1 Introduction

Human milk is a vital source of nutrition and immune protection for newborn babies, providing a complex and dynamic blend of bioactive compounds that support optimal growth and development [1]. The composition of human milk is carefully regulated to meet the specific needs of the infant, providing a unique combination of macronutrients, micronutrients, hormones, growth factors, enzymes and immune components [2]. However, in certain circumstances, human milk can become contaminated with harmful substances, including heavy metals, which may pose potential health risks to the child [3].

Human milk is formed through a complex process involving the mammary glands, which secrete and modify milk components to ensure optimal nutrition and protection for the newborn. Understanding the formation of human milk is essential for comprehending how contaminants may enter milk and subsequently affect the health of the child [4]. The composition of human milk is highly dynamic, varying throughout lactation and adapting to the changing needs of the growing infant [1]. It contains a rich array of proteins, lipids, carbohydrates, vitamins, minerals, and bioactive factors that support the infant's growth, immune function and overall well-being. However, contaminants such as heavy metals can infiltrate human milk [5, 6], potentially compromising its nutritional and protective qualities [3].

However, heavy metals—lead, cadmium, mercury and arsenic—are pervasive environmental pollutants that can find their way into human milk. All these metals can enter the maternal body through various sources, such as food, water, air and occupational exposures. Once absorbed, they can be transferred to the developing infant through breastfeeding, potentially exerting toxic effects on the child's health [7, 8].

The assessment of health risks associated with the consumption of heavy metal-contaminated human milk in children involves several important parameters, including estimated daily intake, HQ and CR. Estimating the daily intake of heavy metals in infants is crucial to determine the potential exposure levels and evaluate their potential health effects. By considering factors such as the concentration of heavy metals in human milk, the volume of milk consumed by the infant and the body weight of the child, the estimated daily intake can be calculated. This information provides valuable insights into the level of exposure and allows for comparisons with established reference values or guidelines [9].

The HQ is another key parameter in assessing health risks. It is calculated by dividing the estimated daily intake of a specific heavy metal by its corresponding RFD, which represents the level of exposure considered safe for long-term consumption. The HQ helps determine whether the intake of a particular heavy metal exceeds the acceptable threshold and indicates the potential for adverse health effects. If the HQ exceeds 1, it suggests a potential risk and warrants further investigation and intervention to reduce exposure.

In addition to the hazard quotient, the assessment of cancer risk is crucial in evaluating the long-term health effects of heavy metal exposure. CR estimates the probability of developing cancer over a lifetime due to the ingestion of a particular heavy metal. It is typically calculated by multiplying the lifetime average daily intake of the heavy metal by its corresponding CSF. The CSF represents the potency of the heavy metal to cause cancer. By comparing the calculated CR with the acceptable risk level, usually expressed as a range of acceptable risk (e.g., 1 in 10,000 to 1 in 1,000,000), the potential CR associated with heavy metal exposure can be assessed.

The assessment of health risks through estimated daily intake, HQ and CR provides valuable information on the potential adverse effects of consuming heavy metal-contaminated human milk in children. These risk assessment parameters aid in identifying populations at higher risk, determining the need for intervention or regulation and guiding decision-making processes to protect the health and well-being of infants. It is essential to continually monitor and assess these risks, considering emerging research, advancements in analytical techniques and the evolving understanding of the toxicological effects of heavy metals on child health [10].

The health effects of consuming contaminated human milk on infants are a matter of significant concern. Heavy metals have the potential to disrupt various physiological processes, including neurological development, immune function and endocrine regulation. Prolonged exposure to elevated levels of heavy metals through breastfeeding may increase the risk of developmental disorders, impaired cognitive function, compromised immune responses and long-term health implications for the child [6].

Therefore, the objective of this systematic review is to comprehensively assess the health risks associated with human milk consumption in children. Specifically, this review will focus on the composition of human milk, the importance of human milk for newborn babies, the mechanisms of heavy metal invasion in human milk and the health effects of consuming contaminated milk on infants [11]. By conducting a systematic review, this study aims to synthesize and analyse the literature on the assessment of health risks associated with human milk consumption in children. The findings will contribute to a comprehensive understanding of the mechanisms by which heavy metals invade human milk, the potential health effects on infants and the implications for public health interventions and breastfeeding practices [3].

Ultimately, this systematic review will provide valuable insights into the assessment of health risks due to human milk consumption in children. The knowledge generated from this review will inform healthcare professionals, policymakers and parents about the potential risks associated with consuming contaminated human milk, facilitating evidence-based decision-making regarding infant feeding practices and the development of strategies to minimize exposure to harmful contaminants [12].

2 Materials and methods

2.1 Literature search

Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed for completion of this study (Fig. 1). The literature studies published between 2013 and 2023 were collected from databases such as ScienceDirect, PubMed, and Google Scholar. The key words used for the search included ‘trace metals,’ ‘heavy metals,’ ‘toxic metals,’ and ‘trace elements’ in combination with ‘mothers’ milk,’ ‘human milk,’ and ‘breast milk’ and by checking the reference list of the published studies.

