Toxic effects of heavy metals on human health.
Heavy metals (HMs) are natural environmental constituents, but their geochemical processes and biochemical equilibrium have been altered by indiscriminate use for human purposes. Due to their toxicity, persistence in the environment and bioaccumulative nature; HMs are well-known environmental contaminants. As result, there is excess release into natural resources such as soil and marine habitats of heavy metals such as cadmium, chromium, arsenic, mercury, lead, nickel, copper, zinc, etc. Their natural sources include the weathering of metal-bearing rocks and volcanic eruptions, while mining and other industrial and agricultural practices include anthropogenic sources. Prolonged exposure and increased accumulation of such heavy metals may have detrimental effects on human life and aquatic biota in terms of health. Finally, the environmental issue of public health concern is the pollution of marine and terrestrial environments with toxic heavy metals. Therefore, because of the rising degree of waste disposal from factories day by day, it is a great concern. Pollution of HMs is therefore a problem and the danger of this environment needs to be recognized.
- Heavy metal
- Health hazard
Heavy metals (HMs) are an environmental threat and are of grave concern worldwide . Rapid industrialization and urbanization have caused heavy metals to contaminate the atmosphere and it is a problem for human health [2, 3, 4]. HMs poses a major environmental threat to living organisms and habitats due to their non-biodegradability, bioaccumulation, environmental stability, persistence and biotoxicity characteristics [5, 6, 7]. In order to prevent microbial activities, they can directly influence the physical and chemical properties of sediment, soils and water . They can also disrupt the natural ecosystem and impact the human body acutely and permanently through the food chain [9, 10, 11]. The non-degradable HMs can also accumulate in the surface sediment for a long time via the food chain’s amplification effect, causing various diseases and complications in the human body [1, 12, 13].
Natural activities (e.g. geological weathering, atmospheric precipitation, wave erosion, wind and bioturbation) and anthropogenic activities (e.g. rapid industrialization, urbanization, agricultural runoff and transport) play a key role in the spread of HMs to the marine habitats of aquatic ecosystems such as rivers and estuaries [2, 4, 14]. In addition, human activities that can produce industrial pollution, the deposition of urban waste and the offensive use of chemical fertilizers and pesticides result in the accumulation and sinking of HMs in aquatic habitat surface sediments [15, 16, 17]. The HMs released into the water column have a negative effect on water quality [11, 18] and on surface sediments that alter environmental parameters such as pH, temperature, bioturbation etc. [19, 20]. Sediment quality can therefore play a critical role in identifying the effects of natural and anthropogenic activities [21, 22, 23]; sediment quality can also provide information on the anthropogenic impact on the ecosystem and guide environmental policy and management .
Farm waste, agricultural runoff, pesticides, solid waste, waste management, effluents from fish processing plants, jute processing, cement manufacturing, oil refining, fertilizer manufacturing, building materials, soup and detergent factories and brickyard waste are the major sources of pollution (Figure 1). Due to the potential risk of HMs in water, soil and sediment through the disposal of the effluents mentioned, this riverine water, sediment and environment may be important.
2. Heavy metals
Any metal or metalloid of environmental significance is a heavy metal ; the term originated in reference to the adverse effects, all denser than iron, cadmium, mercury and lead (Figure 2). It has since been extended to some other similarly toxic metal or metalloid, irrespective of density, such as arsenic, chromium, cobalt, nickel, copper, zinc, arsenic, selenium, silver, cadmium, antimony, mercury, thallium and lead are commonly found to be heavy metals [26, 27].
In a general, collective term that refers to a group of metals and metalloids with an atomic density greater than 4 g/cm3 or 5 times or more than water, the term heavy metal is often referred to as trace elements as they exist in minute concentrations in biological systems .
Heavy metals are classified as “metals that occur naturally and have an atomic number greater than 20” [29, 30]. They are a significant class of contaminants that affect the environment. A serious problem with cultural, ecological and economic consequences is heavy metal contamination in the environment. Because of the toxicity, persistence, and bioaccumulative nature of these materials, research on heavy metals in the atmosphere is an important part of environmental research.
2.2 Sources of heavy metals
Heavy metals may come from natural and anthropogenic processes and end up in various environmental compartments (soil, water, air and their interface). Figure 3 gives information on natural and anthropogenic sources of heavy metals.
