Heavy metals are a group of metals and metalloids that have relatively high density and are toxic even at low concentrations. The criteria used to define heavy metals vary depending on the author and context, but they are generally defined as metals with relatively high densities, atomic weights, or atomic numbers. Some heavy metals are either essential nutrients, such as iron, cobalt, and zinc, or relatively harmless, such as ruthenium, silver, and indium, but can be toxic in larger amounts or certain forms. Other heavy metals, such as arsenic, cadmium, mercury, and lead, are highly poisonous.

Heavy metals are natural components of the Earths crust and cannot be degraded or destroyed. They enter our bodies via food, drinking water, and air. Some heavy metals are essential to maintain the metabolism of the human body, but at higher concentrations, they can lead to poisoning. Heavy metal poisoning could result from drinking-water contamination, high ambient air concentrations near emission sources, or intake via the food chain. Heavy metals are dangerous because they tend to bioaccumulate, meaning they increase in concentration in a biological organism over time, compared to the chemicals concentration in the environment.

Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) . These heavy metals have harmful effects on human health, and exposure to these metals has been increased by industrial and anthropogenic activities and modern industrialization. Contamination of water and air by toxic metals is an environmental concern, and hundreds of millions of people are being affected around the world. Food contamination with heavy metals is another concern for human and animal health.

The toxic mechanism of heavy metals functions in similar pathways usually via reactive oxygen species (ROS) generation, enzyme inactivation, and suppression of the antioxidant defense. However, some of them cause toxicities in a particular pattern and bind selectively to specific macromolecules. Different toxic mechanisms of heavy metals increase our knowledge of their harmful effects on the body organs, leading to better management of animal and human poisonings.