Several heavy metals are found naturally in the earth crust and are exploited for various industrial and economic purposes. According to the Environmental Protection Agency (EPA), lead is considered a carcinogen. Arsenic-induced carcinogenic mechanisms include epigenetic alterations, damage to the dynamic DNA maintenance system and generation of ROS [36, 37]. Certain pesticides, fertilizers and animal feeding operations also release arsenic to the environment in higher amounts. The NMDAR is known to enhance learning and memory mediated by the hippocampus  as this has been confirmed in animal studies in which animals exposed to lead during its developmental process exhibit similar learning deficits comparable to those with the absence or impaired NMDARs [48, 49]. As long ago as 2007, the World Health Organization stated that heavy metals accumulated in the environment, “. Humans get in contact with arsenic through several means which include industrial sources such as smelting and microelectronic industries. More focus will be laid on the various mechanisms that lead to heavy metal toxicity with emphasis on macromolecule and cellular damages, carcinogenesis, neurotoxicity and the molecular basis for their noxious effects. Even though there are no immediate problems, heavy metals keep on accumulating in our body. 1,, Kamgba F. A. Also, lead has been shown to reduce the expression of key presynaptic proteins such as synaptobrevin (Syb) and synaptophysin (Syn) involved in vesicular neurotransmitter release [59, 60]. Vegetables take up heavy metals by absorbing them from contaminated soils, as well as from deposits on parts of the vegetables exposed to the air from polluted environments (Wang et al., 2005). In vivo and in vitro experiments have shown chromate compounds to induce DNA damage in many different ways and can lead to the formation of DNA adducts, chromosomal aberrations, alterations in replication sister chromatid exchanges, and transcription of DNA . Long-term exposure to cadmium leads to its deposition in bones and lungs. It can affect the central nervous function leading to mental disorder, damage the blood constituents and may damage the lungs, liver, kidneys and other vital organs promoting several disease conditions . This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (1) Other heavy metals act as molecular “mimics” of nutritionally essential trace elements; depleting the body from cofactors essential to cellular regeneration. Also, humans may get exposed to cadmium by inhalation through incineration of municipal waste. Certain heavy metals are known to generate free radicals which may lead to oxidative stress and cause other cellular damages (see  for review). Chromium is extensively used in industries such as wood preservation, electroplating, metallurgy, production of paints and pigments, chemical production, tanning, and pulp and paper production. Nickel can promote methylation of promoter and induce the down regulation of maternally expressed gene 3 (MEG3) thereby upregulating hypoxia-inducible factor-1α, two proteins which are known to be implicated in carcinogenesis . Heavy metals exist all around us in our water, air and soil. Olawoyin R(1), Oyewole SA, Grayson RL. Iron-induced free radicals can initiate cancer by the oxidation of DNA leading to DNA damage . People that consume high levels of heavy metals risk acute and chronic toxicity, liver, kidney, and intestinal damage, anemia, and cancer. Some heavy metals are known to have carcinogenic effect. Heavy metals in the environment and their health effects Heavy metals have a density of 6.0 g/cm3 or more (much higher than the average particle density of soils which is 2.65 g/cm3) and occur naturally in rocks but concentrations are frequently elevated as a result of contamination. Others can enter the body by inhalation while others such as lead can be absorbed through the skin. Certain pesticides, fertilizers and animal feeding operations also release arsenic to the environment in higher amounts. They are present in the atmosphere, lithosphere, hydrosphere and biosphere . Heavy metal-bound proteins may be a substrate for certain enzymes. Heavy metals can damage and alter the functioning of organs such as the brain, kidney, lungs, liver, and blood. Heavy metals may also inhibit protein folding. The major sources of lead are drinking water from old plumbing systems or through new dripless faucets made of metal alloys containing lead. However, the main differences between manganism and Parkinson disease is the insensitivity of manganism to levodopa (l-DOPA) administration and also the differences in the symptoms and progression of the disease . Coffee tends to aggravate the brain problem, as caffeine increases stomach hydrochloric acid levels. Cadmium has been shown to inhibit human thiol transferases such as thioredoxin reductase, glutathione reductase, thioredoxin in vitro by binding to cysteine residues in their active sites . The metallic mercury is a shiny silver-white, odorless liquid metal which becomes colorless and odorless gas upon heating. Even though there are no immediate problems, heavy metals keep on accumulating in our body. Long-term exposure can lead to the formation of skin lesions, pulmonary disease, neurological problems, peripheral vascular disease, diabetes mellitus, hypertension and cardiovascular disease . Cadmium inhibits the formation of many enzymes and the action of nutrients in the body, and produces high blood pressure, anemia, liver damage, and metabolic kidney dysfunction and can contribute to osteoporosis, dementia, calcium stones, hypertension, hair loss, low blood hemoglobin levels, etc. The third stage commences between 12 and 96 h after the onset of clinical symptoms and is characterized by hypotension, shocks, lethargy, hepatic necrosis, tachycardia, metabolic acidosis and may sometimes lead to death . 2000 Owerri, Imo State, Nigeria. plant metabolism, hampered due to heavy metal interference with activities of soil microorganisms. Aluminum accumulates in the body over time, and in the brain where it’s considered a prime suspect in various brain diseases, including Alzheimer’s. In babies and young children, the nervous system can be affected making the child less able to think and learn (http://www.epa.gov/mercury/about.htm). The various sources of heavy metals are summarized in Figure 1. Effects of Heavy Metals on Plants: The heavy metals have adverse effects on plants. These transcription factors control the expression of protective genes that induce apoptosis, arrest the proliferation of damaged cells, repair damaged DNA and power the immune system . Side effects may include: 1) Cancers of the skin, lungs, stomach and other organs. Are Heavy Metals in Healthy Foods? These metals have been extensively studied and their effects on human health regularly reviewed by international bodies such as the WHO. Lead can also reduce the expression of brain-derived neurotrophic factor (BDNF), a trans-synaptic signaling molecule that is released from both axons and dendrites which is involved in synaptic development and neurotransmitter release . Furthermore, when these heavy metals are used in the industries for various industrial purposes, some of these elements are released into the air during combustion or into the soil or water bodies as effluents. Experimental studies confirmed that copper is also capable of inducing DNA strand breaks and oxidation of bases via oxygen free radicals . Heavy metal toxicity can either be acute or chronic effects. Reactions of Heavy metals with sulphydryl groups of proteins or enzymes (A) = Intramolecular bonding; (B) = Intermolecular bonding; P = Protein; E = Enzyme; M = Metal. Let’s take a look at some of these heavy metals and their effect on health, beginning with arsenic. Arsenic compounds have been shown in an in vitro cell line study to promote genotoxicity in humans and mice leucocytes . Most of the paints, dyes, soaps, metals, semi-conductors and drugs contain arsenic. Smokers are more susceptible for cadmium intoxication than non-smokers. Heavy metal contamination can have both short and long-term side effects and produce damage to systems and organs that cannot always be completely reversed by treatment in cased of prolonged overexposure. In recent decades, adverse effects of unexpected contaminants on crop quality have threatened both food security and human health. Long-term exposure to cadmium can result in pathological changes in the kidneys. The presence of heavy metals cannot be seen with naked eyes, but can be detected with a water test. More so, repeated long-term contact with some heavy metals or their compounds may even damage nucleic acids, cause mutation, mimic hormones thereby disrupting the endocrine and reproductive system and eventually lead to cancer . For example, some heavy metals inhibit the cellular function, which is to protect the organism from oxidative damage, reducing the effectiveness of this antioxidant defense system for detoxification. 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