Overview
Kortei and colleagues from the University of Health and Allied Sciences, Ho, Ghana measured the elemental content of kaolin clay from Anfoega (Volta Region) using flame atomic absorption spectrophotometry (AAS) and conducted a health risk assessment including estimated daily intake (EDI), hazard quotient (HQ), target hazard quotient (THQ), and carcinogenic risk estimates following USEPA methodology. The clay samples are from the same location as Kortei et al. 2019 (Pan African Medical Journal) and report identical mineral concentrations, indicating the same material was analyzed in both publications. The risk assessment adds HQ and cancer risk calculations not in the 2019 paper. The key finding is that the total target hazard quotient (TTHQ) of 15 and elevated cancer risk values for arsenic (2.82) and nickel (19.38) indicate that consuming this clay poses health risks to pregnant women, despite absolute concentrations being relatively low compared to some other regional studies.
Key numbers
Table 2 (p. 4): Macro mineral concentrations (mg/kg, wet weight)
| Mineral | Mean concentration (mg/kg) | WHO Recommended Daily Intake |
|---|---|---|
| Iron (Fe) | 1.38 +/- 1.5 | 18 mg |
| Copper (Cu) | 2.40 +/- 1.5 | 0.9 mg |
| Zinc (Zn) | 7.74 +/- 1.5 | 11 mg |
| Potassium (K) | 4.01 +/- 1.0 | 3100-3500 mg |
| Magnesium (Mg) | 13.24 +/- 2.2 | 280-350 mg |
| Sodium (Na) | 13.76 +/- 2.1 | 500-2400 mg |
Table 3 (p. 5): Toxic trace element concentrations
| Mineral element | Mean concentration | WHO/FAO PMTDI |
|---|---|---|
| Arsenic (As) | 1.63 +/- 0.03 microg/kg | 3.0 microg/kg BW/day |
| Manganese (Mn) | 4.72 +/- 0.8 mg/kg | 4.9 mg/kg (PMTDI for 60 kg person: 294 mg/day) |
| Lead (Pb) | 0.53 +/- 0.02 mg/kg | 3.0 microg/kg BW/day (PMTDI for 60 kg: 180 microg/day) |
| Nickel (Ni) | 1.85 +/- 0.3 mg/kg | 5.0 microg/kg BW/day (PMTDI for 60 kg: 300 microg/day) |
Note on arsenic units: Table 3 reports arsenic as 1.63 microg/kg (1.63 ppb), while manganese, lead, and nickel are in mg/kg (ppm). The arsenic value is therefore approximately 1,000-fold lower in absolute terms than the other trace metals. This is consistent with the 2019 paper from the same research group and the same clay source; it may reflect methodological sensitivity or the specific form of arsenic in this kaolin source. Other Ghanaian clay studies (Nkansah et al. cited in this paper) report arsenic in the hundreds to thousands of mg/kg range from Kumasi clay, suggesting substantial geographic heterogeneity.
Table 5 (p. 5): Estimated daily intake, hazard quotient, and cancer risk for a 60 kg adult ingesting 70 g clay/day
| Heavy metal | Sample | EDI (mg/kg BW/day) | HQ | THQ | Cancer risk |
|---|---|---|---|---|---|
| Arsenic | Clay | 1.88 | 6.26 x 10^-4 | 2.62 | 2.82 |
| Copper | Clay | 2.767 | 0.069 | 0.067 | n/a |
| Lead | Clay | 0.611 | 1.75 x 10^-1 | 1.69 | 0.052 |
| Manganese | Clay | 5.44 | 38.86 | 0.377 | n/a |
| Nickel | Clay | 2.13 | 106.5 | 10.34 | 19.38 |
TTHQ = 2.62 + 0.067 + 1.69 + 0.377 + 10.34 = 15 (Table 5, caption).
The authors note that TTHQ greater than or equal to 15 implies significant safety concerns for Mn and nickel (Ni) specifically. The carcinogenic risk index for arsenic (2.82) and nickel (19.38) are both above 1 x 10^-4, the threshold the authors use to indicate probable carcinogenicity risk.
Health risk context cited in the paper:
- Lead levels (0.53 mg/kg) were below the range of 549-622.92 microg/kg reported by Nkansah et al. from Kumasi clay; and below the 2.36 mg/kg previously found by the same research group.
- Manganese 2.3-128 mg/kg and nickel 2.9-1400 mg/kg ranges reported from Tanzania (Nyanza et al.) were above the values in this study.
- Nickel hazard quotient (HQ 106.5) is well above 1, indicating likely non-carcinogenic adverse health effects from nickel in this clay at the stated ingestion rate (70 g/day).
Methods (brief)
Clay sampling: Kaolin samples purchased from Anfoega, Volta Region, Ghana. Samples ground to powder by mortar and pestle, sieved to 0.1 mm mesh.
Mineral analysis: Dry ashing method (AAS). About 100 g of sample added to 100 ml distilled water on a shaker at 125 rpm for 12 hours, then filtered. Buck Scientific 210VGP Flame AAS: Cu at 324.8 nm, Fe at 248.3 nm, Mn at 279.5 nm, lead (Pb) at 217.0 nm, Zn at 213.9 nm. Calibration from minimum three standards.
Risk assessment: USEPA exposure parameters: body weight 75 kg (adult), 15 kg (child); ingestion rate 100 mg/day (adult), 200 mg/day (child); exposure frequency 365 days/year; exposure duration 30 years (adult), 6 years (child). Reference doses (RfD) from literature (Table 4): Fe 0.7, K 1000, Zn 0.3, arsenic (As) 1.5 x 10^-4 mg/kg/day. Cancer slope factors from JEFCA and USEPA.
LOD/LOQ: not stated. CRM: not reported. Recovery: not reported.
Evidence Fitness
EF-3 (modeled or limited evidence). The mineral data are primary AAS measurements from a small set of kaolin samples from a single Ghanaian mining and market source. The values are identical to those in Kortei et al. 2019, indicating the same underlying measurements. The risk assessment is a standard USEPA-based HQ/THQ/cancer-risk calculation, transparent in its parameters. The key limitation is the small sample set (4 types) and absence of CRM, LOD, or recovery data. The arsenic unit discrepancy (microg/kg vs mg/kg for other metals) is not explained in the paper and should be treated with caution; the absolute value (1.63 ppb arsenic) is anomalously low compared to other Ghanaian clay studies. The nickel HQ (106.5) and cancer risk index (19.38) are sensitive to the assumed ingestion rate (70 g clay/day) and the reference dose chosen.
Related evidence
- Clay and geophagy
- Elemental minerals and microbial compositions as well as knowledge and perceptions regarding kaolin (clay) consumption by pregnant women in the Ho municipality of Ghana
- Geophagy During Pregnancy in Sub-Saharan Africa: A Review of Cultural Practices and Health Implications.
- Lead
- Arsenic
- Nickel
- Manganese
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