**11. Effects of heavy metals on humans**

The harmful effects of heavy metals on human health have been proven from various aspects, and exposure to these pollutants causes acute and chronic poisoning, as well as various diseases, including nervous disorders, hormone imbalance, respiratory and cardiac disorders, decrease memory, types of cancer and eventually death [30, 127]. The lethality of most heavy metals for humans is estimated in the range of 500–350 mg per day. Heavy metals cause various diseases such as infertility, poisoning, nervous system disorders, breaking chromosomes, premature aging and various cancers in humans [28, 128]. Cancer is the main cause of death in developed and developing countries of the world. The increase in cancer may be caused by the increase in environmental pollution [65]. Heavy metals are one of the most important environmental pollutants [129]. These metals have the greatest impact on the health of citizens due to the occurrence of health risks such as reduced growth of children, kidney diseases, cancer and other adverse effects [130].

Heavy metals accumulate in the vital organs of the human body due to their indestructible and stable nature and lead to serious health disorders. Heavy metals should be considered due to their indestructibility, stability, accumulation in living organs and damage to the health of living organisms [32]. Contact with heavy metals occurs chronically (contact over a long period of time) due to movement in the food chain, but acute poisoning by ingestion or skin contact with heavy metals is rare, but possible. There is [131].

Heavy metals in soils are harmful to human health, especially children [132, 133]. Children have a high rate of heavy metal absorption due to their active digestive system, small body size, developing nervous system, swallowing dust, soil or suspended particles, weak immune system and excessive use of hands [134]. Heavy metals are very harmful to the human body because they do not have any effective elimination mechanism in the body [135]. These metals affect health indirectly by consuming plants that grow in contaminated soil and directly by inhaling and consuming contaminated water [136]. Heavy metals may become a problem for human health and have adverse environmental effects [137]. Heavy metals can enter the human body directly through swallowing and breathing, or reach the earth's surface through atmospheric fallout, and enter the body through water and food after polluting water and soil sources and entering the structure of plants [29].

### *Heavy Metals in Surface Soils and Crops DOI: http://dx.doi.org/10.5772/intechopen.108824*

Some heavy metals are necessary for the continuation of life and activities of animals and they play a significant role in the body. Unnecessary and toxic heavy metals without having a role in the physiological activities of animals, even in low concentrations, cause disturbances in the body system of animals [27]. Among the pollutants in the environment, toxic metals in high concentrations cause poisoning for living organisms. Some metals, especially heavy metals, are of high environmental importance due to their toxic properties and accumulation in living organisms, even in relatively low concentrations [138]. Unlike some organic substances, these toxic metals are not biodegradable and their accumulation in living tissues can lead to death or serious threats to health [139]. Due to the fact that these metals are not decomposed by conventional biological processes, as a result, by accumulating in the tissues of living organisms, they are easily moved through the food chain, hence, by increasing their amounts in the soil over time, to It significantly damages plants. For example, copper and zinc in very low concentrations are essential trace elements for the survival of plant and animal life [140]. Long-term biological durability and remaining in the soil causes the accumulation of these metals in food chains and as a result, potential negative effects for human health. The amount of access to these metals depends on the type of plant and their required amount as micronutrients and the ability of plants to efficiently regulate their metabolism through the secretion of organic acids or protons into the root environment. In addition, the soil properties are effective on the mobility of these metals and therefore regulate their release rate in the soil solution. The ability of plants to absorb metals from the soil, their internal use and detoxification mechanisms have met with increasing popularity [141].

Physiological stages in plants are affected by increasing the concentration of heavy metals around the target plant. These metals cause oxidative stress in the plant, which is one of the harmful effects of this stress in the production of free radicals. In high concentrations of metals, substitution with essential metals occurs and since essential metals play an important role in the formation of pigments and enzymes, therefore the formation of pigments is disturbed and hence the elements It makes the soil unsuitable for plant growth and destroys biodiversity [142, 143]. For example, cadmium metal is one of the most toxic elements for plants and has no biological role. Cadmium metal mainly enters the environment and food chain through industrial processes and phosphate fertilizers. This toxic metal is easily absorbed by the roots and by forming complex complexes with organic compounds such as proteins, it prevents the necessary activity of cells. Cadmium, by increasing the peroxidation of lipids and the production of reactive oxygen species, provides membrane deterioration [100]. Since this metal has two positive charges (bivalent) and competes with elements such as magnesium in chlorophyll and with iron ions which are divalent and replaces them and the chlorophyll molecule in the plant is thus destroys Therefore, photosynthesis is very sensitive to cadmium. The high concentration of essential metals such as copper and zinc also harms the plant. Copper metal reduces plant growth by preventing the absorption of other elements such as calcium, iron and potassium, which are essential plant elements [144, 145].
