A poisonous substance is any chemical or material that can cause harm to living organisms. Toxicants, ARSENIC, toxins, and biocides are all examples of poisonous substances. Toxicants are chemicals that are toxic to living organisms, while ARSENIC is a naturally occurring element that is highly toxic in its inorganic form. Toxins are poisonous substances produced by living organisms, such as bacteria, fungi, and plants. Biocides are chemicals that are used to kill living organisms, such as bacteria, fungi, and viruses.