Toxic substances are chemicals or mixtures of chemicals, either synthetic or natural, that are poisonous to humans, plants, or animals under expected conditions of use and exposure. These substances include pesticides, some industrial chemicals, drugs, hazardous wastes, and radioactive materials. Toxic substances can cause immediate poisoning, behavioural and other nervous system disorders, and other illnesses. Over time, exposure can lead to chronic ailments and disability and can affect the growth of cells, sometimes causing cancer, genetic damage, or mutation Owing to their widespread distribution, use, and disposal, toxic substances pervade the environment.
Toxic effects depend not only on the composition and basic properties of a substance or mixture but also on the dosage, route and conditions of exposure, susceptibility of the organism exposed and other factors.
Technological and social developments have multiplied the hazards to which the population, especially in urban areas, is exposed, such as the harmful effects of chemicals on the central nervous system. More than one-third of the industrial chemicals listed in the American "table of threshold limit values" affect the nervous system at the threshold concentration, or at concentrations twice to ten times greater than the recommended level. Many industrial chemicals, such as carbon disulphide, mercury, manganese, tin, lead compounds, trichloroethylene, decaborane, and carbon monoxide have been shown to be selective neurotoxic agents producing neurological and behavioural disturbances. The critical period of vulnerability is during foetal and immediate postnatal life, and infancy; at these stages of cerebral maturation such compounds can produce serious and irreversible damage. The dosage and duration of exposure to chemical agents is also very important. For example, lead poisoning in children can produce irreparable brain damage with permanent mental retardation. Severe exposure often occurs in children from slum areas of industrialized cities and leads to chronic impairment of the nervous system.