The industrial activities of the last century have caused massive increases in human exposure to heavy metals. Mercury, lead, chromium, cadmium, and arsenic have been the most common heavy metals that induced human poisonings [2]. The bioaccumulation of these heavy metals leads to diverse toxic effects to different tissues and organs, where it disrupts cellular activities such as cellular growth, damage-repairing processes, apoptosis and cell differentiation [2]. Acute or chronic poisonings may occur following exposure through the environment like water, air, and food [4].
The comparison of the mechanisms of action of heavy metals demonstrates similar pathways with essential minerals to induce toxicity in humans which includes ROS generation, compromising antioxidant defence, enzyme deactivation, and oxidative stress [2,3]. Some toxic metals including chromium, cadmium, and arsenic can cause genomic instability [3]. Induction of oxidative stress following the defects in DNA repair and DNA damage by the three metals have been considered as the cause of their carcinogenicity [6]. Several acute and chronic toxic effects of heavy metals may affect different organs and systems in the body. Gastrointestinal and kidney dysfunction, neurological disorders, skin lesions, vascular damage, immune system dysfunction, birth defects, and cancer are examples of the complications of heavy metals toxic effects [3].
Mercury (Hg)
Elemental mercury is liquid at room temperature and can be readily evaporated to produce vapor. It is found in air, water and soil in 3 forms: elemental or metallic mercury (Hg0), inorganic mercury (Hg+, Hg2+), and organic mercury (commonly methyl or ethyl mercury) [4]. Mercury vapor is more hazardous than the liquid form of mercury. Organic mercury compounds such as methyl mercury (Me-Hg) or ethyl mercury (Et-Hg) are more hazardous than the inorganic compounds. Inhaling large amounts of mercury vapor where there are mercury spills due to container breakage can be fatal [7].
Mercury compounds have many applications in the mining process. For example the extraction of gold and industrial processes. In lamp producing factories, Hg is used in the production of fluorescent light bulbs. Me-Hg and Et-Hg were used as fungicides to protect agricultural crops against infections [4]. In addition, Hg had been used in the medicinal industry, however it had been replaced with safer pharmaceutical drugs to prevent hazardous events [7]. Some examples are the diuretics chlormerodrin, merbaphen, and mercurophylline and phenylmercury nitrate (disinfectant) [1].
Lead (Pb)
Lead
is a catastrophic environmental pollutant which has high toxic effects to
various body organs. Even though Pb can be absorbed from the skin, it is mostly
absorbed from respiratory and digestive systems [5]. Pb is highly toxic which
has adverse effects on the neurological, biological, and cognitive functions in
the bodies. Chronic Pb exposure can lead to various disorders in the nervous
system, respiratory tract, urinary, and cardiovascular system due to
immune-modulation, oxidative, and inflammatory mechanisms [10]. Furthermore, Pb
could disrupt the balance of the free-radical and antioxidant activities and
promote chronic inflammation in many organs. Exposure to Pb can alter
physiological functions and leads to different chronic diseases in later life
[5].
Anemia may develop with Pb poisoning via the inhibition of ferrochelatase and δ-aminolevulinic acid dehydratase (ALAD), two enzymes involved in heme biosynthesis [4]. Several antioxidant molecules may have alterations due to Pb exposure and subsequent oxidative stress. Pb has high affinity to the antioxidant molecules which it competes to bind to the antioxidative enzymes and thus decreases the ability of antioxidant defense [4]. The antioxidant functions to detoxifying free radicals could be affected due to Pb exposure. Pb can promote oxidative damage in different organs via direct effect on membrane lipid peroxidation and reducing antioxidant parameters [1].
Chromium (Cr)
Chromium is commonly found in the earth’s crust and seawater. It is a naturally occurring heavy metal in industrial processes. Cr has multiple oxidation states ranging from −2 to + 6, in which the trivalent and hexavalent forms are the most stable forms. Cr (VI) is related to a series of diseases and pathologies while Cr (III) is required in trace amounts for natural lipid and protein metabolism and also as a cofactor for insulin action [3]. Based on the International Agency for Research on Cancer (IARC) report (2018), hexavalent chromium has been classified as a group I occupational carcinogen [6]. The primary route of exposure for non-occupational human populations occurs via ingestion of chromium containing food and water or dermal contact with products containing chromium [6]. Moreover, refractory, metallurgic, and chemical industries release a large amount of Cr into soil, ground water, and air which put humans, animals, and marine life at risk [6]. Cr can cause various diseases via bioaccumulation in the human body which includes dermal, renal, neurological, and GI diseases to the promotion of tumor growth including in different organs [10].