2.2.1 Natural sources
Various natural sources of HMs have been recorded in several studies. Natural emissions of HMs occur under numerous and certain environmental conditions. Volcanic eruptions, sea-salt sprays, forest fires, rock weathering, biogenic sources and particles of wind-borne soil are included in these pollutants. The release of metals from their endemic spheres to different environmental compartments will lead to natural weathering processes. In the form of hydroxides, oxides, sulfides, sulfates, phosphates, silicates and organic compounds, heavy metals can be found.
Lead (Pb), nickel (Ni), chromium (Cr), cadmium (Cd), arsenic (As), mercury (Hg), selenium (Se), zinc (Zn) and copper (Cu) are the most popular heavy metals. While the above-mentioned heavy metals can be present in traces, humans and other mammals still cause significant health problems.
2.2.2 Anthropogenic sources
Industries, irrigation, drainage, mining and metallurgical processes, as well as runoff, also contribute to the release of pollutants into various compartments of the ecosystem. For certain metals, anthropogenic heavy metal processes have been noted to go beyond natural fluxes. In wind-blown dust, metals naturally released are mainly from industrial areas. Car exhaust that releases lead; smelting that releases arsenic, copper and zinc; insecticides that release arsenic and the burning of fossil fuels that release nickel, vanadium, mercury, selenium and tin are some essential anthropogenic causes that contribute significantly to heavy metal pollution in the environment. Because of the everyday manufacture of products to meet the demands of the large population, human activities have been found to contribute more to environmental pollution.
3. Contamination of heavy metals
Harmful trace metals are a significant threat to both aquatic and terrestrial ecosystems . Upon release from both natural and anthropogenic sources, HMs contaminates natural aquatic bodies, sediments and soils. Ultimately, during volcanic eruptions and complex industrial emissions, heavy metals released into the atmosphere often return to the soil and cause water and soil contamination. They either collect in biota or leach down into ground water because heavy metals in the atmosphere are irreversible. There are significant public health effects of the contamination of biota and groundwater with potentially harmful heavy metals. In riverine settings, the degree of heavy metal pollution can be measured by observing the concentrations and distribution of these elements . Figure 4 provides a conceptual schematic of the contamination of the marine (riverine) environment with heavy metals. Various physicochemical and climatic effects affect the overall dynamics and biogeochemical cycling of heavy metals in the atmosphere.
Water is regarded as the life-blood of the biosphere as well. It can dissolve distinct organic and inorganic chemicals and environmental contaminants, as water is a common solvent. They are vulnerable to pollution in both freshwater and marine aquatic environments. Heavy metals are important contaminants in the pollution of the marine environment. Water contamination by HMs is a critical environmental issue that adversely affects plants, animals and human health . Also at very low levels, heavy metals are highly harmful to marine species . These elements may cause significant physiological changes in the body and histopathological changes in the tissues of aquatic organisms, such as fish . There are several sources of heavy metal pollution in water. Two main culprits of this industrialization and urbanization are the increased degree of concentration of heavy metal in water. Heavy metals from factories, municipalities and urban areas are carried by runoff . This release of untreated industrial waste into marine bodies is a significant cause of surface and groundwater contamination . Due to the environmental persistence, bioaccumulation, and biomagnification of food chains and the toxicity of these elements, contamination of water bodies with heavy metals is a global issue .
The contamination of sediments with HMs is a very important environmental problem with implications for marine life and human health. Sediments act as the main source of HMs in the aquatic environment. Their quality can indicate the contamination status of water . Sediments act as a sink and heavy metal source, releasing them into the column of water . Continued heavy metal accumulation in sediments can also contribute to groundwater pollution of these contaminants . Many physicochemical variables such as temperature, hydrodynamic conditions, redox status, organic matter and microbe content, salinity, and particle size influence the adsorption, desorption, and subsequent concentrations of heavy metals in sediments . The distribution of heavy metals in sediments is influenced by the sediment’s chemical composition; grain size and total organic matter content . The pH is an important determinant of the bioavailability of metals in sediments. A decrease in pH increases the competition between metal ions and H+ for sediment binding locations and may contribute to the dissolution of metal complexes, thereby releasing free metal ions into the water column . Higher toxic heavy metals concentrations in riverine sediments can pose an ecological risk to benthos (bottom-dwelling organisms) .
