Heavy metals are elements characterized by high atomic weights and widespread distribution throughout the Earth’s crust. This group includes iron, manganese, cobalt, and zinc, which are essential for the proper functioning of organisms. However, other metals such as mercury, cadmium, and nickel have no beneficial activity, and their presence in the body has been associated with various toxic effects. The ubiquitous presence of these toxic elements in the environment is largely due to anthropogenic activities (industry, mining), which promote their release into the air, water, and soil. Additionally, the use of these metals in various products (electrical and electronic devices, automobiles, ovens, light bulbs) or jewelry has led to an increased risk of exposure. Unfortunately, inadequate disposal and protection practices increase the risk of contact. Toxic heavy metals can enter the body through inhalation, skin absorption, or ingestion and induce symptoms such as ataxia, allergies, reduced motor function, dyspnea, multiple organ failure, and even death, in addition to having the potential to cause harm to future generations. Therefore, the aim of this chapter is to describe the effects of heavy metal poisoning and its impact on the development of allergic events. Cadmium, mercury, and nickel were selected as target elements due to their concomitant involvement in poisoning and allergy.