Journal of Radioanalytical and Nuclear Chemistry

, Volume 295, Issue 1, pp 143–150

Toxic element composition of multani mitti clay for nutritional safety


    • Chemistry Division, Directorate of SciencePakistan Institute of Nuclear Science and Technology (PINSTECH)
  • Y. Faiz
    • Chemistry Division, Directorate of SciencePakistan Institute of Nuclear Science and Technology (PINSTECH)
  • S. Rahman
    • Chemistry Division, Directorate of SciencePakistan Institute of Nuclear Science and Technology (PINSTECH)
  • N. Siddique
    • Chemistry Division, Directorate of SciencePakistan Institute of Nuclear Science and Technology (PINSTECH)

DOI: 10.1007/s10967-012-1876-x

Cite this article as:
Waheed, S., Faiz, Y., Rahman, S. et al. J Radioanal Nucl Chem (2013) 295: 143. doi:10.1007/s10967-012-1876-x


Geophagy of multani mitti (MM) clay is very common in central Pakistan especially amongst women. It was therefore mandatory to establish baseline levels of toxic elements in this clay for its safe dietary consumption by consumers of different genders, age groups and physical states. Instrumental neutron activation analysis and atomic absorption spectrometry techniques were used to determine the nutritional safety of MM clay for oral intake. All quantified toxic elements were detected at trace levels with composition in the descending order; Pb > Br > As > Sb > Hg > Cd. Comparison of these elements in MM clay with other clays shows that As, Cd, and Pb, are lowest in MM clay while its Br and Hg contents are high. Highest weekly dietary intakes of As, Br, Cd, Hg, and Sb were found to be 18, 0.05, 1.6, 9.2 and 1.1 % of the respective recommended provisional tolerable weekly intakes. The findings of this study show that As, Br, Cd, Hg and Sb in MM clay are well below the tolerance levels. However its Pb concentration is very high and may pose health concerns. The data presented in this study can be used as national base level guideline for geophagy of MM clay by men, women (normal, pregnant and lactating) and children.


Atomic absorption spectrometry (AAS)ClayInstrumental neutron activation analysis (INAA)Multani mitti (MM)Provisional tolerable weekly intake (PTWI)Toxic elements


Human beings have been aware of the beneficial health effects of clays since prehistoric times. Many ancient civilizations ingested and used natural clays in small amounts for nutritional and therapeutic purposes [1, 2]. Clays are unique in their mineral composition and help strengthen the body and enhance our natural immune system. Clays are also used in pharmaceutical formulations. The habit of eating earth components including clay is commonly termed as geophagy and can counteract the inadequacy of minerals supply from food. Nature has laid down a balanced ratio of different essential trace minerals in clays depending on their type and geophysical feature. Clay minerals can adsorb and retain harmful and toxic substances in human beings. Since ancient times throughout the world, the beneficial health effects of geophagy were known especially in the remediation of gastrointestinal disorders [1]. Ingestion of clay by pregnant women has often been recommended to overcome deficiencies of some essential elements (especially Ca, Mg, Zn, Fe, Cu, Mn, and Se) [3]. Depending upon the geological appearance and composition of a particular clay it may also contain toxic and heavy metals such as As, Cd, Hg, Pb and Sb. Human exposure to toxic elements has exponentially increased over the last few decades due to combustion of fossil fuels, industrial emissions and waste discharge, use of agricultural fertilizers, insecticides and pesticides, and domestic processes etc. Therefore the potential for long-term adverse health effects is obvious and well documented [4]. These toxic and heavy metals enter our body directly through ingestion and inhalation and can directly influence behavior causing impairment of mental and neurological functions and altering metabolic processes in the body [5].

In Pakistan geophagy of multani mitti (MM) clay is very common especially amongst the women folk and children. People generally use raw MM clay for oral ingestion, as medication, in aesthetic medicine and as body and hair wash. Although therapeutic properties of clays have gained especial attention, it is its toxic contaminants that require scrutiny. It is believed that people who are habitual consumers of unprocessed clay may be at risk for several significant health problems due to intake of toxic elements over long period of time [6]. Moreover any material to be consumed orally or as medication as in pharmaceuticals must not be toxic or have severe side effects. The presence of these elements even in minute concentrations can be a potential threat to the consumer or patient [4]. It is therefore necessary to asses its quality through accurate and precise characterization of the toxic element contents to identify varieties of clays which are suitable for direct consumption.

