Introduction

Since ancient times, humans have explored their environment for plants that could be used to cover all their basic needs: food, shelter, fuel, and health [1]. Phytotherapy is a famous medical modality that uses some plants to treat humans. Although the root of phytotherapy lies in the past, the practice is still meaningful and relevant [2]. In recent decades, the use of medicinal herbs and herbal decoctions increased significantly even in the industrialized countries due to their availability, acceptability, and minimal side effects [3, 4]. Herbal drug preparations mostly include subjecting the plants to treatments, such as extraction, distillation, fractionation, purification, concentration, or fermentation [1, 5].

It is well known that high consumption of herbal medicines with generally high content of heavy metals pose a serious health hazard due to their toxic, carcinogenic, and mutagenic effects [5]. The concentrations of essential and nonessential elements in herbal products depends on many factors, i.e., plant species, soil condition, geographical origin, harvesting time, the applied extraction technique, preparation method, instrument contamination, and storage [3, 4].

The production of Herbal distillates (also known by other names such as floral water, hydrosol, hydrolate, herbal water, and essential water) is currently going on in many countries. In major producing countries such as Iran, Turkey, and Egypt, selected plant organs (leaf, petal, stamen, seed, fruit, and root) are distilled in large closed pots. In Iran, herbal distillates are produced using both Industrial and traditional method. In the traditional method, the distillation pots are usually made of copper, while the collecting vessels and condensation pipes are made of galvanized steel. In the industrial method, most equipment and vessels are made of stainless steel. However, in both methods, soft plant organs such as leaves and petals are directly loaded into the distilling pot, but hard parts such as cumin seed or ginger root, must first be ground. Within the pot, the soft or powdered organs are mixed with water and heated to boiling point. The resulting vapor is then condensate and transferred to the collecting container through pipes. Daily consumption of herbal distillates by local residents is normally less than one teacup (100 ml).

Herbal decoctions are generally prepared from dried or fresh plant organs, i.e., flowers, leaves, seeds, and roots [3]. The usual practice for preparing herbal decoctions in Iran is to add water and boil for few minutes. Then let it steep for 15–20 min. Consumption of herbal decoctions is normally one teacup (125 ml) two or three times a day.

A large number of Shiraz residents (especially the old generation) consume herbal distillates and decoctions to regain their own well-being. Thus, health hazard assessment of these food supplements, as a major exposure pathway, seems necessary. The main purpose of this research is the determination of essential and nonessential elements in 16 most used herbal distillates (prepared by traditional methods) and 5 most consumed decoctions in Shiraz city and to estimate the potential health risks.

Materials and Methods

Sixteen specimens of the most commonly used herbal distillates were purchased from two main distribution centers in Shiraz, Iran. The samples were originally produced in Meymand city (121 km south of Shiraz), which is a major center for herbal distillates production. All samples were collected in 60 ml sterile glass bottles that were previously washed and dried in oven at 180 °C.

Five specimens of the most commonly used herbal decoctions were also purchased and prepared the same way that local residents prepare herbal decoctions, which is using about 4.0 g of dried plant parts (flowers, leaves, and roots) and 250 ml of water in clean, dry stainless steel teapot and bringing to a boil. The mixture boiled for 15 min, and then left to cool at room temperature for another 10 to 15 min. The supernatant was then filtered using 45 μm syringe filters.

In order to determine heavy metals content in herbal residues used for making herbal distillates and herbal decoctions, the residues were dried at room temperature and then ground. 2 g of each sample was used for analysis.

The concentration of essential and nonessential elements in herbal distillates and herbal decoctions samples were determined using inductively coupled plasma-mass spectrometry (ICP-MS). The residues were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). In order to vouch the reliability of the results, appropriate quality assurance procedures and precautions were performed. QA/QC included reagent blanks and analytical duplicates and analysis of the standard reference material (GBW 07604-poplar leaves, GBW 08505-tea, GBW 07605- tea). One-way analysis of variance (ANOVA) was carried out, and significance level was considered at P value ≤0.05. All data were analyzed using SPSS statistics v.21 and Excel 2013 software for Windows.

The characteristics of plants used in this study and the claimed medicinal/health-promoting effects of their distillates and decoctions are presented in Tables 1 and 2.

