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concluded that lead (Pb) could influence behavioural inhibition systems, resulting in an increase of

aggression, and it could also promote tooth decay.

The rate of consumption is also a determining step for toxicity in human health. Metals such

as copper, nickel and zinc are essential trace elements for the body but are harmful if consumed

in excess. Prolonged exposure to copper is linked to abnormalities that severely affect capillaries,

kidneys and liver functions. It also leads to central nervous system irritation along with depression.

System dysfunction impedes growth and development. Excess zinc is related to infertility. Exposure

to nickel among humans enhances the risk of developing respiratory cancer nasopharyngeal

carcinoma (Barta et al. 2019). Excessive mercury harms the neurological system and affects

muscle functioning, body balance, partial blindness and abnormalities in infants (Baby et al. 2010).

According to some studies, mercury may harm both the foetal and embryonic neural systems at

concentrations lower than the WHO’s recommended limits, resulting in learning difficulties, poor

memory and reduced attention spans.

Cadmium (Cd) poisoning damages the liver, brain, lungs, placenta, kidneys and bones. The

severity depends on the amount of exposure (Sobha et al. 2007). Extreme exposure has been

related to mortality and pulmonary oedema. Emphysema, bronchiolitis and alveolitis are the other

consequences of exposure to Cd. Cadmium is associated with many clinical problems, including

heart failure, malignancies, cataract development and lung-related issues. The toxicity symptoms

of arsenic, mercury and lead, depend on the form in which these are consumed. Arsenic promotes

protein coagulation with coenzymes and inhibits adenosine triphosphate (ATP) synthesis. Arsenic

oxides are carcinogenic and serve violent mortality in higher concentrations. Arsenic is associated

with several disorders, such as Guillain-Barre syndrome, which is known as an anti-immune

disorder. In this condition, the patient’s Peripheral Nervous System (PNS) is attacked by the human

immune system, which results in nerve inflammation and muscle weakness.

10.3.2 Effects of Toxic Metals on Plants

A minimum concentration of metal ions is essential for optimal plant growth. The absence of the

same could result in growth deficiencies, whereas excessive access to Cd, As, Hg and Pb are highly

toxic in plants (Okereafor et al. 2020). There are many parameters such as temperature, pH of the

soil and plant growth environment, which decide the plant’s metal absorption (Nagajyoti et al. 2010).

Excessive levels of hazardous metals affect plant growth and development (Musilova et al. 2016).

Different metals like Mn, Pb, Cd, Cr and Co cause slow development in maize plants (Mujtaba

Munir et al. 2020). If exposed to a high concentration, toxic metals may cause oxidative stress, cell

structural deformation and slow photosynthetic activities in the plants. Zn and Cd phytotoxicity

is shown in different plant species such as Brassica juncea by delayed growth and development,

alteration in metabolism and inductive oxidative damage (Okereafor et al. 2020). Cd and Zn at

extremely high concentrations may cause oscillations in the catalytic competence of enzymes in

pea plants (Romero-Puertas et al. 2004). Zinc poisoning has also been observed in plants causing

slow development of roots and shoots and chlorosis in younger leaves. There have been reports

of agricultural yield reductions owing to Ni toxicity, which inhibited specific enzymatic activity

(amylase, protease and ribonuclease), reducing seed germination. Membrane stability, nitrate

reductase and carbonic anhydrase were also impacted by Ni (Yusuf et al. 2012).

Cu is a micronutrient for plants, which is essential for ATP production and CO2 absorption.

Cu is found in several proteins, including plastocyanin which is important in the photo system,

Cytochrome Oxidase and Electron Transport Chain (Demirevska-Kepova et al. 2004). When it

is present in excess, it may cause stress, growth retardation and leaf chlorosis in plants. Plants

suffer from oxidative stress because of elevated Cu levels, which disrupts metabolic pathways and

damages macromolecules. It is also observed that copper poisoning affects Alyssum montanum,

whereas copper and cadmium cause poor germination, small seedling length and roots development

in Solanum melongena (Al Khateeb and Al-Qwasemeh 2014).