semen cassiae, lotus leaf, mulberry leaf, Chinese date, Flos sophorae, and chrysanthemum. The producing areas of these 120 samples were recorded, and six samples were of imported origin, and thus not marked in Supplementary Fig. 1. All MFHTs were sourced from pharmacies in different regions of China.

Trace element contents in teas and infusions

The MFHT samples were dried, crushed, passed through an 80-mesh sieve, sealed in a sample bag, and stored in a desiccator. Dried samples (0.3 g) were digested with 5 mL HNO3 and 2 mL H2O2. The samples were then placed in a microwave digester (TANK PRO, Hanon, Shandong, China) and digested according to the procedure described in Supplementary Table 1. Subsequently, the digestion solution was heated to 150 °C in an acid detector (TK-12, Hanon, Shandong, China) until the red-brown vapor evaporated and was concentrated to 1–2 mL before being transferred to a 10 mL polyethylene terephthalate tube. The volume was increased to 10 mL with 2% nitric acid and refrigerated for testing. The Fe, Mn, Zn, Cu, Cr, Cd, Pb, and Ni contents of the digested samples were analyzed via inductively coupled plasma mass spectrometry (NexION300D, PerkinElmer, Inc., USA). As was analyzed by atomic fluorescence spectrometry (AFS-6790, Haiguang, Beijing, China). The concentration deviations obtained for sample replicates were within ± 5%.

Six samples were randomly selected from the same type of MFHT (a total of 72 samples from the 12 types of MFHTs) for the infusion extraction experiment. By investigating the weight of herbal tea bags in markets and simulating the herbal tea brewing process, 20 g of uncrushed herbal tea sample was weighed and was added to 200 mL of boiled deionized water and steeped for 30 min. These infusion samples were digested, and the concentrations of Fe, Mn, Zn, Cu, Cr, Cd, As, Pb, and Ni were tested using via ICP-MS (NexION300D, PerkinElmer, Inc., USA) employed to analyze the trace element content in the MFHTs.

Nemerow integrated pollution index

The Nemerow integrated pollution index (NIPI) is used to evaluate the quality of the soil and water environment, and reflect the concentration of heavy metals in the environment and Chinese medicinal materials (Yari et al. 2020). The advantage of this index over other indicators is that it highlights the influence of high concentrations of trace elements on Chinese medicinal materials (Huang et al. 2018).

The NIPI is calculated using Eq. (1) for each sampling station (Yang et al. 2011),

$$\mathrm{NIPI}=\sqrt{\frac{{\mathrm{Pi}}_{\mathrm{ave}}^{2}+{\mathrm{Pi}}_{\mathrm{max}}^{2}}{2}}$$
(1)

where Piave represents the average pollution index of element i and Pimax shows the highest rate of element pollution index i. The amount of pollution is also based on the value of the index calculated in five pollution classes listed in Supplementary Table 2.

In this study, the NIPI was employed to estimate the extent of contamination of MFHTs with limited trace elements (Pb, As, Cd, Cu, Cr, Ni).

Health risk assessment

Based on the concentrations of nine trace elements in the infusions, the potential risk of exposure to these elements by consumption of MFHTs was estimated. For nonvolatile trace elements, oral ingestion is a common exposure pathway for humans consuming MFHTs. The methods published by the United States Environmental Protection Agency (U.S. EPA 1989, 2004) were employed for the health risk assessment.

The estimated daily intake (EDI) via oral ingestion of each trace element (Fe, Mn, Zn, Cu, Cr, Cd, As, Pb, Ni) present in the mixture was determined using the following equation,

$$\mathrm{EDI}\;\left(\mathrm\mu\mathrm g/\mathrm{kg}-\mathrm d\right)=\left(\mathrm C\times{\mathrm D}_{\mathrm I}\times{\mathrm E}_{\mathrm F}\times{\mathrm E}_{\mathrm D}\right)/\left(\mathrm{BW}\times\mathrm{AT}\right)$$
(2)

where C is the trace element concentration in the infusion (μg/mL), DI is the daily intake of herbal tea (500 mL) based on an average daily consumption of a bottled beverage, exposure frequency (EF) is 300 d/yr, exposure duration (ED) is 10 yr, body weight (BW) is the adult average body weight (65 kg), and averaging time (AT) is ED × 365 d.

