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.