Introduction

Use of herbal medicine is a traditional way to cure ailments and has been applied for more than five millennia in several civilizations (Petrovska 2012). Even today, plant material continues to play a major role in primary health care as therapeutic agents in many developing countries. For example, Ocimum sanctum and Azadirachta indica are known since ages to cure diseases (Dimitrova 1999; Wiart 2006; Petrovska 2012). Besides using herbal medicines directly to treat many diseases, natural products have contributed enormously to the development of important therapeutic drugs used currently in modern medicine (Govindaraghvan and Nikolaus 2015). It is estimated that about 25% of all modern medicines are directly or indirectly derived from higher plants (Bodeker et al. 2005; Ziarati 2012). Rapid industrialization and unorganized urbanization may cause accumulation of toxic substances such as metals and pesticides in soil, water, and air (Kishan et al. 2014; Kumar et al. 2015; Rodriguesa et al. 2017) Heavy metals can enter the body either through food, air, or water and bio-accumulate over a period of time on chronic exposure (Srogi et al. 2002; Duruibe et al. 2007; Neha et al. 2017; Kumar et al. 2018). The relative toxicity of heavy metals to living beings follows the following pattern: Hg > Cu > Zn > Ni > Pb > Cd > Cr > Sn > Fe > Mn > Al (Anna et al. 2015). Plants are sessile and rely on their environmental conditions for growth and development. They accumulate heavy metals in their harvestable parts via root uptake, foliar adsorption and decomposition of specific elements (Kishan et al. 2014). Contamination and accumulation of toxic elements in herbal medicines depends on factors like species, harvesting time, level and duration of contaminant exposure, cultivation, processing, topography, geographical origin, and storage of the plant material (Arpadjan et al. 2008). Heavy metals enter into food chain through their accumulation in plants where they find place through uptake and transport via transporters of essential elements according to their structural analogy (Chauhan et al. 2017). The uptake of contaminants in plants occurs through the root system, which provides a wide surface area for the absorption (Ouyang 2002). The transfer of bound ions from the extracellular space to the hydrophobic environment of the membrane into the cell is facilitated by the transmembrane structures. Translocated metals become complexed and sequestered in cellular structures of shoots (Lasat et al. 1998). Translocation of metal-containing sap occurs from the root to the shoot in a process that is mainly controlled by root pressure and leaf transpiration (Khatoon et al. 2017). However, pathways involved in the transfer of POPs from air to plants involve gaseous exchange, particle-bound deposition, and wet deposition (McLachlan and Horstmann 1998). Conditions such as hydrophobicity, water solubility, and vapor pressure govern penetration of POPs into the vegetation. In addition, environmental characteristics, such as temperature, organic content of the soil and plant species, also influence the pollutant uptake mechanism (Khatoon et al. 2017).

The possibility of transmission of toxic heavy metals to humans and animals through the application of herbs grown in polluted area is a major concern. To get desirable therapeutic benefits, quality of the raw herbs should be ensured in terms of metal contamination. WHO (2007) advocates that herbs and herbal products should not be used without qualitative and quantitative analysis of their heavy metal contamination (Kishan et al. 2014). Physiochemical properties, safety, and efficiency of the herbal commercial products can be augmented by following good agriculture and collection practice (GACP), good manufacturing practice (GMP) before and during the manufacturing processes, and good laboratory practice (GLP) (Govindaraghvan and Nikolaus 2015).

Apart from metals, herbal drugs may also contain pesticide remainders, through faulty agricultural practices, such as spraying, handling of soils during farming, and administering fumigants during storage (Kunle et al. 2012; Mishra et al. 2007). Pesticides are used in agriculture to protect plants from insects, pests, and pathogens and to improve the quality and amount of harvest, but the accumulation of pesticide residues in edible parts of plants may cause toxic and allergenic effects on human health (Srivastava et al. 2006a, b). The prime adverse effects associated with the chronic exposure to organic pollutants are nervous disorders, including headache, dizziness, tremor, discoordination, and/or convulsions (Shaban et al. 2016). The maximum residue limits (MRLs) for pesticides have been set in the Commission Regulation and the Pesticide database of the European Union (Reinholds et al. 2017). There are quite meager evidence available on the quality and safety of medicinal herbs and their products sold in the local market. This study aims to determine the level of heavy metal and pesticides contamination in some frequently used herbs.

Material and methods

Sample collection

Twenty raw medicinal plant materials (Table 1) were collected from local market of Lucknow, U.P., India. Plant materials were washed with tap water followed by distilled water and after drying ground manually and subjected to analysis.

Table 1 Medicinal plants investigated for metal and pesticides contamination and their medicinal uses (Rastogi and Mehotra 1991, 1993)
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Metal analysis

The samples were digested in solution of nitric acid and perchloric acid mixture (1:3; v/v). One gram accurately weighed ground sample and 20 ml of aquaregia solution was placed into a 50-ml conical flask. The samples were heated on a hot plate at 70–80 °C until a clear solution was obtained. 0.1 N nitric acid was prepared for volume make-up of the digested sample. The metals content were analyzed on an atomic absorption spectrophotometer (AA 240 FS, Varian).