Non-carcinogenic risk was expressed using hazard quotient (HQ), which is the arithmetic product of EDI and trace element-specific reference dose (RfD) (U.S. EPA 2004). International oral reference doses of trace elements (RfDo, μg/kg-d) used in this study were 700 for Fe, 140 for Mn, 300 for Zn, 40 for Cu, 3 for Cr, 0.3 for Cd, 0.3 for As, 3.8 for Pb, and 20 for Ni. The reference values were adapted from the U.S. EPA Integrated Risk Information System (IRIS) (2019) and US EPA Regional Screening Levels (RSLs) (2018). U.S. EPA (2018) and U.S. EPA (2019) generally, HQ > 1 (HQ < 1) indicates that adverse health effects are likely (not likely) to occur (U.S. EPA 1989). Exposure to two or more contaminants has additive and/or interaction effects. Therefore, HQsum has been created to quantify the risks posed by more than one contaminant in a single product, calculated as,

$${HQ}_{sum}={\sum }_{n=1}^{n}{HQ}_{n}$$
(3)

where HQn is the targeted hazard quotient for the nth term of contaminant.

Results and discussion

Trace elements in MFHTs

Trace elements in MFHTs are summarized in Supplementary Table 3. The maximum (and average) concentrations of trace elements in the 12 types of MFHTs are as follows (in mg/Kg): Fe: 7915.92 (782.85), Mn: 1549.76 (205.68), Zn: 217.09 (52.93), Cu: 44.73 (16.97), Pb: 19.27 (2.61), Ni: 41.76 (5.32), Cr: 172.84 (12.39), As: 4.60 (1.08) and Cd: 4.85 (0.30). The average concentrations of Fe (4396.57 mg/Kg) and Cr (71.37 mg/Kg) in dandelion and Cd (1.71 mg/Kg) in honeysuckle were much higher than those in all the MFHTs. In addition, the concentrations of Fe, Mn, Zn, and Cu in the 12 types of edible MFHTs were much higher than those of Cr, Ni, Pb, As, Cd, and the Cr and Ni contents were significantly higher than Pb, As, and Cd contents. The MFHTs exhibited higher mean concentrations of Cd, As, Pb, and Cr (0.30, 1.08, 2.61, and 12.39 mg/Kg) than traditional teas (0.04, 0.19, 0.70, and 4.58 mg/Kg) (Hadayat et al., 2018).

Based on the basic characteristics of MFHTs as both food and medicine, it is required that Pb ≤ 5 mg/kg, As ≤ 2 mg/kg, Cd ≤ 0.3 mg/kg, Cu ≤ 20 mg/kg, Cr ≤ 2 mg/kg, and Ni ≤ 1 mg/kg (Pharmacopoeia of the People's Republic of China (2005) and National Food Safety Standard Limits of Contaminants in Food (GB 2762–2017)). Due to the lack of standards for Fe, Mn, and Zn, the over-standard rates of these three trace elements are not discussed here. The over-standard rates of Cr and Ni were 82.5% and 100% in all 120 samples, respectively, whereas those of Cu, Cd, Pb, and As were 31.6%, 22.5%, 12.5%, and 11.6%, respectively. The exceedance of Cr and Ni in the 12 types of MFHTs was higher than that of the heavy metal pollutants (Cu, Cd, Pb, As) in Chinese medicinal materials. In addition, Cu and Cd severely exceeded the standard in honeysuckle (50% and 80%, respectively), dandelion (90% and 90%, respectively), Sophorae flower (Cu was 100%), and chrysanthemum (50% and 60%, respectively).

In this study, the NIPI was used to investigate the pollution risk and raise awareness on potential trace element pollution (Pb, As, Cd, Cu, Cr, Ni) in MFHTs (Table 1). All the NIPIs were greater than 1, which indicates that all 12 types of MFHTs were contaminated. The high NIPIs of dandelions and Flos sophorae (25.96 and 9.06, respectively) indicate severe pollution with limited trace metals. Honeysuckle, mulberry leaf, lotus leaf, semen cassia, and chrysanthemum were also substantially contaminated (NIPI > 3). The NIPIs of the flowers and leaves of the MFHTs were significantly higher than those of the other parts, which indicates that the limited trace elements are more easily concentrated in flowers and leaves.