Table 1 Characteristics of plants used for extracting herbal distillates
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Table 2 characteristics of plants used in preparing herbal decoctions
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Risk Assessment

Potential health risks of heavy metals in herbal products can be assessed by the same methods used for assessing carcinogenic or noncarcinogenic risks. Hazard quotient (HQ) and hazard index (HI) are introduced by US Environmental Protection Agency for noncancer risk assessment [3, 6–9].

$$ \begin{array}{c}\hfill \mathrm{H}\mathrm{Q}=\frac{\mathrm{ADD}}{\mathrm{RFD}}\hfill \\ {}\hfill \mathrm{ADD}=\frac{C\times \mathrm{I}\mathrm{R}}{\mathrm{BW}}\hfill \\ {}\hfill \mathrm{H}\mathrm{I}={\displaystyle \sum}\mathrm{H}\mathrm{Q}={\mathrm{HQ}}_1+{\mathrm{HQ}}_2+\cdots +{\mathrm{HQ}}_n\hfill \end{array} $$

Where, ADD is the average daily intake of metals (μg/kg-day), RFD is the daily intake reference dose (μg/kg-day), C is the mean concentration of metals in the herbal products (μg/g), IR is the average consumption rate of herbal products (kg/person-day), and BW is the body weight (kg). The RFD values are shown in Tables 3 and 4.

Table 3 Concentration of essential elements in the analyzed herbal distillates and regulatory limit values (μg/kg except where noted)
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Table 4 Concentration of nonessential elements in analyzed herbal distillates and regulatory limit values (μg/kg except where noted)
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Results and Discussion

Herbal Distillates

The concentration of essential and nonessential elements in the analyzed herbal distillates is summarized in Tables 3 and 4, respectively. Measured concentrations showed a wide variability due to differences in botanical structure, soil composition in which the plants are cultivated, irrigation water, distillation equipment, and containers. The concentration of Cr, Mn, P, Sr, and V were below detection levels in all analyzed samples. The concentration of toxic elements in the herbal distillates were well below food and agriculture organization of the United Nations/world Health Organization (FAO/WHO) provisional tolerable daily intake, American Herbal Products Association (AHPA), minimum risk level (MRL) from the US Agency for Toxic Substances and Disease (ATSDR), and WHO permission limit for finished herbal products in most samples (Tables 3 and 4) [10–15].

Metals such as Co, Ni, Fe, Mn, Cu, Cr, and Zn are generally categorized as essential element, and play an important role in biological systems. As the difference between the essentiality and toxicity ranges is very narrow for heavy metal ions, essential metals can also produce toxic effects if the metal intake is high; whereas nonessential elements such as Cd, Pb, As, Al, and Rb are toxic even at very low concentrations for human health [3, 16, 17].

High zinc and copper content in herbal distillates prepared by the traditional method is probably the result of copper pots and galvanized pipes used in the distillation process. However, considering the Zn and Cu deficiency in the developing countries, where people’s diet rely more on vegetables [18], the consumption of herbal distillates may provide the needed Zn and Cu, especially for those living in the developing countries and also for vegetarians if taken at recommended levels.

Rose water displayed the highest Cu (243. 8 μg/kg) and Fe (600 μg/kg) contents. Other determined elemental concentrations in Rose water include: Se (25.4 μg/kg), Ca (1070 μg/kg), Na (950 μg/kg), Al (0.04 μg/kg), and Si (1.07 μg/kg). Crataegus distillate had the highest level of Zn (11,980 μg/kg), Co (1.3 μg/kg), Rb (15.4 μg/kg), and U (1.1 μg/kg). Pussy willow distillate displayed the highest molybdenum (8.3 μg/kg) and bismuth (0.8 μg/kg) contents. The highest level of As (6.5 μg/kg) and Sn (7.7 μg/kg) had been shown in Starflower distillate. Date palm flower distillate revealed the highest Pb (19.9 μg/kg) and Ni contents (21.1 μg/kg). Finally, the highest contents of Cd, K, and Mg were seen in Sycamore, Fenugreek, and the herbal mixture, respectively.

Calcium is an important element in bone and tooth development, regulating blood pressure and enzyme regulation [4]. Herbal distillates such as Rose water and Orange blossom distillate with high Ca content could be profitable for those who cannot use animal-based Ca sources. Relatively high concentration of K, Cu, Rb, and Mg in Common nettle and Fenugreek distillates correlates with the metal content of other herbal distillates with the reputation of beneficially influencing glucose levels in diabetes [4, 19, 20]. The reputation of Crataegus distillate, Peppermint distillate, Pussy willow distillate, and Rose water in treating skin disease and upset stomach is probably due to the high zinc content. Zn is known to play an important role in treating skin irritation and mitigating ulcer [4]. Cancer and associated mortality decrease in humans with consumption of selenium (Se) is already reported in the literature [21–23]. The result of this study indicated that consumption of only 100 ml/day of Rose water and Orange blossom distillate may provide 5 and 4 % of recommended Se requirement (55 μg/day), respectively. So, these two herbal distillates could also be considered anticancer due to relatively high Se content.