Heavy metals and metalloids are released into soils from activities and sources such as manufacturing activities, mine tailings, high metal waste disposal, leaded gasoline and paints, fertilizer land application, animal manures, sewage sludge, pesticides, irrigation of waste water, residues of coal combustion and petrochemical spillage, resulting in soil contamination by heavy metals . Most HMs does not typically experience microbial or chemical degradation and thus, after being released to the atmosphere, their total concentrations last in the sail for a long time. The composition of the parent rock, the degree of weathering and physical, chemical and biological characteristics of soil and environment conditions are factors influencing the presence and distribution of heavy metals in soils . Compared to virgin soils and soils with low inputs, substantial heavy metal enrichment has been recorded in soils receiving more fertilizer input and Cu fungicide . Soils may be polluted with heavy metals from heavy vehicular activity on roads in urban areas. In urban areas, soil samples have elevated levels of Pb, of which 45–85% is bioaccessible . The bioavailability of heavy metals in soils is of great importance for their environmental fate and for their plant uptake. Different HMs has different soil bioavailability and this bioavailability depends on the speciation of metals and the different soil physicochemical characteristics.
Aquatic biota is exposed to heavy metals by water, sediments and food on various routes . Different toxic HMs released to freshwater bodies from various natural and anthropogenic sources are introduced to freshwater fish. Heavy metal pollution of fish has become a major global concern because it poses a danger to fish and poses health hazards to buyers of fish . Assessment of the bioaccumulation of HMs in fish species from various aquatic ecosystems is very significant . Assessing the amount of heavy metals in fish tissues is important for the conservation of marine environments and the human consumption of fish . There are high levels of unsaturated fatty acids and low cholesterol levels in fish. They are a major protein source .
It is advantageous to use edible fish in human diets and is also recommended in healthy diets. Contamination of fish by toxic heavy metals is considered a risk to human health and has raised concerns about their consumption, especially among more vulnerable groups of people, such as women, children and people at risk of other diseases.
Heavy metal bioaccumulation in freshwater fish depends on different factors, including the characteristics of the fish and the external environmental factors. Fish-related factors include fish age, size (weight and length), feeding habits and physiology of the body, while external environmental factors include water column metal concentration and bioavailability, water physicochemical properties and other climatic factors. Depending on the structure and function of the tissues, the degree of accumulation of heavy metals in the various tissues of fish is usually different. Metabolically active tissues such as the gills, liver and kidneys typically have higher heavy metal accumulations than other tissues such as the skin and muscles. The comparatively higher accumulation of heavy metals in metabolically active fish tissues is normally explained by the induction/occurrence in these tissues of metal-binding proteins called metallothioneins (MTs) upon exposure to heavy metals. Fish gills have been observed as the target tissue for heavy metals such as Ni to accumulate and eliminate . While fish muscles are the tissue of poor heavy metal accumulation , from a human consumption point of view, they are essential. Trace metal bioaccumulation in fish muscles is typically species-specific .
The bioaccumulation of toxic HMs in freshwater fish has significant environmental, ecological and social consequences; it affects the carnivorous species and human by eating fish [57, 58]. Waterborne HMs are absorbed into fish and penetrate the human body through the food chain, thereby impacting human health . In addition, poisonous HMs affects the health and well-being of fish as well.
4. Trophic transfer of heavy metals
As HMs in the atmosphere is permanent, they accumulate in living organisms and are passed in the food chains from one trophic stage to another. The degree to which heavy metals are deposited in biota depends on their accumulation rate and their removal rate from the body.
Heavy metals, i.e. water, sediments and soil, may enter the body of an organism directly from the abiotic system, or may enter the body of the organism from its food/prey. For example, heavy metals may reach the body of the fish directly from water or sediments through the gills/skin of the fish or from the food/prey of the fish through its food canal. Over successive trophic levels in a food chain, the concentration of a heavy metal can increase or decrease. The retention of heavy metals in an organism’s body depends on a variety of factors, such as the speciation of the metal concerned and the physiological mechanisms established by the organism for heavy metal control, homeostasis and detoxification. Because of their lipophilicity, methylated forms of heavy metals like Hg are accumulated to a greater extent in biota and also biomagnified in food chains. In metal-rich environments, such plants have the potential to survive and are called metallophytes. Special mechanisms for dealing with higher heavy metal concentrations in soil have been established by these special plants and are classified into three groups, i.e. excluders, markers, and hyper accumulators . To define the trophic transfer of heavy metals, certain words are used (Figure 5).