Fig. 1
figure 1

Flow diagram (PRISMA chart) showing the selection and search process for data collection. Approximately 380 articles were retrieved from the data search. After removing 107 duplicated downloads, 181 articles were excluded based on the initial screening as they did not contain relevant data related to the presence of heavy metals in human milk. The remaining 92 articles underwent a thorough review, and 70 of them were excluded due to insufficient or insignificant data. Finally, 22 articles were identified as primary sources for the study

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2.2 Inclusion and exclusion criteria

The articles identified during the search process went through the selection process. The inclusion criteria included (a) full text articles published in English; (b) studies that reported the presence of metals in breast milk which are also reported from different population of world; and (c) studies that specifically contained the mean values of the heavy metal concentrations. Exclusion criteria included duplicate articles, books, review articles, and articles that reported heavy metal presence but did not include the mean values. The needed information from the articles included the mean values of heavy metals in the concentration year of study and country of study. The concentrations reported in the studies were all in different units, such as ng/dL, and μg/L, which were unified to mg/L. For countries with more than one study, mean was calculated by averaging the means from individual studies.

The selected articles were thoroughly analyzed and synthesized to extract relevant information on the presence of heavy metals in human milk. The extracted data were then organized in tables to facilitate data presentation and comparison.

The quality of the included studies was assessed to ensure the reliability and validity of the data. Quality assessment criteria included study design, sample size, data collection methods, and reporting clarity. Studies that met rigorous quality standards were given higher consideration in the data synthesis.

The findings from the selected articles were interpreted and discussed to provide a comprehensive review of the presence of heavy metals in human milk. The conclusions drawn from the data synthesis will shed light on the current state of possible health risk in children due to consumption of breast milk.

2.3 Human health risk assessment after extracted data on heavy metals level in human milk

2.3.1 Estimated Daily Intake (EDI)

The EDI of heavy metals due to exposure to breast milk was calculated using the formula [13].

$${\text{EDI}}\,{ = }\,\frac{{{\text{C}}_{i} \, \times \,{\text{RDA}}}}{{{\text{Body}}\,{\text{Weight}}}}$$

The recommended dietary intake (RDA) of human milk is 0.67 L/day [14]. The average body weight of children is 8.15 kg [15], and Ci represents the reported average value of heavy metals.

2.3.2 Hazard Quotient (HQ)

The formula used for the calculation of the hazard quotient included the EDI and the RfD, and 10–3 was the unit conversion factor [16].

$${\text{HQ}}\, = \,\frac{{{\text{EDI}}}}{{{\text{RfD}}}}\, \times \,10^{ - 3}$$

2.3.3 Carcinogenic Risk (CR)

For calculation of CR, we used exposure frequency (Efr) 360 days/year, exposure duration (ED) 0.5 years, average time (AT) 2 years, and CSF (mg/kg/day) and EDI (mg/kg/day) [17].

$${\text{CR}}\, = \,\frac{{{\text{Efr}}\, \times \,{\text{ED}}\, \times {\text{EDI}}\, \times {\text{CSF}}}}{{{\text{AT}}}}\, \times \,10^{ - 3}$$

The values of Rfd and CSF used in calculation are mentioned in Table 1.

Table 1 Oral responsive dose and carcinogenic slope factor (adapted from [18])
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3 Results and discussion

The reported values, EDI, the HQ and CR due to the heavy metals present in breast milk are discussed below.

3.1 Aluminium

The highest concentration of aluminium in breast milk was found in Spain with reported values Ci (Fig. 2P) estimated daily intake EDI (Fig. 3P) and hazard quotient HQ (Fig. 4N), (0.034 mg/kg, 0.003 mg/kg/day, 2.8E-06), followed by Iran with Ci (0.12 mg/kg) (Fig. 2N), EDI (0.0099 mg/kg/day) (Fig. 3N), HQ (9.9E-06) (Fig. 4P), Sweden with Ci (0.185 mg/kg) (Fig. 2B), EDI (0.015 mg/kg/day) (Fig. 3B), HQ (1.52E-05) (Fig. 4B) and Brazil with Ci (0.20 mg/kg) (Fig. 2O), EDI (0.017 mg/kg/day) (Fig. 3O), HQ (1.66E-05) (Fig. 4M).

Fig. 2
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A–P The reported mean values of heavy metals (mg/L) calculated from different studies presented in different countries

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Fig. 3
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A–P The graphs give the values of EDI (mg/kg/day) of heavy metals in the infants

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Fig. 4
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A–P The calculated hazard quotient due to the consumption of the heavy metal in breast milk on the infant’s body

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The recommended daily allowance of aluminium is not set. The minimal risk level (MLR) for aluminium was 26 mg/kg/day [30], the estimated daily intake calculated was significantly lower, and the health risk calculated was significantly low, indicating minimal health risk.