Overall, considering the proven Zn, Cu, and Fe deficiency in Shiraz [24–27], Rose water with high concentration of essential elements such as Cu, Fe, Se, K, Mg, Na, and Zn seems to be more beneficial for health among the investigated herbal distillates. On the other hand, considering the fact that Ni content of Shiraz soils are high [28], Date palm flower distillate with high nonessential and toxic element contents such as Pb and Ni, if not harmful should be considered a non beneficial herbal distillates.

Herbal Decoctions

The concentration of essential and nonessential elements in the analyzed herbal decoctions is summarized in Tables 5 and 6, respectively. It can be seen that all analyzed herbal decoctions contain macroelements in the range of milligrams per kilogram and microelements in the range of microgram per kilogram. The elemental concentrations vary widely, and compared with herbal distillates, all elements except Zn and Cu displayed higher concentrations. Generally speaking, elements known to be essential for good health, have five general physiological role i. e., Ca, P, and Mg play important roles in bone and membrane structure; Na and K are essential for water and electrolyte balance; Zn, Cu, Se, Mg, and Mo are important in the metabolism of cholesterol and cardiovascular health problems. Cr and Mn in herbal decoctions are known to have some therapeutic properties against diabetic and cardiovascular diseases [29, 30].

Table 5 Concentration of essential elements in herbal decoctions and regulatory limit values (in μg/kg except where noted)
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Table 6 Concentration of non-essential elements in herbal decoctions and regulatory limit values (in μg/kg except where noted)
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The lowest K (3.4 mg kg−1), P (0.01 mg kg−1), and Mg (37.9 mg kg−1) contents occur in Lemon verbena, and the highest occurs in Chamomile, being 1130, 44.2, and 114.7 mg kg−1, respectively. Ca content varied between 79.2 mg kg−1 (Valerian) and 200.7 mg kg−1 (Lavender), while Na ranged from 107.3 mg kg−1 (Starflower) to 289.5 mg kg−1 (Chamomile).

Concentration of microelements in herbal decoction revealed that decoctions are generally rich in Mn, Zn, Fe, Cu, Sr, Ni, and Rb. Presence of Fe, Mn, Cu, and Zn in these herbal products makes them useful nutritional supplements to human health, especially for vegetarians and residents of Shiraz, where a deficiency of these elements is already proven [18, 24–27]. On the other hand, the concentrations of As, Pb, and Cd in some samples exceeded the WHO permissible limit for finished herbal products [13]. Hence, the best benefit for human health lies in obtaining the correct amount of supplementation in the right form at the right time.

Valerian had the highest concentration of Fe and Mn (400 and 1200 μg/kg, respectively), while Starflower displayed the highest Zn concentration (930 μg/kg), and Chamomile the highest Cu concentration (144.6 μg/kg).

The concentration of Cr and U were below the detection limits in all samples. On the contrary, As, Rb, Sr, Sn, Ni, V, and Co were detected in the majority of the samples (Tables 5 and 6). The high concentrations of Al, Pb, and V occur in Lavender (500, 9.8, and 3.5 μg/kg, respectively). The highest concentration of arsenic, as a toxic element occurs in Valerian (12.1 μg/kg). Chamomile and Valerian contained the highest (87.9 μg/kg) and lowest (22.3 μg/kg) concentrations of Ni, respectively. Soil condition, ambient pollution, species, and irrigation water play significant roles in elemental composition of herbal decoctions [18].

Elemental concentrations of herbal raw materials used in making herbal decoctions are presented in Tables 7 and 8. In order to compare elemental concentrations in herbal raw materials and herbal extracts, the extraction ratio was calculated (%). Extraction ratio (ER) is defined as:

Table 7 Concentration of essential elements in raw herbal materials used for making herbal decoction (mg/kg)
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Table 8 Concentration of nonessential elements in raw herbal materials used for making herbal decoction (mg/kg)
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$$ {\mathrm{ER}}_a = \frac{C_{\mathrm{extract}}}{C_{\mathrm{plant}}}\times 100 $$

Where, a is the specific element, C extract is concentration of a in herbal decoctions, and C plant is concentration of a in herbal raw materials [31]. Figure 1 shows the extraction ratios.

Fig. 1
figure 1

Extraction ratio of a essential and b nonessential elements

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The ER varied from 0.01 to 37.21 %. As shown in Fig. 1, for all elements except “Na” ER is below 10 % and shows low extraction ratio [31]. The ER depends on plant species, plant organs used for making decoctions and geochemical behavior of elements.

Comparison of elemental content of Starflower raw material with its distillate and decoction is illustrated in Fig. 2. The general decreasing order of most elements is: raw Starflower > Starflower decoction > Starflower distillate. Based on Fig. 3 (ionic charge vs. ionic radius), Sn+4, Se+4, As+3, As+5, and Be+2 plot in the middle zone, where cations have sufficient density of charge to bind strongly to O2− and form insoluble stable oxides/hydroxides [32]. As already mentioned, high zinc content in herbal distillates probably comes from galvanized pipes of the distillation line. Hence, it displayed lower concentration of these five elements in decoctions than in distillates.