5. Quantification of trophic transfer
The degree or extent of accumulation of HMs in biota has been quantified using some terminology. Bioconcentration factor (BCF), bioaccumulation factor (BAF), bioaccumulation coefficient (BAC) and so on are some of these quantitative terms. Below, some of these terms are discussed.
5.1 Bioconcentration factor (BCF)
Bioconcentration Factor is representative of the degree of heavy metal enrichment in an organism as opposed to that in its habitat. The ratio of the concentration of a heavy metal in the tissue of an organism to its concentration in the abiotic medium is known as’ (water and sediments).
It is calculated by the following equation :
The alternative words transfer factor (TF), metal transfer factor (MTF), accumulation factor (AF), bioaccumulation factor (BAF), and biota sediment accumulation factor (BSAF) are used by some writers and measured accordingly. All these indexes, however, indicate the extent of a heavy metal’s accumulation in the organism compared to that in the world where it grows/lives.
5.2 Bioaccumulation coefficient (BAC)
Bioaccumulation Coefficient is calculated by the following equation :
Where, Cplant is the metal concentration in plant and Csoil is the metal concentration in soil.
The values of BCF, BAF, BAC, etc. are obviously dependent on the concentration of the HM in the organism and the environmental medium in question. In view of the fact that the values of these indices are inversely related to the concentration of metals in the environmental medium (water, sediments and soil), the values of these indices should be used with caution to measure the contamination of heavy metals in biota. For example, because of the lower metal content in the environment of the former fish, the BCF value of a heavy metal in the muscles of a fish living in less polluted water may be higher than that of a fish living in more contaminated water. The bio-concentration factor (BCF) values of seven common HMs in crop grains have been shown to decrease exponentially with average soil metal concentrations .
6. Effects of heavy metals in human health
A big source of exposure for the human population to heavy metals is food and drinking water. The intensification of HMs in industrial and agricultural activities has led to much industrialization, modern urbanization and rapid economic development around the world . These activities can cause pollution of toxic HMs in water, air, and soils. Heavy metal-contaminated media contribute to the bioaccumulation of these elements from the environment in the human food chains, eventually reaching the human body. There are some health impacts of HMs on the human body depending on the amount and duration of exposure (Figure 6).
Among different HMs, Cd is considered as seventh most hazardous and noxious metal that causes an indirect oxidative stress and have carcinogenic and mutagenic effect resulting severe health problem in human body such as kidney damage, prostate dysfunction, bone diseases and cancer [64, 65]. It can also responsible for kidney dysfunction and proteinuria if Cd has prolonged exposure to kidneys.
Arsenic is also causes skin damage, cancer and marked problems with circulatory system . When As cross its permissible limit in drinking water is also causes developmental abnormalities, neurobehavioral sicknesses, cardiovascular diseases and hearing sickness, together with anemia, leukopenia, eosinophilia and carcinoma [67, 68].
Chromium is known as highly toxic HM as it can cross the cell membrane via sulfate transport system and causes denaturation and mutation of nucleic acids and proteins. It also creates critical health issues like skin problems, nasal irritation, hear impairment and lung carcinoma [65, 69, 70].
Lead can also create different health problems like decrease in intelligent quotient, memory loss, infertility, mood swing, weakness of joints, nausea, insomnia, anorexia, or even death . Young and infants are more sensitive to Pb poisoning than adults. Details about effects of different HMs on human health were described in Table 1.