3.2 Cobalt

The reported values of heavy metals in the case of cobalt were observed in South Africa with the lowest Ci (3.60E-05) (Fig. 2H), followed by India with Ci (0.39) (Fig. 2I) and Sweden with Ci (5.90E-05), and the highest reported values were in Iraq with Ci (288.52) (Fig. 2M). Similar patterns were followed by EDI and HQ South Africa (2.96E-06 and 9.87E-06), Sweden (4.85E-06 and 1.62E-05), India (0.032 and 0.11), and Iraq (23.72 and 79.06). The daily recommended daily allowance of cobalt was 0.012 mg/kg/day [31]. Using this value as a reference, Iraq (0.032 mg/kg/day) (Fig. 3M) was the only country with a higher EDI than the recommended dietary intake. This leads to a higher hazard quotient in 79.06 Iraq (Fig. 4O). High cobalt intake can lead to anaemia [32]. Cobalt toxicity can even lead to complex syndromes, with neurological disruptions such as impaired hearing and visual function and cardiovascular and endocrine disruption [33].

3.3 Copper

Copper in breast milk was reported in the case of Egypt with the lowest Ci (0.00076) (Fig. 2K), followed by Iraq Ci (0.18), Iran (0.27), South Korea (0.27) (Fig. 2A), Australia (0.22) (Fig. 2F), Spain (0.37) and Sweden Ci (0.47). The arrangement of countries in the increasing order of EDI and HQ was as follows: Egypt (6.25E-05 and 1.56E-06), Iraq (0.015 and 0.0004), Iran (0.022 and 0.0005), South Korea (0.022 and 0.0006) (Figs. 3A and 4A), Australia (0.018 and 0.0004) (Fig. 3F), Spain (0.03 and 0.0008), and Sweden (0.04 and 0.001). The daily recommended allowance value of copper for infants was found to be 0.2 mg/kg/day [34]. The EDI in all countries was lower than the recommended daily allowance and a lower HQ.

3.4 Iron

The reported values of iron in milk in increasing order were as follows: Egypt had the lowest Ci (0.002), followed by Australia (0.05), Sweden (0.39), Poland (0.34) (Fig. 2L), South Korea (0.40), Iran (0.42), Brazil (0.76), Spain (0.68), and India (1.18). The trends of EDI and HQ followed similar patterns, where Egypt (EDI: 0.00022, HQ: 3.11E-07) had the lowest and India (EDI: 0.097, HQ: 0.0001) had the highest. The EDI and HQ for the remaining countries were as follows: Australia (EDI: 0.0039, HQ: 5.55E-06) (Fig. 4F), Sweden (EDI: 0.028, HQ: 3.98E-05), Poland (EDI: 0.032; Fig. 3L, HQ: 4.58E-05; Fig. 4L), South Korea (EDI: 0.033, HQ: 4.73E-05), Iran (EDI: 0.035, HQ: 4.93E-05), Brazil (EDI: 0.06, HQ: 8.889E-05), and Spain (EDI: 0.056, HQ: 7.97E-05). The recommended dietary allowance level of 0.27 mg//kg/day is the limit of iron that an infant can tolerate [35]. Based on these values, the EDI levels of iron were lower than the recommended dietary allowance in all observed countries. Thus, possessing a lower health risk due to contamination but inadequate levels of Fe can affect different life processes. Low iron levels in the diet can affect different physiological functions in the body, as they are involved in many different processes across the body. However, infants who are 2–4 months old and born after complete parturition time have sufficient iron build stores in their body. Iron deficiency can occur in infants at 6–9 months as they start consuming solid food. If this solid food does not contain adequate amounts of iron, it can lead to deficiency in infants, leading to ammonia.

3.5 Mercury

The mean reported values of mercury in breast milk had the following pattern in increasing order: Brazil Ci (0.00042), Saudi (0.0012), Nigeria (0.0029) (Fig. 2G), Spain (0.0056), and Ghana (0.0076). The calculated EDI and HQ values were as follows: Brazil (EDI: 3.5E-05, HQ: 4.93E-08), Nigeria (EDI: 0.00024, HQ: 3.41E-07; Figs. 3G, 4G), Saudi Arabia (EDI: 9.8E-05 and HQ: 1.39E-07; Figs. 3D, 4D), Spain (EDI: 0.00046, HQ: 6.58E-07), and Ghana (EDI: 0.00062, HQ: 8.93E-07). The daily recommended allowance of mercury by the WHO [36] was 0.571 mg/day. The EDI of all the countries was lower than the RDA, hence a lower health risk or hazard quotient.