Pesticides estimation

Ten grams of the sample was suspended in 50 ml of petroleum ether-acetone mixture (1/1 v/v) and shaken on an Excella E24 incubator shaker series for 3 h. The extract was filtered and concentrated to exactly 1 ml by using rotatory evaporator IKA RV 10 and nitrogen stream respectively. The samples were dissolved in a mixture of acetone and n-hexane (1:1; v/v). Column cleanup was done with anhydrous sodium sulfate and Florisil (activated magnesium silicate).50 ml of eluting solvent (n-hexane: ethyl acetate: dichloromethane in the ratio of 70:15:15) was used for each sample. The samples were run on GC (Agilent 7890 A) equipped with ECD detector.

To control analytical quality, reagent blanks and sample replicates were used during the analysis to assess contamination and accuracy. Stock standards were used from Merck, India, traceable to the National Institute of Standards Technology (NIST) to establish calibration curves. Recovery studies of metal determination were performed using CRMs to display the efficiency of the methods used. The recovery rates were ranged between 86 and 103%.

Result and discussion

Range and mean concentration of the five metals viz. cadmium, chromium, copper, iron, and lead analyzed in 20 medicinal plants has been summarized in Table 2. The total metal concentration in selected plant material ranged between 44.73 and 385.15 μg g−1. In most of the samples, the metal concentration was found to be beyond the WHO (2007) standards. Results revealed the expected influence of ambient environmental conditions (soil and air) on the total metal profile as the lower level of concentration of investigated metals was found in herbs growing in relatively clean regions than in those growing in polluted areas. The concentration of Pb and Cr in various medicinal herbs ranged between 13.26–54.47 and 17.63–58.63 μg g−1 which was far above the standards prescribed by WHO, i.e., 10 and 2 μg g−1 respectively. The concentration of Pb and Cr found in medicinal plants warns about the risk associated with the ingestion of contaminated herbal medicines (Haider et al. 2004). Correlation coefficient of lead with chromium (R2 0.7611) indicates the common source of origin of these metals. The presence of lead in plant material indicates the probable influence of vehicular activity on metal concentration in ambient environment (Khan et al. 2008).

Table 2 Heavy metal concentration (μg g−1) in different medicinal plants (mean ± SD)
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Among the pesticides analyzed, the total DDT found in medical plants ranged between 0.63 and 7.14 μg g−1 (Table 3). The maximum concentration of total DDT was found in Lantana camara (7.14 μg g−1), the minimum in Asparagus racemosus(0.63 μg g−1). Industrial effluents and waste material from pesticides factories release DDT to terrestrial and aquatic environments. Usually, DDT evaporates from soil and surface water into air, while some is broken down by photodegradation or by the microorganism in soil or surface water. When DDT is broken into soil, it usually forms DDE or DDD (Hellawell 1988). Thus, the presence of DDT or its metabolites shows that the sites where these herbs were grown were heavily contaminated with DDT, whereas the availability of DDE and DDD indicates soil contamination due to spraying of pesticides for pest control which subsequently underwent photodegradation.

Table 3 Concentration and range of DDT and its metabolites (μg g−1) detected in medicinal plant materials
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Among the different HCH isomers characterized, it has been observed that α-HCH predominates the total HCHs profile (Table 4). Lindane and other HCH isomers are highly resistant to microbial and chemical degradation and thus persist in the environment for prolonged duration. The range of total HCH found in medical plants was ranged between 1.63 and 6.44 μg g−1. The maximum (6.44) and minimum (1.63 μg g−1) concentrations of total HCH were found in Rosa rubiginosa and Tinospora cordifolia respectively.

Table 4 Concentration and range of HCH isomers (μg g−1) detected in different medicinal plant materials
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For the concentration of heavy metals, DDT and HCH detected in the samples were well above the permissible limits prescribed by WHO (2007). Findings warrant urgent attention of the quality assurance agencies to detect the level of toxic content in the herbal plant material before processing it for herbal formulation.

Overall, the results of heavy metal and pesticide analysis showed that the contaminants are present in varied concentrations in the 20 medicinal herbs commonly sold in the local market of Lucknow. Variation in the metal and pesticide accumulation may be assigned to the different anatomical and chemical characteristic of particular plant species including stage of growth, soil type, and the type of metals absorbed (Verma et al. 2007; Olowoyo et al. 2012; Orisakwe et al. 2012). Furthermore, contamination could occur during storage and/or at the point of sale. Therefore, it is not only the growth conditions but harvesting and processing also adds to the metal and pesticides contamination.

Results of the study revealed that the content of heavy metals and pesticides detected in the medicinal plant samples were above the permissible limits. Sources of heavy metal and pesticide contamination in herbs could be linked to water used in irrigation, polluted soils, fertilizers, pesticides, industrial emissions, transportation, harvesting, and storage processes. It is evident that there is an urgent need to implement a regular monitoring and testing program on the quality of the local and imported herbs sold in the market. Awareness among the suppliers and consumers should be disseminated to prevent collection of medicinal herbs growing near contaminated sites to prevent health risk associated with their consumption.