**3. Toxicity of heavy metals**

Heavy metals: Fe, Co, Cu, Mn, Mo, Se, Zn, Cr, and Cd, as well as Hg, Pb, and As, have a high density compared to water and are present in trace amounts in different matrices. Heavy metals could cause toxicity at low concentrations, hence their heaviness and toxicity are associated [23]. Heavy metals have a negative impact on the soil, water, air, plants, animals, and humans. High heavy metal concentrations in soil can impact soil quality, particularly pH, color, porosity, and natural composition, as well as crop yield and the extinction of many species of normal flora and animals. Their buildup in the water creates significant issues for humans and ecosystems due to a decrease in drinking water quality and cleanliness, as well as a decrease in water

supplies for all life forms. Increased acid rain, corrosion, eutrophication and haze, decreased agricultural yields, and a shortage of oxygen can be caused by heavy metal contamination in the air. They can damage plants' roots and leaves, disrupt important biochemical processes such as photosynthesis, impair mineral absorption, damage chlorophyll, and inhibit root growth and development, leading to a reduction in total plant growth [24].

Heavy metal toxicity in animals shows itself with reduced body weight, renal damage, liver damage, shorter life span, increased oxidative stress, changes in cell composition, and DNA damage. In humans, they can cause renal illness, liver disease, lung complications, and some cancers [25]. Heavy metals accumulate in organs and soft tissues when they are not metabolized by the body, causing them toxic. Ingestion of contaminated food or water, inhalation, or skin absorption have all been routes they enter the human body. One of the most common ways for heavy metals to enter the bodies of animals is via ingestion. These metals can have inhibitory, stimulatory, or toxic effects on specific biochemical processes, resulting in a variety of health problems in the nervous system (Alzheimer's, Parkinson's, depression, and dementia), the bone system (bone mineralization), and the reproductive system. reactive oxygen species (ROS) can also cause DNA and RNA damage and malignancies of the lungs, skin, and bladder, as a consequence of its production. The amount of exposure, the length of exposure, the pollutant concentration, and the organisms exposed to it, as well as the metal's type and oxidation state, determine its toxicity [26].

### **3.1 Heavy metal toxicity amplification in plants and animals**

Heavy metals persist in nature, posing a threat to human health in addition to their detrimental impacts on plants and wildlife. For example, lead (Pb) is one of the most toxic heavy metals, with a soil retention time of 150–5000 years and a reported concentration retention period of up to 150 years [27]. Plants that grow in heavy metal-contaminated areas tend to collect higher concentrations of heavy metals in the food chain. The main route for heavy metals into animal and human tissues would be through contaminated food, making them vulnerable to a variety of diseases.

Several heavy metals such as Cr, Cd, Pb, Hg, and Al are highly toxic even in trace concentrations, even though they are nonessential and have no physiological role [28]. Both essential and nonessential heavy metals damage plants in comparable ways, caused by low biomass accumulation, chlorosis, growth and photosynthetic inhibition, altered water balance and nutrient assimilation, and senescence, which leads to plant death.

#### **3.2 Potential Human Exposure to heavy metals**

Each year, over 300,000 employees are exposed to heavy metals and metal-containing compounds in the workplace. One of the biggest sources of health concern is industrial worker exposure to the high risk of Cr-induced diseases. Cr is anticipated to be released into the environment at a rate of 33 tons per year, which poses a significant carcinogenic risk. The Occupational Safety and Health Administration (OSHA) in the United States recently established a "safe" level of 5g/m3 for an eight-hour timeweighted average. The overall human population's atmospheric levels range from 1 to 100 ng/cm3 , while levels near Cr-related industries can exceed this range [10].

#### *Heavy Metal Contamination in the Coastal Environment and Trace Level Identification DOI: http://dx.doi.org/10.5772/intechopen.106653*

Nonoccupational exposure occurs when heavy metal-containing meals and water are consumed. Whenever it comes to chromium contamination, recorded levels range from 1 to 3000 mg/kg in soil, 5 to 800 g/L in seawater, and 26 g/L to 5.2 mg/L in rivers and lakes. The amount of chromium in food varies wildly depending on how it is processed and cooked. Fresh foods contain chromium levels ranging from 10 to 1,300 g/kg [29]. Employees in chromium-related industries can be exposed to two orders of magnitude higher levels of chromium than the general population.