Characterization of toxic elements in MM is imperative to establish baseline of these elements for nutritional safety. In this study toxic elements have been quantified in MM clay using instrumental neutron activation analysis (INAA) together with atomic absorption spectrometry (AAS). As children and pregnant women are at greater risk of exposure to toxic elements due to ingestion of MM clay, this work takes into consideration the intake of MM clay in these subjects along with men, women and lactating women.


Sampling and sample preparation

MM clay reserves are located 45 km North West of Dera Ghazi Khan District in Pakistan. The samples were collected in clean plastic bags from this site and brought to the Pakistan Institute of Nuclear Science and Technology (PINSTECH) in Islamabad where sample preparation and analyses were performed. Sample processing and preparation was done in a laminar flow fume hood. The samples were crushed and sieved to obtain homogenized powder with particle size of <0.125 mm. The processed samples were stored in pre-cleaned polyethylene bottles. The homogeneity of the prepared MM material was checked through analysis of Mn contents in 100 mg samples. The measurement variation was found to be <6 % around the mean values confirming the homogeneity of the preparation.

INAA methodology

Two IAEA reference materials (RMs) (IAEA soil S-7 and IAEA Marine sediment SD-M-2/TM) were used to ensure traceability of the results. The same RMs were also used as multi-elemental standards for quantification of toxic elements in the sample using INAA. Approximately 100 mg of MM samples in triplicate along with IAEA RMs were packed and sealed in polyethylene capsules. Taking into consideration the elements to be quantified different suites of samples were prepared and packed in reactor rabbits. All irradiations of these targets were performed adjacent to the reactor core of the 27 kW tank-in-pool type miniature neutron source reactor. The thermal neutron flux density at this site was 1 × 1012 cm−2 s−1. All samples after desired cooling were transferred to pre-cleaned and pre-weighed polyethylene counting capsules for analysis.

High purity germanium detector (Canberra Model AL-30) coupled to a PC-based Intertechnique Pro-286e Multichannel Analyzer (MCA) was used for quantification of MM samples in accordance with the optimized radio-assay scheme. The system resolution was 1.9 keV at 1332.5 keV peak of 60Co with peak to Compton ratio of 40:1. “Intergamma, version 5.03” software was used for obtaining all spectra. An indigenous computer program was used for the calculations and statistical data treatments [7].

AAS methodology

Approximately 1 g of MM clay sample was taken in 100 ml digestion flask with air condenser (30 cm long) attached to it and 10 ml of aqua regia was added to it. For analysis of soil and clay samples instead of total digestion usually pseudo total digestion is performed with boiling aqua regia under reflux. The contents were heated on a hot plate at 85 °C for leaching for 2h. After cooling, drop wise 1.5 ml of H2O2 was added at 60 °C until completion of reaction [8]. The leachate was filtered and solution was made up to 10 ml in a volumetric flask with de-ionized water. The digested samples were analyzed in triplicate. The applied procedure was validated by simultaneous processing and analysis of IAEA S-7 and IAEA SD-M-2/TM RMs for Cd and Pb. Quantification of Cd, and Pb, was carried out using a Hitachi model Z-2000 Polarized Zeeman Atomic Absorption Spectrometer (AAS) coupled with software based data handling facility.

Results and discussion

MM clay has been studied for its toxic elemental content using INAA and AAS techniques. For the quantification of As, Br, Hg and Sb by INAA, two radio-assay protocols presented in Table 1 were employed. In the first scheme the targets were irradiated for 1h and after subsequent cooling for 1–2 days 76As, 82Br and 122Sb were determined. The second scheme quantified 203Hg where the sample was irradiated for 5 h followed by 2–3 weeks of cooling time. The observed elemental interferences were handled as mentioned in our earlier works [9, 10]. Cd and Pb in MM samples were determined using AAS.
Table 1

Nuclear data and irradiation conditions

Isotope used

Half life

γ-ray used (keV)

Irradiation time (h)

Cooling time

Counting time


21.3 h



1–2 day

30 min


35.3 h



1–2 day

30 min


2.70 day



1–2 day

30 min


46.6 day



2–3 weeks

2 h

Corte et al. (1986) [40]