Cadmium (Cd)
Cadmium occurs naturally in soil and minerals such as sulfide, sulfate, carbonate, chloride, and hydroxide salts as well as in water. High Cd level in water, air, and soil is linked with industrial activities which could be a significant human exposure to Cd, thus the ingestion of Cd contaminated food is one of the major exposure [10]. Moreover, cigarette smoking is also connected as one of the factors of Cd exposure, which is capable of elevating Cd concentrations in blood and urine [3]. Presence of Cd in contaminated water could disturb the necessary mechanisms in the body, potentially resulting in acute or chronic diseases [2,3]. Cd is classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans (Group 1) [6]. Occupational exposure to Cd may occur in alloy, battery, and glass production and in electroplating industries. Due to the importance of the subject, Cd level in the air is routinely monitored in some countries [2]. Unlike low GI absorption, Cd is more efficiently taken from the lungs via inhalation of industrial dust. Acute or chronic inhalation of Cd in industrial areas might lead to renal tubular dysfunction and lung injuries. Cd blood concentration in smokers is almost twice higher than that of non-smokers [2].
Arsenic (As)
Arsenic as a harmful heavy metal is one of the main risk factors for public health. Sources of As exposure are occupational or via the contaminated food and water. It has a long history of use, either as a metalloid substance or as a medicinal product [7]. As is present as a contaminant in food, water, and the environment. Arsenic exists in the forms of metalloid (As0), inorganic (As3+ and As5+), organic, and arsine (AsH3). The order of increasing toxicity of As compounds is defined as organic arsenicals < As0 < inorganic species (As5+ < As3+) < arsine [8,9]. Primary As absorption is from the small intestine, followed by skin contact and inhalation. Following distribution to many tissues and organs in the body including the lungs, heart, kidneys, liver, muscles, and neural tissue, As is metabolized to monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in which the latter is the predominant form in the urinary excretion of As [8].
Acute and chronic As toxicity is related to the dysfunctions of numerous vital enzymes. Similar to the other heavy metals, As can inhibit sulfhydryl groups containing enzymes which leads to their dysfunction [9]. Moreover, As inhibits the pyruvate dehydrogenase by binding to the lipoic acid moiety of the enzyme. Pyruvate dehydrogenase inactivation can block the Krebs cycle and inhibits oxidative phosphorylation. As a result, ATP production decreases, resulting in cell damage [7]. Furthermore, the damage of capillary endothelium by As increases vascular permeability, leading to vasodilation and circulatory collapse [7].
To avoid heavy metal
exposure:
- Always wash the foods before consumption in order to remove the
chemicals such as pesticides and fungicides.
- Check the functionality of the water filter regularly.
- Eat a variety of nutritious foods.
- Consume supplements that help to remove heavy metals from the body
- Take wholesome and natural foods that help to remove heavy metals
from the body such as cilantro, chlorella, garlic, lemon water, etc.
Angel, Nutritionist
ANNA HOO CLINIC
References:
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- Engwa, G. A. et al. (2019) ‘10.5772@Intechopen.82511.Pdf’, Mechanism and Health Effects of Heavy Metal Toxicity in Humans, p. 23.
- Gorini, F., et al. (2014). The role of heavy metal pollution in neurobehavioral disorders: a focus on autism. Rev. J. Autism Dev. Disord. 1 (4), 354–372. doi:10.1007/s40489-014-0028-3
- Kianoush, S., et al. (2015). Recent advances in the clinical management of lead poisoning. Acta Med. Iran 53, 327–336.
- Kim, T. H., et al. (2020). Exposure assessment and safe intake guidelines for heavy metals in consumed fishery products in the Republic of Korea. Environ. Sci. Pollut. Res. Int., 27, 33042–33051. doi:10.1007/s11356-020-09624-0
- Koedrith, P., et al. (2013). Toxicogenomic approaches for understanding molecular mechanisms of heavy metal mutagenicity and carcinogenicity. Int. J. Hyg. Environ. Health 216 (5), 587–598. doi:10.1016/j.ijheh.2013.02.010
- Lee, M.-R., et al. (2017). Blood mercury concentrations are associated with decline in liver function in an elderly population: a panel study. Environ. Health 16 (1), 17. doi:10.1186/s12940-017-0228-2
- Li, L., and Chen, F. (2016). Oxidative stress, epigenetics, and cancer stem cells in arsenic carcinogenesis and prevention. Curr. Pharmacol. Rep. 2 (2), 57–63. doi:10.1007/s40495-016-0049-y
- Liaw, J., et al. (2008). Increased childhood liver cancer mortality and arsenic in drinking water in northern Chile. Cancer Epidemiol. Biomarkers Prev. 17 (8), 1982–1987. doi:10.1158/1055-9965.epi-07-2816
- Lin, X., et al. (2018). Connecting gastrointestinal cancer risk to
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