|Heavy metals||EPA regulatory limit for drinking water (ppm)||OSHA limit for workplace air (μg/m3)||Toxic Effects||References|
Nausea and vomiting
Decreased production of red and white blood cells
Abnormal heart rhythm
Damage to blood vessels
A sensation of “pins and needles” in hands and feet also cause muscle damage
Darkening of the skin and the appearance of small “corns” or “warts” on the palms, soles and torso
Diffuse or spotted hyper-pigmentation of the skin
Benign skin lesions (hyperkeratosis) and cancer of the skin
Liver disease reflected by abnormal porphyry metabolism
Chronic inhalation can cause lung cancer
Chronic exposure via water can cause cancer of internal organs, particularly the urinary bladder, lung, liver, and kidney
Affects essential cellular processes such asoxidative phosphorylation and ATP synthesis
|[28, 63, 71, 72]|
Gastrointestinal dysfunction (vomiting, abdominal cramps, diarrhea)
Difficulties in breathing
Increased or decreased blood pressure
Numbness around the face and
High blood pressure
Changes in heart rhythm and cardiac arrhythmias
Respiratory failure and
|[63, 73, 74]|
Cause acute bronchitis or even chronic disease, such as emphysema or pulmonary fibrosis and lung cancer
Severe irritation in the stomach, leading to vomiting and diarrhea
Buildup kidney disease, lung damage, and fragile bones
Kidney tubular dysfunction
Osteoporosis (elderly women with iron deficiency)
Lung cancer (only in chronic inhalation)
Affects calcium regulation in biological systems
|[75, 76, 77, 78, 79, 80, 81]|
|Chromium (Cr)||0.1||0.5 to 1000|
Irritation to the lining of the nose
Nose ulcers and runny nose
Breathing problems, such as asthma, cough, shortness of breath, or wheezing
Allergic reactions consisting of severe redness and swelling of the skin
Damage to liver, kidney circulatory and nerve tissues
Arygria, a blue-gray discoloration of the skin and other body tissues
Mild allergic reactions such as rash, swelling, and inflammation in some people
Lung and throat irritation and
|Mercury (Hg)||2.0||100 for organic Hg|
50 for metallic Hg
Loss of memory
Disturbance of vision
Fetal death and abortion
Damage the brain resulting irritability, shyness, tremors, changes in vision or hearing, and memory problems
Damage kidneys and
Gastrointestinal dysfunction (nausea, vomiting, diarrhea)
Increases in blood pressure or heart rate
Skin rashes and
|[28, 83, 84, 85, 86, 87]|
Severely damage the brain and kidneys
Miscarriage in pregnant women
Damage the organs responsible for sperm production
In children causes impaired development, reduced intelligence, short-term memory loss, disabilities in learning and coordination problems, risk of cardiovascular disease
Effects in central nervous system, including impaired motor function and cognitive function and even seizures, coma, and death with markedly elevated blood lead levels
Impaired heme synthesis
Chronic kidney disease
Impairment of cognitive function
Lead produces tumors in experimental animals, but there is not enough evidence to regard lead as a human carcinogen
|[75, 88, 89, 90, 91, 92]|
Produce selenosis; major signs of selenosis are hair loss, nail brittleness, and neurological abnormalities
Dietary exposure of around 300 μg/day affects endocrine function
Impairment of natural killer cells activity
|Copper (Cu)||1.3||Brain and kidney damage|
Stomach and intestine irritation
|[63, 75, 88]|
|Nickel (Ni)||0.2||Allergic skin diseases such as itching|
Cancer of the lungs, nose, sinuses, throat through continuous inhalation
Immunotoxic, neurotoxic, genotoxic effect
|[75, 94, 95]|
Heavy metals and metalloids are critical environmental contaminants for both marine and terrestrial environments and ecosystem. The danger of an environmental chemical is a function of its persistence, toxicity and bioaccumulative ability in the environment. Due to these three characteristics: persistence, bioaccumulation, and toxicity; HMs are considered as harmful. The most dangerous heavy metals and metalloids that are environmentally important include Cr, As, Cd, Pb, Hg, Ni, Cu and Zn. In aquatic and terrestrial food chains, the trophic transition of these elements has major consequences for different lives of ecosystem and human health. So contamination of environment of heavy metals is of great concern, and its treatment from the soil and water around industrial areas is needed urgently in every place. The assessment and monitoring of the concentrations of potentially toxic heavy metals and metalloids in the various environmental segments and in the resident biota is very significant. A detailed environmental chemistry and ecotoxicology analysis of dangerous heavy metals and metalloids demonstrates that steps should be taken to mitigate the effects on human health and the environment of these elements.