The quality assurance of the elemental analysis was ascertained by comparison of the results of two control material, IAEA S-7 and IAEA SD-M-2/TM obtained using INAA and AAS with the certified data. The accuracy of results helps to ensure the trueness and bias of the adopted methodology in accordance with ISO-5725 standards [11]. Table 2 shows good agreement between the IAEA certified values and results obtained in the current study using INAA and AAS. The overall precision for the measurement of all elements when analyzed through paired difference test at 95 % confidence level was found to be acceptable and in range of these RMs [12, 13]. Relatively high error was observed for Cd measured by AAS which may be due to the fact that Cd in IAEA S-7 is not certified while in IAEA SD-M-2/TM the certified concentration of this element is low with large reported error. Hg quantified by INAA also shows lower precision as in IAEA S-7 only information value is reported for this element. Moreover high variation coefficient (νc) of Hg in both standards could be due to its low concentration and low count rate for both standards.
Table 2

Quality assurance data for IAEA matrix RMs (values expressed in μg g−1 unless otherwise specified)




Certified value

95 % CI

Our value ± Unc.

Certified value

95 % CI

Our value ± Unc.




12.8 ± 1.01



18.9 ± 1.22




7.80 ± 0.62



64.7 ± 3.59




1.40 ± 0.17



0.130 ± 0.02




0.047 ± 0.006



0.061 ± 0.01




58.0 ± 3.61



23.5 ± 1.51




1.65 ± 0.14



1.04 ± 0.10

CI confidence interval, () information values

aDetermined by AAS

Toxic elements quantified in MM clay using INAA and AAS are presented in Table 3. The concentrations of all elements (As, Br, Cd, Hg, Pb and Sb) are reported on dry weight basis and are the averages of several determinations. INAA of MM clay shows good precision for As and Br with νc < 10 % around the mean values. However for Hg and Sb relatively high νc > 10 % is observed. Concentration of Pb determined by AAS shows very good precision with νc < 5 % while for Cd νc is high (14.8 %). The composition of toxic elements in MM is quantified in the descending order; Pb > Br > As > Sb > Hg > Cd where all measured toxic elements are present in trace amounts.
Table 3

Global comparison of MM clay with other reported clays (values expressed in μg g−1 unless other wise specified)








MM Clay

2.46 ± 0.16

2.93 ± 0.25

0.101 ± 0.015

0.137 ± 0.017

8.30 ± 0.30

0.423 ± 0.050










SW-y-2 (SC)39

20.63 ± 3.14

0.436 ± 0.076


14.6 ± 0.6

1.39 ± 0.19

NIST-SRM 679 (BC)40

9.7 ± 0.4

1.0 ± 0.1







Sikor15 (BC)




Comparison of our results for toxic elements in MM clay has been made with other clays. Table 3 presents results on these elements from Brazilian white and green clays (GC) used for pharmaceuticals and cosmetics, SW-y-2 Na rich bentonite from Wyoming, Ohio red clay (RC), NIST-SRM 679-brick clay, Muddy clay (MC) from eastern Gulf of Finland and Sikor-backed clay from Bangladesh [1417]. Assessment of this data shows that As (2.46 μg g−1), Cd (0.101 μg g−1), and Pb (8.30 μg g−1), are lowest in MM while its Br (2.93 μg g−1), and Hg (0.137 μg g−1) content are high. However As in MM clay falls in the range cited for white clays (WC). RC and MC contain very high levels of As with respective concentrations of 14.6 and 11.8 μg g−1. Cd levels in MC are comparable to MM Clay and Sikor (BC) has relatively high concentration of this element. Pb content in SW-y-2 (SC), MC, and BC (20.6, 19.2, and 23.2 μg g−1 respectively) are comparable and almost twice the levels of MM clay. Br levels in some of the GC are the lowest with a range of 0.5–3.3 μg g−1 and therefore Br levels in MM clay and WC fall in this range. Hg with range of 0.12–0.021 μg g−1 and Sb with range of 0.20–0.57 in MC are the lowest. Concentration of Sb in MM (0.423 μg g−1), GC 0.48 μg g−1) and SC (0.436 μg g−1) are comparable while its value in RC (1.38 μg g−1) and BC (1.0 μg g−1) is high. On the whole the composition of MM clay is close to Bangladeshi sikor (BC). The variation in elemental concentrations in different clays may be attributed to differences in mineralogy, depth of the sample site, geographic location and its age. Comparison of clays shows that MM clay contains low levels of As, Cd, Pb and Sb while it’s Br and Hg levels are higher.