Fig. 2
figure 2

Elemental content of Starflower raw material and its distillate/decoction

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Fig. 3
figure 3

Ionic potential of the studied elements

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Herbal Residues

Rhizome, leaf, and flowers of seven plant species used for making herbal distillates and herbal decoctions were analyzed. Variable concentrations of essential, nonessential, and toxic elements were determined (Tables 9 and 10, respectively). Although the analyzed elements displayed a wide range of concentration, all were significantly higher than corresponding concentration in herbal distillates and herbal decoctions, reflecting the low extraction ratio from raw herbal materials.

Table 9 Concentration of essential elements in herbal residues (mg/kg)
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Table 10 Concentration of nonessential elements in herbal residues (mg/kg)
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In order to identify the similarities in elemental contents of herbal residues, hierarchical cluster analysis (HCA) of herbal residues using furthest neighbor method for calculating cluster distances and applying Euclidean distance as a measure of between plant samples was carried out. Figure 4 shows the results as a dendrogram. All samples were classified according to elemental concentration in to three groups. Group 1, Rose and Lavender; group 2, Fumitory and Chicory; group 3, Lemon verbena, Date palm flower, and Dill. Okem et al. [33] believe that similarity of elemental concentrations in herbal extracts is either the result of using the same plant organs or similar accumulation routs [33]. As shown in Fig. 4, classification is more based on the plant organs from which the herbal products were prepared than the preparation methods. Cluster 1 relates to distillates and decoctions prepared from flowers. Cluster 3 contains herbal residues from the leaves and flowers of the medicinal plants. While, cluster 2 does not relate to any specific plant organs. Based on HCA, and Tables 9 and 10, plant samples in group 1 have displayed high levels of heavy metals, while group 3 plants did not display high metal content.

Fig. 4
figure 4

Dendrogram of herbal residues used for making herbal distillates and decoctions

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Risk Assessment

Important factors involved in the absorption of elements from ingested food include elemental concentrations, amount and types of the food consumed, and the pH of the food [3]. The WHO recommended limits for As, Cd, Cr, and Pb in processed herbal products are 10, 6, 20, and 20 μm/kg, respectively. Also, United Nations Food and Agriculture Organization (FAO) and WHO provisional tolerable weekly intake of As, Pb, and Cd is 15, 25, and 7 μm/kg per body weight, respectively (Tables 3, 4, 5, and 6) [10–13, 34]. No permissible limits for all metals in medicinal plants are set by WHO. The reason is the essentiality of many of these metals as dietary micronutrients for humans [34]. The low concentration of the potentially toxic elements in the analyzed herbal distillates (except date palm flower distillate) indicates that their daily consumption as dietary supplement poses no threat to human health. However, Pb, Cd, and As concentration in herbal decoctions is a different matter and exceeds WHO permissible limits in some samples. Hence, their long-term consumption may cause serious problems for human health. Possible health threats of herbal distillates and decoctions are presented in Tables 11 and 12, respectively. As a rule, HQ and HI greater than 1, needs consideration [3]. Daily intake of herbal distillates and decoctions for a 70-kg individual is assumed to be approximately 100 and 250 ml, respectively.

Table 11 HQ and HI for the analyzed herbal distillates
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Table 12 HQ and HI for the analyzed herbal decoctions
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As shown in Tables 11 and 12, HQs and HI for all measured elements in every herbal distillates and herbal decoctions (except Valerian) are well below 1, meaning that daily intake of metals through consumption of herbal distillates does not pose a significant potential health hazard for a normal adult. Nevertheless, HI for Valerian is between 1 and 5 which is of a concern [35].

Conclusion

In this research, the elemental concentrations of herbal distillates and decoctions consumed by Shiraz residents were measured to determine their potential health risk. The results indicated that different herbal products and distillate/decoction residues display a wide variety of elemental concentrations. Comparison of elemental content between raw herbal material and herbal distillate/decoction of the same species indicated that there is no direct correlation between concentration of elements in plant organs and its products. The reason is apparently the fact that elemental concentration of herbal products apart from abundance in raw herbal materials, also depends on the ionic potential of each element, preparation method, and contamination from preparing instruments and storage containers. Considering the low elemental concentration of herbal products (especially herbal distillates), it seems that most attributed health benefits are apparently more related to organic compounds rather than elemental concentrations. Finally, although phytotherapy may provide essential elements for people especially in the developing countries, but adequate precautions (e.g. controlling type and dose of consumption) seems necessary to avoid metal toxicity.