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In biology , poisons are substances that cause disturbances in organisms, usually by chemical reaction or other activity on the molecular scale, when an organism absorbs a sufficient quantity .The fields of medicine (particularly veterinary) and zoology often distinguish a poison from a toxin, and from a venom. Toxins are poisons produced by organisms in nature , and venoms are toxins injected by a bite or sting (this is exclusive to animals ). The difference between venom and other poisons is the delivery method. Industry , agriculture , and other sectors employ poisonous substances for reasons other than their toxicity. Most poisonous industrial compounds have associated material safety data sheets and are classed as hazardous substances. Hazardous substances are subject to extensive regulation on production, procurement and use in overlapping domains of occupational safety and health , public health , drinking water quality standards, air pollution and environmental protection. Due to the mechanics of molecular diffusion, many poisonous compounds rapidly diffuse into biological tissues, air , water , or soil on a molecular scale. By the principle of entropy, chemical contamination is typically costly or infeasible to reverse, unless specific chelating agents or micro-filtration processes are available. Chelating agents are often broader in scope than the acute target, and therefore their ingestion necessitates careful medical or veterinarian supervision. Pesticides are one group of substances whose toxicity to various insects and other animals deemed to be pests (e.g., rats and cockroaches) is their prime purpose .