3.6 Manganese

The reported mean values of manganese in breast milk in increasing order were Egypt (0.00025), South Africa (0.00066), Australia (0.0014), Sweden (0.003), Brazil (0.0058), Spain (0.011), Saudi Arabia (0.016), South Korea (0.058), and Iraq (1801.4). The EDI and HQ of each country in order of increasing values were as follows: Sweden (EDI: 0.00025, HQ: 3.5E-07), Egypt (EDI: 2.1E-05, HQ: 2.94E-08), South Africa (EDI: 5.46E-05, HQ: 7.79E-08; Figs. 3H, 4H), Australia (EDI: 0.00011, HQ: 1.61E-07), Brazil (EDI: 0.00048, HQ: 6.91E-07), Spain (EDI: 0.00087, HQ: 1.26E-06), Saudi Arabia (EDI: 0.0013, HQ: 1.90E-06), South Korea (EDI: 0.0048, HQ: 6.87E-06), and Iraq (EDI: 148.09, HQ: 0.21). The adequate daily allowance of manganese was 0.003 mg/day [37]. The comparison of the recommended daily allowance and EDI showed that Iraq had an EDI of 148.09 and the highest HQ (0.21). Manganese in an important element obtained from foods and dietary supplements. It works in the functioning of many enzymes, such as manganese superoxide dismutase, arginase, and carboxylase, that perform important functions, such as reactive oxygen species scavenging, bone formation, immune response, and reproduction [38]. High levels of manganese can affect the central nervous system, leading to hearing loss, muscle spasm, and loss of balance. More symptoms of manganese toxicity can lead to depression, delusions, headaches, lower extremity weakness, and short-term memory loss [39]. The higher intake of manganese can also lead to impaired neuromoter symptoms such as Parkinson’s disease [38].

3.7 Molybdenum

The molybdenum concentration in breast milk was observed only in Australia and Sweden with Ci (0.00037 mg/L and 0.0035 mg/L, respectively). The EDI of Australia was 3.04E-05 mg/day and that of Sweden was 0.00028 mg/day, while the HQ in Australia was 6.08E-06 and that in Sweden was 5.75E-05. The RDA of molybdenum was 0.002 mg/day [40]. The comparison of the RDA and EDI showed that the EDI of both countries was lower than the RDA. Acute molybdenum toxicity is rare, but it can occur with industrial mining and metalworking exposure. In healthy people, consumption of a diet high in molybdenum usually does not pose a health risk because molybdenum is rapidly excreted in urine. Molybdenum toxicity is generally characterized by achy joints, gout-like symptoms, and abnormally high blood levels of uric [41].

3.8 Selenium

The mean recorded values in human milk for selenium were lowest in South Korea, with Ci (0.01), followed by Australia (0.01), Brazil (0.01), Sweden (0.01), India (0.02), and Spain, which had the highest Ci (0.04). The arrangement of the countries in the increasing order of Ci, EDI, and HQ were as follows: South Korea (EDI: 0.00086 and HQ: 0.00017), Australia (EDI: 0.0011 and HQ: 0.00023), Brazil (EDI: 0.0012 and HQ: 0.00024), Sweden (EDI: 0.0010 and HQ: 0.0002), India (EDI: 0.0018 and HQ: 0.00037; Figs. 3I, 4I), and Spain (EDI: 0.0036, HQ: 0.00073). The RDA of selenium is 0.015 mg/day [42]. The risks of higher selenium intake can be hair and nail loss or brittleness. The increased selenium can also lead to over activation of selenoprotein expression, which can lead to inhibited capacity for DNA repair, leading to carcinogenic outputs, delayed lesion progression, increased apoptosis, and decreased proliferation [43,44,45].

3.9 Zinc

The mean reported value of zinc in breast milk in increasing order was as follows: Egypt had the lowest Ci (0.002), followed by South Korea (0.49), Iran (0.97), Spain (1.40), Australia (1.4), Sweden (3.471), India (7.14), and Iraq with the highest Ci (212). The estimated daily intake and the hazard quotient for zinc in all the countries were as follows: Egypt (EDI: 0.00018, HQ: 6.33E-07), South Korea (EDI: 0.040, HQ: 0.00013), Iran (EDI: 0.079, HQ: 0.00027), Spain (EDI: 0.115, HQ: 0.00038), Australia (EDI: 0.114, HQ: 0.00038), Poland (EDI: 0.181, HQ: 0.0006), India (EDI: 0.59, HQ: 0.002), Sweden (EDI: 0.28, HQ: 0.0009), and Iraq (EDI: 17.42, HQ: 0.058). Based on the tolerable upper intake levels of 4 mg/day [46], all countries except Iraq had lower values of estimated daily intake compared to the tolerable upper intake levels. The EDI and HQ in Iraq were 17.42 and 0.058, respectively. Zinc is essential for many cellular processes, such as catalytic activities, immune functions, protein and DNA synthesis, cell division and cell signalling. Zinc permits growth and development during pregnancy, infancy, and other stages of life [47]. The richest souses of zinc include meat, fish, seafood, beans, nuts, and whole grains. It is stored in skeletal muscle and bone. It is passed on to the breast milk during lactation. High amounts of nausea, headache, vomiting, and loss of appetite. High amounts of zinc can interfere with copper absorption and immune functions and lower HDL cholesterol levels [48]. Amounts higher than 50 mg/day can lead to toxicity and neurodysfunction, affect the absorption of magnesium and cause magnesium deficiency [49].