As toxic elements are reported to accumulate in the body, the relevance of these elements in MM clay through dietary intake has also been evaluated and compared with the provisional tolerable weekly intakes (PTWI) set by the Joint FAO/WHO Expert Committee on food additives (JECFA) [18]. It was important to compare our values with PTWI as in this way regular ingestion of MM clay can be scrutinized for its consumption over a long period of time without appreciable health risk. Geophagy of MM is more popular in children and women, especially in pregnant and lactating women. It is reported that most of the toxic elements in pregnant and lactating women can transfer from the mother to the fetus and through milk to newborn placing the health of the unborns and newly born baby at risk [19]. Considering this fact, PTWI values have been used to estimate its values for men (MMM), women (MMW), pregnant women (MMP), lactating women (MML) and children of ages 4–8 (MMC1) and 9–13 (MMC2) with respective estimated body weights of 70, 55, 65, 60, 20 and 40 kg. Weekly contribution to PTWI value, in which the percentage numbers are showed as the ratio of the intakes by geophagy of the MM clay to the PTWI values for each subject groups are presented in Table 4. The weekly intakes of characterized elements have been calculated on the basis of domestic survey with fixed amount of MM clay ingested by each category.
Table 4

Weekly intake of toxic elements through consumption of MM clay in Pakistan (All intakes expressed in μg per week unless specified)

Toxic elements

PTWI μg week−1

MMM 15 g week−1 bw = 70

Weekly contribution to PTWI value

MMW 30 g week−1 bw = 55

Weekly contribution to PTWI value

MMP 70 g week−1 bw = 65

Weekly contribution to PTWI value

MML 50 g week−1 bw = 60

Weekly contribution to PTWI value

MMC1 5 g week−1 bw = 20

Weekly contribution to PTWI value

MMC2 10 g week−1 bw = 40

Weekly contribution to PTWI value















































































Arsenic (As)

As is ubiquitous in our environment and is a poison in large amounts. It is the third most prevalent element in the earth’s crust. As has been declared as a human carcinogen by the International Agency for Research on Cancer (IARC) since 1980 [20]. Excessive exposure to this element poses health problems such as various types of cancer, cardiovascular diseases, diabetes and neurological disorders as well as dermal effects [20]. According to the National Research Council excessive As intake may pose greater risk for both cancer and non-cancer effects in infants and children. Numerous other disorders are linked to this element including respiratory problems, nervous system effects, low IQ and reproductive effects. Pregnancy complications have also been related to As intake such as fetus abnormalities, abortions, stillbirths, premature deliveries and reduced birth weight of babies. Even low level As-exposures have been reported to affect human health especially in malnourished people [21]. Trace amounts of As were quantified in MM clay with concentration of 2.46 μg g−1. The calculated intake values for As in MMM, MMW, MMP, MML, MMC1 and MMC2 presented in Table 4 are 36.9, 73.8, 172, 123, 12.3 and 24.6 μg respectively. Figure 1 shows that intake of As by each category of MM consumer contributes to about 3.6, 9.1, 18, 13.9, 4.2 and 4.2 % of the recommended PTWI values which is equivalent to 14.7 μg kg−1 body weight per week [18, 22]. The estimated dietary exposures of As through intake of MM clay for all the population groups studied are well below the PTWI and are unlikely to constitute a risk to health. However pregnant and lactating women should be careful in consuming large amounts of MM clay.
Fig. 1

Comparison of weekly As intake through ingestion of MM by different subjects with respective PTWI values

Bromine (Br)

Bromine is naturally present at very low levels in food articles. Elemental bromine is toxic Once ingested it is very slowly excreted from the body causing conditions of intoxication called bromism with adverse health effects. This state is characterized by neurological, psychiatric, dermatological, delirium, psychomotor retardation schizophrenia and possibly endocrine effects [23]. Consuming dietary articles with high Br content can increases its concentration in our body which competes with iodine receptors; eventually depleting the iodine of body and introducing a deficiency condition of “brominated thyroid” [24]. Apart from increased risk of thyroid gland iodine deficiency it also leads to an increased risk of breast, ovary and prostate cancers. Br can cause headache, fatigue, weight-gain, heart and kidney diseases. Trace levels of Br were determined in MM with concentration of 2.93 μg g−1. Weekly dietary intakes of 44.0, 87.9, 205, 147, 14.7 and 29.3 μg were calculated for MMM, MMW, MMP, MML, MMC1 and MMC2 respectively (Table 4). A comparison is made with PTWI values that conform to 7,000 μg per kg body weight per week. The respective intakes MM by each type of consumer contribute to a very small fraction of 0.009, 0.023, 0.045, 0.035, 0.010 and 0.010 % of these estimated PTWI value as presented in Fig. 2. The contribution of Br in MM is well below the tolerance levels of this element and hence, this clay is safe from nutritional safety point of view for all types of consumers.
Fig. 2