Human exposure to heavy metals seems to be mostly by inhalation, with the lung being the primary target organ. However, significant human exposure to heavy metals has also been recorded through the skin [30]. For example, exposure to chromium found in cement is implicated in the worldwide incidence of dermatitis among construction workers. Exposure to Cr(VI)-containing chemicals in the workplace and the environment has been linked to multi-organ toxicity in humans, including kidney impairment, allergy and asthma, and lung cancer.

Enormous concentrations of heavy metals in the air would irritate the nose lining and lead to ulcers. The numerous health problems documented in animals after swallowing heavy metals at detectable levels include stomach irritation and ulcers, sperm loss, anemia, and endocrine abnormalities [8]. Some people who are extremely sensitive to heavy metals have experienced allergic reactions such as edema and severe redness of the skin. In both humans and animals, toxic substances in drinking water have been related to an increased risk of stomach cancer. Ingestion of extremely high concentrations of these compounds by humans, whether it be by accident or on purpose, has resulted in severe respiratory, cardiovascular, gastrointestinal, hematological, hepatic, renal, and neurological consequences, in people who died or survived as a result of medical care. Strong evidence of heavy metal carcinogenicity in humans and terrestrial mammals has also been reported by Mahurpawar [25] and Tchounwou et al. [10].

#### **3.3 Heavy metal toxicity and carcinogenicity**

Heavy metal toxicity is mostly determined by their oxidation state and lipophilicity. Even though the processes of biological interaction are unknown, the chemicals can penetrate through cell membranes, and their intracellular reduction to reactive intermediates may be connected to toxicity variation. It can be absorbed to some extent through the lungs, gastrointestinal tract, and even intact skin. It is considered a detoxification process when harmful substances are reduced at a distance from the target site for toxic or genotoxic action; nevertheless, when the compound is reduced in or near the cell nucleus of target organs, it may help activate their toxicity [31].

Under physiological conditions, heavy metals can be reduced by hydrogen peroxide (H2O2), ascorbic acid, and glutathione reductase to form reactive intermediates such as thiol radicals and hydroxyl radicals. Any of these heavy metal species could disrupt cellular integrity and function by attacking DNA, proteins, or membrane lipids [32].

With animal research, many harmful effects of heavy metals on mammals have been reported [25, 33]. After subcutaneous treatment of Cr, rats developed severe urea nitrogen, creatinine elevations, proteinuria, an increase in blood alanine aminotransferase activity, and hepatic lipid peroxide production. In similar studies, Sahu et al. [34] found that chromium induced kidney impairment in rats when administered as a single subcutaneous injection. Giving Cr to rats in water caused hepatic

**Figure 2.**

*Mechanism of heavy metal induced carcinogenicity (Source: Hartwig [37]).*

mitochondrial and microsomal lipid peroxidation, as well as increased urine lipid metabolite excretion, including malondialdehyde.

In humans, heavy metals have been linked to harmful health consequences. Respiratory malignancies have been recorded among workers exposed to Cr, Sn, Pb, and As-containing substances in the workplace following the epidemiological research [35]. DNA strand breakage in peripheral lymphocytes and lipid peroxidation products in urine in exposed employees reinforce the proof of heavy metal-induced human toxicity. Oxidative damage is assumed to be the cause of genotoxic impacts, such as chromosomal abnormalities and DNA strand breaks. According to the current study by Singh et al. [36], non-oxidative processes appear to have a role in carcinogenesis. Carcinogenicity appears to be linked to inhalation of less soluble chemicals such as Sn and Cr. Surface charge, phagocytization capacity, crystal modification, and size are all features of chromium that may be significant in predicting cancer risk, according to epidemiological studies (**Figure 2**).