Table 1 The average value of normalized Nemerow Integrated Pollution Index (NIPI) of limited trace elements of MFHTs samples
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Human health risks associated with trace elements in 12 types of MFHTs

Oral ingestion (drinking MFHTs) is the main pathway of trace element exposure. The trace element-specific non-carcinogenic risks via oral ingestion (HQ) are shown in Fig. 1. The average HQ values in 120 samples were in the following order: As > Mn > Cr > Cd > Pb > Fe > Cu > Ni > Zn. Except in lotus leaf, As had the highest average HQ in the other 11 types of MFHTs, followed by Cr and Mn. In addition, 16.7% and 83.3% of the HQ values of As in honeysuckle and dandelion samples, respectively, were slightly greater than 1, suggesting that exposure to As via oral ingestion of MFHTs might pose more severe non-carcinogenic health risks than exposure to other trace elements.

Fig. 1
figure 1

The non-carcinogenic risks of exposure to all trace elements in 12 types of MFHTs. (The cycles represent suspicious outliers. The five black short straight-line in one plot from the top to the bottom are the maximum, the 75th percentile, the median, the 25th percentile, and the minimum value, respectively. The red line is the mean value)

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To further understand the risk of exposure of the public, non-carcinogenic risks of exposure to all trace elements in each sample (expressed as total HQsum) were calculated by adding the HQ values of each single trace element. The total non-carcinogenic risks of trace elements in 12 types of MFHTs were calculated using the amount of herbal tea consumed by a person in a day, and the HQsum values are displayed in Fig. 2. The total HQsum values ranged from 5.88E–02 to 2.40E + 00, with mean and median values of 5.37E–01 and 3.68E–01, respectively. The average total HQsum values were in the following order (Fig. 3): dandelion (1.51) > honeysuckle (1.21) > mulberry leaf (0.57) > lotus leaf (0.54) > chrysanthemum (0.53) > Flos sophorae (0.46) > Chinese-date (0.45) > Momordica grosvenori (0.29) = Rosa rugosa (0.29) > matrimony vine (0.23) > Scaphium scaphigerum (0.22) > semen cassiae (0.14). Total HQsum values of 66.6% (honeysuckle), 83.3% (dandelion), and 16.6% (lotus leaf samples) were greater than 1. The results imply that honeysuckle and dandelion teas may be harmful to human health when consumed daily because of the excessive trace element exposure. Therefore, the non-carcinogenic risks caused by the elevated trace element concentrations in honeysuckle and dandelion teas should be investigated by the agencies for prevention and control of diseases. The results suggest reducing the frequency and intensity of dandelion and honeysuckle tea consumption to decrease the non-carcinogenic risks of exposure to trace elements.

Fig. 2
figure 2

Risk contributions of varied trace elements in MFHTs. (The cycles represent suspicious outliers. The five black short straight-line in one plot from the top to the bottom are the maximum, the 75th percentile, the median, the 25th percentile, and the minimum value, respectively.The red line is the mean value)

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Fig. 3
figure 3

Distribution characteristics of the trace element( Fe, Cu, Pb, Cr, Ni, Mn and Zn) content in the soil and the MFHTs. The red arrows represent the trend of the trace elements in soil and the MFHTs. The gray area represents the content of trace elements not exceeding the standard

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The contributions of the total HQ values (mean values) of trace element species to the total HQsum values were in the following order: As (53.7%) > Mn (11.1%) > Cr (10.4%) > Cd (6.8%) > Pb (5.8%) > Cu (3.5%) > Ni (3.3%) > Fe (3.2%) > Zn (2.3%). Among the nine trace elements, only the contribution of Cr to the total HQsum value (10.4%) and the over MCL rate (82.5%) was consistent(Supplementary Table 4). It is unusual that most target trace elements had negligible risk contributions, which did not match their over MCL rates. As had a low over MCL rate (11.6%) and contributed 53.7% to the total HQsum value, whereas trace elements with very high over MCL rates (Ni [100%], Cu [31.6%], and Cd [22.5%]) contributed less than 10% (3.3%, 3.5%, and 6.8%, respectively). Mn, which has no recommended value in the Chinese medicinal material standards, contributed 11.1%, following As. These results suggest that exposure to As and hidden presence of Cr and Mn via consumption of MFHTs on a daily basis poses high non-carcinogenic risks, and is thus a public health concern.

