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Poisons - in biology, poisons are substances that, when absorbed by an organism in sufficient quantities, usually cause interference in the organism through chemical reactions or other activities at the molecular level.

In medicine (especially veterinary) and zoology, poisons are usually distinguished from toxins and venoms. Toxins are produced by natural organisms, and the venom is injected by biting or ST injury (this is exclusive to animals). The difference between venom and other poisons lies in the delivery method.

The use of toxic substances in industry, agriculture and other sectors is not due to their toxicity. Most toxic industrial compounds have material safety data sheets that are classified as hazardous. Hazardous substances are subject to extensive control in overlapping areas related to occupational safety and health, public health, drinking water quality standards, air pollution and environmental protection production, procurement and use. Due to the mechanism of molecular diffusion, many toxic compounds spread rapidly to biological tissues, air, water or soil on a molecular scale. According to the entropy principle, unless there is a specific chelating agent or microfiltration method, chemical contamination is usually expensive or irreversible. Chelating agents usually have a wider range of action than acute targets, so their intake requires careful medical or veterinary supervision.

Pesticides are a class of substances whose main purpose is to be toxic to various insects and other animals (such as rats and cockroaches) that are considered pests. Natural pesticides have been used for this purpose for thousands of years (for example, concentrated salt is toxic to many). The bioaccumulation of chemically prepared agricultural insecticides is a problem worthy of attention for many species, especially birds, whose main food source is insects. Selective toxicity, controlled application and controlled biodegradation are major challenges in the development of herbicides and pesticides and in the field of chemical engineering as a whole, because all life forms on earth have potential biochemical properties. Organisms that are exceptionally good at adapting to the environment are classified as extremophiles, and in most cases they exhibit very different sensitivities.

Toxic substances entering the food chain, whether from industrial, agricultural or natural sources, may not be immediately toxic to the first organisms that ingest the toxin, but will be further concentrated in predators in the food chain, especially predators and omnivores. Especially with regard to fat soluble poisons, they tend to be stored in biological tissues rather than discharged from urine or other water-based wastewater.

Two common cases of acute natural poisoning are theobromine poisoning in cats and dogs and mushroom poisoning in humans. Dogs and cats are not natural herbivores, but the chemical defenses of cocoa can still be deadly. Many omnivores, including humans, easily eat edible fungi, so many fungi have evolved into decisive inedibility, in which case it is a direct defense.

In addition to food, many poisons can easily enter the human body through the skin and lungs. In addition to its corrosiveness, hydrofluoric acid is also a notorious contact poison. Naturally occurring acid gases are notorious, quick acting atmospheric poisons (released by volcanic activity or drilling equipment). Irritants from plant contact, such as ivy or toxic oak, are usually classified as allergens rather than toxic substances. The role of allergens is not to poison itself, but to make the body's natural defense against itself. Poisons can also enter the body through teeth (in contested dental misconduct), defective medical implants, or injections (which are the basis of lethal injections in the death penalty).

In 2013, there were 3.3 million accidental human poisoning incidents. Of the 120000 deaths in 1990, 98000 were worldwide. In modern society, suspicious death cases have attracted the attention of the coroner's office and forensic investigators. Arsenic is a naturally occurring environmental poison. Its human industrial concentrate was nicknamed inheritance powder. In medieval Europe, it was common for monarchs to employ private food tasters in the dawn of pharmacists to prevent Royal assassinations.

Since the isolation of natural radium by Marie and Pierre Curie in 1898 and the subsequent nuclear physics and technology, more and more attention has been paid to radiotoxicants. These are related to ionizing radiation, which is a completely different toxic model from chemical active poisons. In mammals, chemical poisons are usually passed from mother to offspring through placenta during pregnancy or from breast milk during lactation. In contrast, radiation damage can be passed on from mother or father to offspring through gene mutation, and can be passed on to offspring again if it is not the direct cause of death or sterility in abortion or childhood. Atmospheric RA is a kind of natural radiotoxicant with increasing influence, because human beings are able to accommodate the dangerous concentration of RA by transferring their life style from hunter gatherer to more and more closed structure through cave dwelling. The poisoning of Alexander Litvinenko in 2006 was a novel use of radioactive assassination, presumably to avoid routine investigations of chemical poisons.

Poisons widely distributed in the environment are called pollution. These are usually of human origin, but pollution may also include harmful biological processes, such as toxic red tides, or rapid changes in the natural chemical environment attributable to invasive species, which are toxic or harmful to prior ecosystems, especially if they are linked to human economic values or established industries, such as shellfish fishing.

The science of ecology and environmental resource management studies the environmental life cycle of toxic compounds and their complex, dispersed and highly correlated impacts.