3.10 Arsenic

Arsenic is naturally found in the form of a trivalent atomic state. It is found naturally in air, water, food and oceans, and the concentration of arsenic in Earth's crust is 1.5 to 5 mg/kg. The concentrations vary with location and bioaccumulation in food. The major region of arsenic poisoning is contaminated drinking water [50]. The calculated mean reported value of arsenic in South Africa was lowest Ci (0.000166), followed by Brazil (0.0003), Sweden (0.0005), Nigeria (0.0006), Spain (0.0009), Iran (0.001), Ghana (0.027) (Fig. 2J), Lebanon (0.14) (Fig. 2C), and Cyprus with the highest Ci (0.73). The EDI, HQ, and CR calculated based on the reported values arranged in the increasing order had the following sequence South Africa (EDI: 1.36E-05; HQ: 4.54E-05; CR: 1.16E-15; Fig. 5I), Brazil (EDI: 2.38E-05; HQ: 7.95E-05; CR: 6.18E-15), Sweden (EDI: 4.52E-05; HQ: 0.00015; CR: 4.21E-14) (Figs. 6, 7, 8), Nigeria (EDI: 4.93E-05; HQ: 0.00016; CR: 5.47E-11), Spain (EDI: 7.39E-05; HQ: 0.00025; CR: 1.85E-13), Iran (EDI: 8.47E-05; HQ: 0.00028; CR: 2.77E-13), Ghana (EDI: 0.0022; HQ: 0.0073; CR: 4.82E-09; Figs. 3J, 4J), Cyprus (EDI: 0.06; HQ: 0.20; CR: 9.86E-05; Figs. 3E, 4E), Lebanon (EDI: 0.011; HQ: 0.04; CR: 7.31E-07; Fig. 5H). Based on the recommended daily allowance of 0.13 mg/day, [36] the EDI of all countries was significantly lower than the RDA. These countries having estimated intake levels below the recommended daily allowance had a significantly lower carcinogenic risk.

Fig. 5
figure 5

A–K Calculated carcinogenic risk due to the consumption of heavy metals in breast milk on the infant’s body

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Fig. 6
figure 6

The graphs gives a comparative view of EDI (mg/kg/day) values of different countries

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Fig. 7
figure 7

Comparison of HQ across different countries in infant

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Fig. 8
figure 8

Comparative representation of CR value of different countries

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3.11 Cadmium

The reported values of Cd breast milk in South Africa were lowest with Ci (1.30E-05), followed by Sweden (8.60E-05), Poland (0.00011), Spain (0.0004), Brazil (0.00035), Egypt (0.0017), Ghana (0.0012), Nigeria (0.0033), Lebanon (0.00087), Cyprus (0.45) (Fig. 2E), and Iraq with the highest Ci (57.33). The Estimated daily intake, hazard quotient, and carcinogenic risk calculated from the reported values had the following trends, South Africa (EDI: 7.1E-06; HQ: 7.1E-06; CR: 1.2E-17; Fig. 5I), Sweden (EDI: 7.07E-06; HQ: 7.06E-06; CR: 1.23E-17; Fig. 5A), Poland (EDI: 9.04E-06; HQ: 9.04E-06; CR: 2.5E-17), Spain (EDI: 3.3E-05; HQ: 3.3E-05; CR: 1.23E-15; Fig. 5B), Brazil (EDI: 2.87E-05; HQ: 2.87E-05; CR: 8.26E-13; Fig. 5C), Egypt (EDI: 0.00014; HQ: 0.00014; CR: 9.13E-14; Fig. 5F), Ghana (EDI: 0.0001; HQ: 0.0001; CR: 3.58E-11; Fig. 5G), Nigeria (EDI: 0.00027; HQ: 0.00027; CR: 6.92E-10; Fig. 5J), Lebanon (EDI: 7.15E-05; HQ: 7.15E-05; CR: 1.27E-14), Cyprus (EDI: 0.037; HQ: 0.037; CR: 0.0018; Fig. 5K), Iran (EDI: 4.7; HQ: 4.7; CR: 3.62; Figs. 5D, 6, 7, 8). The RDA value for cadmium was 0.006 mg/day [51]. The comparison of EDI and recommended daily allowance showed that the following countries had a higher EDI than the recommended allowance and had higher HQ and CR Lebanon Cyprus, and Iraq.

Some studies reported that cadmium can travel up the food chain and in the mother’s body through food such as vegetables and fruits, sea foods, cereals, from smoking, and drinking water. The contamination of cadmium in these sources can be due to smelting operations, volcanic eruptions, industrial waste such as anticorrosive materials, and nickel cadmium battery waste discharged into rivers and agricultural lands [52]. Early exposure to cadmium can lead to decreased neurodevelopment. Higher exposure to cadmium leads to the development of learning disability and mental development index hypertension [53,54,55].