With the expansion of the influence of TCM, taking traditional Chinese medicine is not limited to China. CHMs have been exported to 185 countries and regions, mainly Japan, South Korea, Indonesia and the United States. There is a large and promising world market for CHMs (Lin et al. 2018). Meanwhile, due to the wide presence of heavy metals and harmful elements in the environment, more research has been done on trace elements in medicinal plants such as globe artichoke in Uruguay, Cetraria Islandica from the European market, and herbal teas collected in the US (Wu et al. 2022). Therefore, the study of MFHT is not only limited to the Chinese region, but also applicable to the habit of drinking MFHT in many regions and countries around the world.

Enrichment of trace elements in MFHTs

Effect of MFHTs on trace element content

Supplementary Table 3 shows the significant differences in the content of each trace element in different types of MFHTs. The Fe and Cr content was higher in dandelion (mean values of 4396.57 mg/kg and 71.37 mg/kg, respectively) than that in other types of MFHTs (mean values less than 2000 mg/kg and 20 mg/kg, respectively) and did not vary considerably. The highest Mn content in MFHTs was in lotus leaf (mean of 965.78 mg/kg); the Mn content in Momordica grosvenori, honeysuckle, dandelion, and mulberry leaf ranged from 400–200 mg/kg, and it was less than 100 mg/kg in the other types of MFHTs. The Zn content in Scaphium scaphigerum and semen cassiae (79.80 mg/kg and 78.24 mg/kg, respectively) was higher than in the other types of MFHTs. The Cu content was high in dandelion and Flos sophorae (> 20 mg/kg) and low in Chinese date tea (< 10 mg/kg), whereas the other MFHTs contained 10–20 mg/kg Cu. The Ni content was higher in dandelion and Flos sophorae (both mean values greater than 10 mg/kg), and the average Ni content in other types of MFHTs was less than 5 mg/kg. The Cd content in honeysuckle (mean value of 6.57 mg/kg) and the As content in dandelion (mean value of 2.57 mg/kg) were much higher than the Cd and As contents in other types of MFHTs. The highest average Pb content in the 12 types of MFHTs was over 4 mg/kg in Scaphium scaphigerum, whereas the lowest one was only 0.79 mg/kg in matrimony vine. The Kruskal–Wallis test in SPSS software was used to analyze the differences in the contents of the nine trace elements in all types of MFHTs(Supplementary Table 5). All the herbal tea types showed significant differences (p < 0.05) in the contents of the nine trace elements (Fe, Mn, Zn, Cu, Cr, Cd, As, Pb, and Ni). Thus, herbal tea is one of the main factors influencing the enrichment of different trace elements in different MFHTs.

Effects of producing areas and environmental/geological factors on trace element content

The experimental data revealed some variations in the content of each trace element in MFHTs from different producing areas. The Kruskal–Wallis test in the SPSS software was used to analyze the differences in the producing areas of MFHTs for the nine trace element(Supplementary Table 6). The samples from different producing areas did not show significant variations (p > 0.05) in Cd and As contents, unlike in the other seven trace elements (p < 0.05). The chemical composition of Chinese herbs varies greatly by the producing area, owing to the differences in the geographical location, soil type, and geochemical environment of the producing area (Zhu et al. 1990; Yang and Chen 2015). The differences in the trace element content of MFHTs from different producing areas in this study also show that the geological environment impacts the growth and chemical composition of MFHTs (Nagajyoti et al. 2010). The contents of trace elements Cr, Fe, Ni, Cu, Zn, Mn, and Pb in the 12 MFHTs were influenced by the combination of the producing area and the type of MFHTs, whereas the enrichment of Cd and As was mainly controlled by the type of MFHTs.