Comparison of weekly Br intake through ingestion of MM by different subjects with respective PTWI values

Cadmium (Cd)

Cadmium is considered a toxic element with no biological function in humans. This element is widely present in the environment and causes various health problems in the general and exposed population. Main route of Cd exposure in humans is through diet. It is transported from the gastrointestinal tract by the blood to the liver and finally accumulates in the kidneys [25]. Consequently Cd has been classified as carcinogenic to humans by the IARC [26]. The major long-term toxic effects of low-level cadmium exposure are renal injury, obstructive pulmonary disease, osteoporosis, and cardiovascular disease. Cd has been associated with cancers of the lung, prostate, pancreas, and kidney as it is a strong human carcinogen [27]. High intake to Cd causes acute gastrointestinal effects with increased salivation, choking, vomiting, abdominal cramps and diarrhoea leading to kidney damage [23]. High exposure of Cd is considered responsible for a clinical disease termed as Itai-itai disease in Japan. The characteristic symptoms of this disease are multiple fractures of bones and damaged kidneys. Cd in MM clay was quantified as a trace element with concentration of 0.101 μg g−1 by AAS technique. Table 4 shows the calculated dietary intakes of this element are 1.52, 3.02, 7.07, 5.05, 0.51 and 1.01 μg for MMM, MMW, MMP, MML, MMC1 and MMC2 respectively. Figure 3 shows that these Cd intakes through ingestion of MM clay by the studied subjects contribute to a minor fraction of about 0.31, 0.79, 1.55, 1.20, 0.36 and 0.36 % respectively to the estimated PTWI values for each type of subject. These PTWI values have been calculated considering its recommended value of 7.0 μg kg−1 body weight per week. The calculated intake values for MM clay are far below the weekly tolerance level, therefore the estimated intake of Cd through ingestion of MM clay can be considered fairly undisruptive for its dietary safety by all consumer categories.
Fig. 3

Comparison of weekly Cd intake through ingestion of MM by different subjects with respective PTWI values

Mercury (Hg)

Mercury, a heavy metal, is widespread and persistent in the environment. All forms of Hg whether metallic, inorganic or organic forms are perilous for human health However, Hg released into the air or water becomes methylated which is its highly toxic state [23, 28]. Hg is considered cytotoxic, immunotoxic, neurotoxic, reproductive and developmental toxin and causes cardiovascular damage and disease. Excessive Hg levels can cause permanent neurologic and kidney impairment [28]. Diverse cognitive, personality, sensory, and motor disturbances have been reported for this element. Prominent symptoms due to excessive Hg intake include muscle tremors, emotional disturbances, insomnia, memory loss, neuro-muscular changes, swelling of salivary glands, excessive flow of saliva, loosening of teeth, headaches, and performance deficits in tests of cognitive function [29, 30]. This element is also related to immunological effects causing nephrotic syndrome. Hg can cause hypertension as it affects the hormone metabolism, vasoconstriction and renal tubular function [31]. In MM clay Hg is present as a trace element (0.137 μg g−1). Table 4 shows that the estimated weekly intakes for MMM, MMW, MMP, MML, MMC1 and MMC2 are 2.06, 4.11, 9.59, 6.85, 0.69 and 1.37 μg respectively. These very low amounts of Hg in the studied groups were compared to estimated PTWI of 1.61 μg kg−1 body weight per week. Figure 4 shows that our intake values for Hg contribute to about 1.8, 4.6, 9.1, 7.1, 5.3 and 5.7 % respectively to the weekly PTWI. The percentage intake of Hg through consuming MM clay by all consumers is well within the safe weekly dietary limits.
Fig. 4

Comparison of weekly Hg intake through ingestion of MM by different subjects with respective PTWI values