3.12 Chromium

The reported values of chromium in breast milk in Sweden were lowest Ci (3.00E-05), and those in South Africa (0.0006), Brazil (0.0056), Iran (0.0052) and Spain had the highest Ci (0.016). The order of EDI, HQ, and CR were as follows: Sweden (EDI: 2.46626E-06; HQ: 8.22E-07; CR: 1.87E-17) (Figs. 6, 7, 8), South Africa (EDI: 5.66E-05; HQ: 1.89E-05; CR: 2.27E-13), Brazil (EDI: 0.00046; HQ: 0.00015; CR: 1.25E-10), Iran (EDI: 0.00042; HQ: 0.00014; CR: 9.52E-11), and Spain (EDI: 0.0013; HQ: 0.00044; CR: 2.5E-09) (Figs. 6, 7, 8). Chromium is an important trace element that performs many important functions in the body. The adequate intake of chromium needed in the body for an infant was 0.0002 mg/day [56]. Compared to the MLR for chromium 0.005 mg/kg/day [57], the estimated daily intake was lower in all countries.

3.13 Nickel

The reported values of nickel in breast milk in Egypt were lowest (1.4E-05), (Fig. 2K), followed by Brazil (0.006), and Spain (0.025), and Sweden (0.96). The EDI, HQ and CR followed the pattern of Egypt (EDI:1.2E-06; Fig. 3K), (HQ: 5.7E-08; Fig. 4K), (CR: 5.8E-21), Brazil (EDI: 0.0005; HQ: 0.2.5E-05; CR: 4.74E-13), Spain (EDI: 0.0021; HQ: 0.0001; CR: 3.44E-09) (Figs. 6, 7, 8), and Sweden (EDI: 0.079; HQ: 0.004; CR: 1.9E-06) The tolerable upper intake level of nickel was 0.2 mg/day [58]. The EDI levels of all countries were lower than the tolerable upper intake levels.

3.14 Lead

The mean reported values in breast milk for lead in South Africa were lowest for Ci (0.00012), Sweden (0.0015), Egypt (0.0029), Spain (0.0052), Brazil (0.0067), Nigeria (0.013), Ghana (0.013), Lebanon (0.018), Saudi (0.047) (Fig. 2D), India (0.075), Iraq (0.223), and Iran (0.064), and Cyprus had the highest Ci (1.19). The calculated values of EDI, HQ and CR were as South Africa (EDI: 1.028E-05; HQ: 2.57E-06; CR: 2.10E-19) (Figs. 6, 7, 8), Sweden (EDI: 0.00012; HQ: 3.08E-05; CR: 3.63E-16) (Figs. 6, 7, 8), Egypt (EDI: 0.00024; HQ: 6.0E-05; CR: 2.7E-14) (Fig. 3K), Spain (EDI: 0.00043; HQ: 0.0001; CR: 1.15E-14) (Figs. 6, 7, 8), Brazil (EDI: 0.00055; HQ: 0.00014; CR: 3.23E-10), Nigeria (EDI: 0.0011; HQ: 0.00027; CR: 2.4E-13), Ghana (EDI: 0.001; HQ: 0.0002; CR: 2.8E-13), Lebanon (EDI: 0.0015; HQ: 0.00037; CR: 6.47E-13), Saudi (EDI: 0.00001; HQ: 2.6E-06; CR: 2.1E-19), India (EDI: 0.0061; HQ: 0.0015; CR: 4.54493E-08). Iraq (EDI: 0.02; HQ: 0.0046; CR: 1.2E-09; Fig. 5E), Iran (EDI: 0.0004; HQ: 0.0001; CR: 9.52E-11) (Figs. 6, 7, 8), Cyprus (EDI: 0.097; HQ: 0.024; CR: 1.8E-07) (Figs. 6, 7, 8). The EDI (estimated daily intake) of all countries was lower than the RDA of lead of 0.040 mg/kg/day [36]. Hence, there is a significantly lower health risk and lower carcinogenic risk.

3.15 Carcinogenic risk prominent countries

The increased levels of heavy metals in breast milk resulted from heavy metal contamination in the soil in the regions of Iraq (Fig. 8). High and moderate levels of heavy metals in the land in Kurdistan regions, Iraq, were reported by Hamad et al. [59]. Heavy metal contamination in dust from sand storms due to anthropogenic reasons was also reported in middle and south Iraq regions [60]. Studies from Iraq have also reported heavy metal contamination in vegetables [61] and meat [62] in the region. These accumulated heavy metals in food items can become concentrated in the human body and be passed on into milk. However, it might have severe health issue in human body.

