**2. Toxicity of heavy metals**

The pathway of exposure for heavy metals is mainly through inhalation, dermal contact, and ingestion. The individual metal exhibits its own specific signs of toxicity [6]. The severity of health effects is dependent on time and dose, the type of heavy metal, and its chemical form. The nature of effect may be toxic, mutagenic, neurotoxic, teratogenic, or carcinogenic [6]. Many studies reported that heavy metals affect cell organelles and interact with cell components causing cell damage and apoptosis. Even at a low level of exposure, they induce multiple organ damage. Intoxication of heavy metals also leads to damage to the major systems in the body and may lead to an increased risk in developing cancers [7]. Metal ion pollution is highly persistent, and most of them are nonbiodegradable. The presence of various heavy metals such as chromium (Cr), lead (Pb), zinc (Zn), arsenic (As), copper (Cu), nickel (Ni), cobalt (Co), cadmium (Cd), and mercury (Hg) causes disturbances in circulatory, gastrointestinal, and nervous systems. They also affect various organs and lead to blindness, deafness, brain damage, loss of fertility, cancer, and many other severe health problems that ultimately cause death of the individual [7–9].