From the perspective of biogeochemistry, the accumulation of trace elements in plants is related to the geochemical distribution and migration of elements, geological and metallogenic background, and the chemical composition and properties of soil parent materials (Zhang et al. 2020). Therefore, the differences between the producing areas of MFHTs depend on one or more environmental/geological factors. Therefore, it would be incorrect to analyze the correlations between the producing areas and trace elements; instead, individual factors controlling the environmental geology of the producing area should be considered. In this study, the temperature, soil background, and rainfall data reported in the China Statistical Yearbook (2021) for different producing areas were analyzed to assess the influence of environmental/geological factors on the contents of trace elements in MFHTs.

Soil is the direct donor of trace elements and provides organic substrates and mineral nutrients essential for the growth and development of MFHTs (Kertesz and Frossard 2015). Several environmental/geological factors such as soil background values, rainfall, and temperature affect the migration and release of trace elements from soils, changing the effective state of elements in soil, which, in turn, determines the accumulation of trace elements in MFHTs (Zhu et al. 1990).

Soil background value

The content of trace elements such as Fe, Mn, and Cu showed a significant correlation with the soil parent rock type (Rezapour et al. 2014). The geological background influences the plant emblematic elements through the soil. Our results showed a close relationship between the trace element content in the soil and the MFHTs (Fig. 3). The trace element contents of the 12 MFHTs varied with the background values of soil trace elements in the growing areas; the contents of Fe, Mn, Cu, Pb, Cr, and Ni in the MFHTs tended to increase and then decrease with the increase in the soil background values, whereas the Zn content did not change. However, trace element contents in MFHTs and soil did not show a significant correlation. The enrichment of most trace elements in MFHTs occurs at the median of the soil background values, rather than at the maximum or minimum values.

The regression equation between the content of each trace element in MFHTs and the dominant factors was established using the content of each element in MFHTs as the dependent variable (Y) and the trace elements in soil and MFHTs as independent variables(Selement and Melement) (Table 2). Stepwise regression analysis revealed that trace elements in the 12 MFHTs were influenced by both the trace element content of the soil and that of the MFHTs. Cr and Cu in MFHTs were not only affected by Cr and Cu in their respective soil, but also affected by other trace elements in soil and MFHTs, for example, Cu in MFHTs was inhibited by Cu in soil and Mn in MFHTs, and promoted by Fe, Zn, Ni in MFHTs and Fe, Pb in soil. However, Feand Mn in MFHTs was only affected by other elements in the soil and iMFHTs, whereas Zn, Pb and Ni contents in MFHTs were not influenced by the relevant trace elements in soil. This result further implies that no significant correlation exists between the soil background value of each trace element and its corresponding value in MFHTs. In addition, the results indicate that a trace element in an MFHT was affected by the synergistic or antagonistic effect of each trace element in the soil and in MFHTs, which together affected the enrichment of that element in MFHTs.

Table 2 Results of step wiseregression analysis
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Rainfall

Most trace metals in soil pore water can be absorbed by the root system of MFHTs(Norini et al. 2019). Rainwater serves as a carrier in the migration of trace elements (Carrillo-González et al. 2006), thus affecting the release of trace elements in soil and thus the content of each trace element in MFHTs. The relationship between the content of trace elements in different MFHT-producing areas and the annual rainfall (Fig. 4)showed that the higher contents of Cr, Fe, Ni, Cu, Zn, Mn, and Pb in MFHTs were observed in areas with annual rainfall of 600–1600 mm than in areas with lower (< 400 mm) or higher annual rainfall (> 1600 mm). At an annual rainfall between 800 and 1380 mm, rainfall exhibited a significant effect on the enrichment or leaching of trace elements (Afrifa et al. 2010), and the extent of the effect varied for each element, with some elements reaching maximum enrichment or leaching level. At the annual rainfall < 400 mm, soil water content is low and migration of trace elements is somewhat restricted; at > 1600 mm, the infiltration or leaching of trace elements and surface runoff constrain their geochemical behavior; thus, at annual rainfall amounts of < 400 m and > 1600 m, the relative content of each trace element in MFHTs was low.

