Herbs and herbal products ideally need to be free of pesticides, fumigants and other hazardous contaminants. At best they may be controlled for the absence of unsafe levels. Herbal drugs are prone to pesticide residues, which accumulate from agricultural practices, such as spraying, treatment of soils during cultivation and administration of fumigants during storage.

Presence of contaminants or residues in herbal drugs can either be accidental or intentional. They may be classified into physicochemical contaminants and biological contaminants. A variety of agrochemical agents and some organic solvents may be important residues in herbal medicines. Contamination should be avoided and controlled through quality assurance measures such as GAP for medicinal plants and GMP for herbal medicines. Chemical and microbiological contaminants can result from the use of human excreta, animal manures and sewage as fertilizers. As per WHO guidelines on GAP for medicinal plants, human excreta must not be used as a fertilizer and animal manures should be thoroughly composted. The level of some contaminants and residues present at the stage of harvesting may change as a result of post-harvest processing (e.g. drying) in herbal preparations such as extracts and in finished herbal products during the manufacturing process.

Hazardous contaminants/residues likely to be present in herbal drugs are discussed below.

1. Toxic metals and non-metals: Presence of lead, cadmium, mercury, chromium, copper, arsenic or nitrite in herbal drugs is attributed to many causes including environmental pollution (contaminated emissions from factories, leaded petrol, contaminated water from rivers, lakes and sea, pesticides etc.). They can pose clinically relevant dangers for the health of the user. The potential intake of the toxic metal can be estimated by the level of its presence in the product and the recommended or estimated dosage of the product. This potential exposure can then be put into a toxicological perspective by comparison with the Provisional Tolerable Weekly Intake values (PTWI) for toxic metals which have been established by the Food and Agriculture Organization of the WHO.A simple, straightforward determination of heavy metals can be found in many pharmacopoeias and is based on colour reactions with special reagents such as thioacetamide or diethyl dithiocarbamate. The amount present is estimated by comparison with a standard. Instrumental analyses have to be employed when the metals are present in trace quantities, in admixture or when the analyses have to be quantitative. The main methods commonly used are atomic absorption spectrometry (AAS), inductively coupled plasma analysis (ICPA) and neutron activation analysis (NAA). The use of herbal medicines is not generally expected to contribute significantly to the exposure of the population to heavy metal contaminants. Still it is recommended that their levels be minimized.
2. Pesticides, agrochemical residues and other persistent organic pollutants: Organic chemicals such as synthetic aromatic chlorinated hydrocarbons generated inadvertently as by-products of combustion or industrial processes and persistent pesticides such as dioxin aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex are likely to remain in herbal medicines.Insecticides, fungicides, nematocides, herbicides, ascaricides, molluscicides, rodenticides and fumigants such as ethylene oxide, ethylene chlorhydrin, methyl bromide and sulfur dioxide are classes of pesticide residues that may accumulate in herbal drugs. Agricultural practices like soil treatment, spraying, fumigation during storage are the sources of such contamination. Of these, only chlorinated hydrocarbons and related pesticides and a few organophosphorous pesticides (e.g. carbophenothion) have a long residual action. Though use of these pesticides is widely discontinued, residues like DDT may still remain in the environment. Copper-based pesticides, though essential in terms of plant nutrition, at higher levels of ingestion (70 mg/day) can have serious adverse effects on health. Also copper is strongly bio-accumulated in nature and therefore is likely to persist in herbal materials. Though most pesticides have very short residual action, herbal materials need to be tested for the presence of organically bound chlorine and phosphorous as a preliminary screening method. Even though there are no serious reports of toxicity due to the presence of pesticides and fumigants, herbal products have to be controlled for their presence to safe levels.TLC and GC methods are available for the determination of organochlorine and urea derivatives, enzymatic methods for organophosphorous compounds, colorimetric methods for urea derivatives and spectroscopic techniques for paraquat triazines and heavy metals. Toxic residues in herbal drugs may be substantially reduced or eliminated by the use of infusions of the dried plant material by the extraction of the useful plant constituents. Storage at 30°C has been shown to reduce rapidly the ethylene oxide residues in senna pods to tolerable levels.HPLC and GC are the principal methods for the determination of pesticide residues when coupled with MS. Samples are extracted by a standard procedure, impurities are removed by partition and/or adsorption and the presence of a moderately broad spectrum of pesticides is measured in a single determination. However these techniques are not universally applicable. Some pesticides are satisfactorily carried through the extraction and clean-up procedures, others are recovered with a poor yield and some are lost entirely. Following chromatography, the separations may not always be complete. Pesticides may decompose or metabolize and many of the metabolic products are still unknown. As a result of the limitations in the analytical technique and incomplete knowledge of pesticide interactions with the environment, it is not yet possible to apply an integrated set of methods that is satisfactory in all situations.Generally the methodology should be adapted to the type of herbal material being tested, and modifications may be necessary for different samples – including seeds, leaves, oils, extracts and finished products – and for samples containing different quantities of moisture. Also, the spectrum of pesticides to be tested for is dependent on the specific pesticides used on the herbal material and the history of use of persistent pesticides in the region.It is therefore desirable to test herbal materials of unknown history for broad groups of compounds rather than for individual pesticides. Various methods are suitable for this purpose. Pesticides containing chlorine in the molecule, for example, can be detected by the measurement of total organic chlorine; insecticides containing phosphate can be measured by analysis for total organic phosphorous, whereas pesticides containing arsenic and lead can be detected by measurement of total arsenic or total lead, respectively. Similarly, the measurement of total bound carbon disulphide in a sample will provide information on whether residues of the dithiocarbamate family of fungicides are present.Importantly, where such general methods are employed, care must be taken to ensure that results are not adversely affected by contributions from certain plant constituents containing the targeted elements. If the pesticide to which the herbal material has been exposed is known or can be identified by suitable means, an established method for the determination of that particular pesticide residue should be employed. General aspects of analytical methodology and commonly recommended procedures for the qualitative and quantitative determination of important pesticide groups are as per WHO guidelines and pharmacopoeias.
3. Solvents: Solvents used in herbal drug processing can be detected as residues in herbal preparations and finished products. Depending upon their potential risk (benzene: class I, methanol, hexane: class II, ethanol: class III), they have to be controlled through GMP and QC.
4. Radioactive contamination: A certain amount of ionizing radiation including nuclides occurring naturally in the earth and atmosphere is unavoidable. WHO in collaboration with several other international organizations has developed guidelines to prevent widespread contamination by radionuclides in the dangerous consequence of a nuclear accident such as in Chernobyl and Fukushima. Taking into consideration the quantity of herbal medicine normally consumed, this is an unlikely health risk. Thus at present no limits are proposed for radioactive contamination.