Increased heavy metal contamination in some parts of Indian soil was also reported by studies carried out all over India. They reported increased heavy metal concentrations in the agricultural soil due to uncontrolled fertilizer use and contamination irrigation water from the river [63,64,65]. The heavy metal uptake by plants from the soil in India has been reported by many researchers. Heavy metal deposition in crops was also reported, and the order of heavy metal uptake by the crops irrigated by Kali River water was Mn > Fe > Zn > Cu > Cr > Ni > Pb > Cd [66]. Heavy metal contamination of crops, cereals, and vegetables has been reported in India by different researchers, which can be the reason for increased heavy metal accumulation in the human body and human milk [64, 66, 67].

In Cyprus, heavy metal contamination in dust was reported by Mussa et al. [68]. They found increased levels of heavy metals in the dust content of urban regions. Heavy metal accumulation, the main food source, Olea europaea L., in Cyprus was found along with heavy metal contamination in Gonyeli lake water [69, 70].

The above reports of metal contamination in the soil, plants and water around all three countries suggest that these heavy metals present in the soil and water can be bio-accumulated and biomagnified in the food chain, leading to increased health risk and CR for infants (Fig. 9). However, as the reported values are from the different populations of world, therefore, it might be the factor for such variation of estimated values of different countries. For instance, in this study, the reported values of heavy metals in Spain were reported from the mining area. Therefore, the mining has the great influence to the local ecosystem so that some heavy metals transported in breast milk through the food chain.

Fig. 9
figure 9

Transfer and deposition of heavy metals in breast milk and their impacts on the infant’s body

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The heavy metals entering the human body can affect different physiological functions by generating free radicals, leading to increased oxidative stress in the cells by disrupting lipid membrane systems [71]. They also impact the structure of enzymes, proteins, nucleic acids and DNA, leading to impaired functioning. Heavy metals in the human body ultimately lead to an increase in reactive oxygen species (ROS). These ROS induce a series of reactions eventually leading to oxidative damage and disruption of antioxidative defences, thus leading to apoptosis and necrosis in the body tissues [72].

Aluminium primarily affects the central nervous system [73]. It can interfere with the function of various enzymes and proteins, disrupt the blood–brain barrier, and induce oxidative stress [74]. Prolonged exposure to aluminium can lead to neurotoxicity, which is associated with conditions like Alzheimer's disease [73]. It can also cause bone disorders and has been linked to developmental delays in infants [75]. Cobalt can interfere with cellular respiration and DNA synthesis. It induces oxidative stress and can replace other essential metals in metalloenzymes, disrupting their function [76]. Excessive cobalt exposure can lead to cardiomyopathy [77], respiratory issues, and thyroid dysfunction [78]. Chronic exposure is also associated with skin conditions like dermatitis [79]. Copper toxicity primarily results from the generation of reactive oxygen species (ROS), leading to oxidative stress. It can also interfere with iron metabolism and disrupt cellular homeostasis [80]. High levels of copper can cause gastrointestinal distress [81], liver [82] and kidney damage [83], and neurological symptoms such as tremors and cognitive deficits. Wilson's disease, a genetic disorder, exacerbates copper accumulation leading to severe health problems [81,82,83]. Iron toxicity is mediated through the production of ROS, leading to oxidative damage to lipids, proteins, and DNA. Iron overload can disrupt mitochondrial function and cellular respiration. Excess iron can cause liver cirrhosis, heart disease, diabetes, and neurodegenerative conditions. Acute iron poisoning in infants can be fatal, leading to severe metabolic acidosis and organ failure [84, 85]. Mercury forms covalent bonds with thiol groups in proteins, disrupting their function. It can cross the blood–brain barrier and the placental barrier, leading to significant neurotoxic effects. Mercury exposure can result in neurological deficits, cognitive impairments, and motor dysfunction. Prenatal exposure is particularly dangerous, leading to developmental delays and neurodevelopmental disorders [86]. Excess manganese can accumulate in the brain, affecting the basal ganglia. It interferes with neurotransmitter synthesis and mitochondrial function, leading to oxidative stress. Manganese toxicity is primarily associated with neurotoxicity, manifesting as manganism, a Parkinson's-like syndrome with symptoms such as tremors, difficulty walking, and cognitive deficits [87]. Selenium toxicity involves the replacement of sulfur in amino acids, leading to the production of abnormal proteins. It also induces oxidative stress. Selenium toxicity, known as selenosis, can cause gastrointestinal upsets, hair loss, white blotchy nails, and neurological damage [88]. Zinc can induce the production of ROS and interfere with the absorption and utilization of other essential metals, such as copper and iron. While zinc is essential, excessive zinc can lead to immune dysfunction, gastrointestinal distress, and disruption of iron and copper metabolism [89, 90]. Arsenic interferes with cellular respiration by inhibiting mitochondrial enzymes. It induces oxidative stress and can cause DNA damage. Chronic arsenic exposure can lead to skin lesions, cardiovascular disease, neurotoxicity, and an increased risk of cancers, particularly skin, lung, and bladder cancer [91]. Cadmium primarily induces toxicity through oxidative stress and disruption of calcium signalling. It can replace zinc in various enzymes, disrupting their function. Cadmium exposure can lead to kidney damage, bone demineralization, and lung damage. It is also classified as a human carcinogen, primarily causing lung cancer through inhalation [92]. Hexavalent chromium (Cr VI) is particularly toxic as it can penetrate cell membranes and cause oxidative stress and DNA damage. Chromium exposure can lead to respiratory issues, skin ulcers, and an increased risk of lung cancer. Chronic exposure can also affect kidney and liver function [93]. Nickel can induce oxidative stress and hypersensitivity reactions. It can interfere with cellular metal homeostasis and enzyme function. Nickel exposure can lead to allergic dermatitis, respiratory issues, and an increased risk of lung and nasal cancers. Chronic exposure can also affect cardiovascular health [94]. Lead interferes with neurotransmitter release, disrupts calcium signalling, and induces oxidative stress. It can accumulate in bones, affecting bone metabolism and haematopoiesis. Lead exposure can cause cognitive deficits, developmental delays in children, anaemia, hypertension, and kidney damage [95,96,97] (Fig. 10). It is particularly detrimental to the developing nervous systems of foetuses and young children [98].