Lead (Pb)

Lead is a toxic metal and its exposure causes numerous health problems, affecting nearly every system of the body and hence is considered a cumulative toxicant. It produces broad range of physiological, biochemical and behavioral dysfunctions [32]. Severe health effects occur through its dietary intake as it accumulates in highest concentrations in liver, kidney and bone. This element is a well-documented toxin for neurological, haematological, gastrointestinal, cardiovascular and renal systems. Chronic exposure to Pb in adults causes anemia, renal dysfunction, peripheral neuropathy, hypertension, reproductive dysfunction and Alzheimer’s disease [33]. Neurotoxic behavior is particularly receptive in childhood Pb exposure that influences the development of the central nervous system in young children [34]. Pb introduces multiple neurobehavioral and cognitive defects, including behavioral problems and decreased intelligence quotient [33]. The Pb concentration in MM clay was found to be 8.3 μg g−1 using AAS technique. The estimated weekly intakes of Pb through ingestion of MM clay by MMM, MMW, MMP, MML, MMC1 and MMC2 are 125, 249, 581, 415, 41.5 and 83 μg respectively (Table 4). These intakes have not been compared to the Pb PTWI values since JECFA on the basis of Pb dose response analysis declared that the established PTWI value for this element (25 μg kg−1 week) is no longer considered health protective and was withdrawn [35]. It is however important to note that in comparison to other toxic elements quantified in MM clay, Pb contributes to a relatively high intake especially for pregnant and lactating females. Taking into account the Pb intake from all other weekly dietary sources, MM clay contributes to a fairly larger budget for Pb. It is therefore comprehended thought that further work will be carried out to monitor blood Pb levels in regular consumers of MM clay.

Antimony (Sb)

Antimony is a potentially toxic trace element with no known biological role. However Sb along with other toxic metals can disturb the cellular defense mechanisms of the human body including reproductive disorders and chromosome damage resulting in mutagenic changes [36]. Specific symptoms of intoxication are generally accompanied by stomach pain, nausea, vomiting, diarrhoea, conjunctivitis, dermatitis, bronchitis, dry throat headache, coughing, anorexia, insomnia and vertigo [3739]. Continuous intake may cause more serious health effects such as lung diseases, myocardial symptoms, joint or muscle pain, stomach pain, diarrhea, severe vomiting, stomach ulcers and death. Sb is present in MM clay as a trace element (0.423 μg g−1). Calculated weekly Sb intake through consumption of MM clay by MMM, MMW, MMP, MML, MMC1 and MMC2 groups as presented in Table 4 are 6.35, 12.7, 29.6, 21.2, 2.12 and 4.23 μg respectively. Figure 5 shows a comparison of Sb intake values from MM clay with estimated PTWI values. The calculations have been made with respect to 42 μg kg−1 body weight per week. These intakes with respect to the estimated PTWI values for each type of subject are 0.22, 0.55, 1.08, 0.84, 0.25 and 0.25 % respectively. Our study shows that weekly intake of MM clay contributes to fairly safe levels of Sb.
Fig. 5

Comparison of weekly Sb intake through ingestion of MM by different subjects with respective PTWI values


Different toxic elements in MM clay were characterized and discussed to ascertain its safe ingestion by consumers of all ages and conditions. INAA technique was used to characterize As, Br, Hg and Sb while Pb and Cd were determined by AAS. All measured toxic elements were detected at trace levels with composition in descending order as Pb > Br > As > Sb > Hg > Cd. Comparison of our results with other clays shows that As, Cd, and Pb, are lowest in MM clay while Br and Hg contents are highest. To check the nutritional safety of MM clay for ingestion the concentration data for these elements were compared with recommended PTWI values. Keeping in mind various consumers it was estimated that weekly contribution of As, Br, Cd, Hg, and Sb are <18, 0.05, 1.6, 9.2, and 1.1 % respectively for their PTWI value. It is assessed from these findings that although As, Br, Cd, Hg and Sb in MM clay are within the safe and tolerable levels however relatively high intake of As and Hg can have toxicological significance. Moreover Pb concentration in MM clay is also fairly high which may cause severe health concerns. It is therefore recommended that regular monitoring of blood Pb levels of all MM clay consumers be carried out on a regular basis to check for their dietary exposures to lead.

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© Akadémiai Kiadó, Budapest, Hungary 2012