Fig. 4
figure 4

Distribution characteristics of the trace element( Fe, Cr, Zn, Cu, Mn, Ni and Pb) content in the annual rainfall and the MFHTs. The red arrows represent the trend of the trace elements in the annual rainfall and the MFHTs, while the red boxes represent the higher content of the trace elements. The gray area represents the content of trace elements not exceeding the standard

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The distribution of seven trace elements in MFHTs also varied under different rainfall conditions. The content of Cu and Zn in MFHTs increased with increasing annual rainfall, and the highest content of both elements was observed in areas with annual rainfall of 1600 mm. The content of Fe, Mn, Cr, Ni, and Pb in MFHTs first increased and then decreased with annual rainfall, where MFHTs with higher contents of Fe and Cr were mainly distributed in the area with annual rainfall of 400–800 mm. The annual rainfall in areas with high Mn and Pb content was 800 mm, whereas that in areas with high Ni content was 1200 mm. This result implies that the mineral phase composition, macronutrient composition, and trace element geochemical behavior of the soil all change systematically and synergistically under different rainfall conditions, but variations in rainfall affect the form of the element and the transformation process in soil, further leading to variations in the producing areas of MFHTs.

Temperature

Temperature affects soil fertility and thus soil plants and microorganisms (Heinze et al. 2017). In addition, the solubility of gases, inorganic salts, and other substances in the soil; the diffusion of water and gas; and the activity of exchange ions are also influenced by temperature (Stotzky and Pramer 1972). Figure 5 shows that the trace element contents in MFHTs tended to increase and then decrease with the annual average temperature of the production areas, and the higher contents occurred at 15–20 °C, indicating that the suitable temperature range for trace element enrichment in MFHTs was 15–20 °C. Soil microbial activity increased with temperature (Stotzky and Pramer 1972), in addition to accelerated soil solution chemistry, and promotion of organic matter decomposition and nutrient conversion in a suitable temperature range, which, in turn, affected the effectiveness of soil trace elements in medicinal plants.

Fig. 5
figure 5

Distribution characteristics of the trace element( Fe, Mn, Zn, Cu, Cr, Pb and Ni) content in the annualaverage temperature and the MFHTs. The red boxes represent the higher content of the trace elements in the annualaverage temperature and the MFHTs. The gray area represents the content of the trace elements not exceeding the standard

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Conclusion

Nine trace elements (Fe, Mn, Zn, Cd, Cr, Cu, As, Pb, and Ni) in 12 types MFHTs were studied, and health risks associated with tea consumption were assessed by analyzing 120 samples originating from 18 provinces in China. In addition, the factors affecting the trace element enrichment in traditional MFHTs were explored. The contents of Fe, Mn, Zn, and Cu were much higher than those of Cr, Ni, Pb, As, and Cd in 12 types of MFHTs. The exceedance of Cr (82%) and Ni (100%) in 12 MFHTs was higher than that of Cu (32%), Cd (23%), Pb (12%), and As (10%). The NIPI values of dandelions and Flos sophorae were considerably high (25.96 and 9.06, respectively), which indicated severe pollution by limited trace metals. Honeysuckle, mulberry leaf, lotus leaf, semen cassia, and chrysanthemum were highly contaminated (NIPI > 3).

The results of the health risk assessment showed that As, Cr, and Mn posed high non-carcinogenic risk in the 12 MFHTs. Especially, honeysuckle and dandelion teas may be hazardous to the human health owing to the trace element exposure through their daily consumption. The other 10 MFHTs can be safely consumed daily.

The enrichment of Cr, Fe, Ni, Cu, Zn, Mn, and Pb in MFHTs was influenced by the type of tea and the producing area, whereas As and Cd were mainly controlled by the MFHT type. Environmental factors such as soil background values, rainfall, and temperature were shown to affect the enrichment of each trace element in MFHTs from different producing areas. Although we performed a series of studies on the enrichment factors of trace elements in MFHTs, we did not examine the actual soil and temperature data or the other environmental/geological factors for each MFHT sample. The relationship between geochemistry and trace element enrichment in MFHTs can be further investigated by conducting field studies. Further research is required to expand this study to include more data from other areas and other types of MFHTs.