Fig. 10
figure 10

Mechanistic approach on the impact of heavy metals in the human body. Heavy metals are discharged into the environment during anthropogenic activities, and plants and animals then absorb them from the soil and water. The ingestion of heavy metals contaminated foods by the mother may result in tainted breast milk. When an infant consumes contaminated breast milk, free radicals are created, which can cause disruption of lipid membranes, enzyme structures, oxidative systems, and DNA. Apoptosis and necrosis are the final results of these structural disturbances

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3.16 Limitations of the study

While our study provides valuable insights into the contamination levels of heavy metals in human milk across various countries, some limitations should be acknowledged.

One of the primary limitations of this study is the statistical method employed to estimate the overall mean concentration of heavy metals. Although this method was intended to provide an approximate representation of heavy metal contamination in human milk for each country, it does not fully capture the variability within and between populations. The estimation method may oversimplify the complexity of the data, potentially leading to inaccuracies in reflecting the true pollution status. This limitation underscores the need for more robust statistical approaches in future studies to improve the precision of such estimates.

The data used in this study were collected from multiple sources, each with varying methodologies, sample sizes, and demographic characteristics. This heterogeneity may have introduced biases or inconsistencies in the results, which could affect the generalizability of the findings. Additionally, the lack of standardized reporting across studies further complicates the aggregation of data and the interpretation of the overall results.

Our study did not account for potential geographical and temporal variations in heavy metal contamination. Differences in environmental factors, industrial activities, and regulatory frameworks between countries or regions may have influenced the levels of contamination reported. Similarly, temporal changes in contamination levels due to shifts in environmental policies or industrial practices were not considered, which may limit the applicability of our findings to current conditions.

By acknowledging these limitations, this review aim to provide a balanced interpretation of the findings and emphasize the importance of cautious interpretation and further investigation in this field.

4 Conclusion and future perspective

The systematic review on the assessment of health risks due to human milk consumption in children has provided valuable insights into the potential hazards associated with heavy metal contamination in human milk. The review revealed that heavy metals, such as lead, mercury, cadmium, and arsenic, can be present in human milk, posing potential health risks to infants through the assessment of EDI, HQ and CR. This review has highlighted the importance of evaluating the potential impacts of heavy metal exposure on child health. The estimated daily intake of heavy metals in infants was found to be influenced by several factors, including milk concentration, consumption volume, and the child's body weight. HQ values indicated the potential risks of exceeding safe levels, with certain populations being more susceptible to adverse effects. CR assessments further demonstrated the need for vigilant monitoring and regulation of heavy metal contamination, as exposure to certain metals was associated with an increased risk of cancer in children.

These findings underscore the importance of continuous monitoring and evaluation of heavy metal concentrations in human milk to ensure the safety and well-being of infants of different population throughout world. It is crucial to implement effective strategies for reducing heavy metal exposure in breastfeeding mothers and promoting awareness of potential risks. Additionally, interventions and regulatory measures should be implemented to minimize the entry of heavy metals into the human milk supply. Further research is needed to enhance our understanding of the toxicological effects of heavy metals in children and develop targeted mitigation strategies. Long-term cohort studies and prospective monitoring programs can provide valuable data on the health outcomes associated with heavy metal exposure through human milk consumption. Moreover, efforts should be made to identify potential sources of contamination and implement preventive measures to reduce heavy metal exposure. Overall, this systematic review emphasizes the significance of health risk assessment in relation to heavy metal contamination in human milk. By identifying potential risks, informing interventions, and guiding regulatory measures, this knowledge can contribute to safeguarding child health and promoting the well-being of infants who rely on human milk as their primary source of nutrition. Continued research and vigilance are essential to ensure the provision of safe and nutritious human milk to support optimal growth and development in children.