persistent organic pollutant persistent organic pollutant persistent organic pollutants (pops) are organic compounds that are resistant to degradation through chemical, biological, and photolytic processes. they are toxic chemicals that adversely affect human health and the environment around the world. because they can be transported by wind and water, most pops generated in one country can and do affect people and wildlife far from where they are used and released. part of a series onpollutionair pollution from a factory air acid rain air quality index atmospheric dispersion modeling chlorofluorocarbon exhaust gas haze indoor air internal combustion engine global dimming global distillation ozone depletion particulates persistent organic pollutant smog aerosol soot volatile organic compound biological biological hazard genetic introduced species (invasive species) digital information electromagnetic light ecological overillumination radio spectrum natural ozone radium and radon in 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intention to eliminate or severely restrict their production, by the international community at the stockholm convention on persistent organic pollutants in 2001. most pops are pesticides or insecticides, and some are also solvents, pharmaceuticals, and industrial chemicals. although some pops arise naturally (e.g. from volcanoes), most are man-made. the "dirty dozen" pops identified by the stockholm convention include aldrin, chlordane, dieldrin, endrin, heptachlor, hcb, mirex, toxaphene, pcbs, ddt, dioxins, and polychlorinated dibenzofurans. consequences of persistence pops typically are halogenated organic compounds (see lists below) and as such exhibit high lipid solubility. for this reason, they bioaccumulate in fatty tissues. halogenated compounds also exhibit great stability reflecting the nonreactivity of c-cl bonds toward hydrolysis and photolytic degradation. the stability and lipophilicity of organic compounds often correlates with their halogen content, thus polyhalogenated organic compounds are of particular concern. they exert their negative effects on the environment through two processes, long range transport, which allows them to travel far from their source, and bioaccumulation, which reconcentrates these chemical compounds to potentially dangerous levels. compounds that make up pops are also classed as pbts (persistent, bioaccumulative and toxic) or tomps (toxic organic micro pollutants). long-range transport pops enter the gas phase under certain environmental temperatures and volatilize from soils, vegetation, and bodies of water into the atmosphere, resisting breakdown reactions in the air, to travel long distances before being re-deposited. this results in accumulation of pops in areas far from where they were used or emitted, specifically environments where pops have never been introduced such as antarctica, and the arctic circle. pops can be present as vapors in the atmosphere or bound to the surface of solid particles (aerosols). a determining factor for the long-range transport is the fraction of a pop that is adsorbed on aerosols. in adsorbed form it is – as opposed to the gas phase – protected from photo-oxidation, i.e. direct photolysis as well as oxidation by oh radicals or ozone. pops have low solubility in water but are easily captured by solid particles, and are soluble in organic fluids (oils, fats, and liquid fuels). pops are not easily degraded in the environment due to their stability and low decomposition rates. due to this capacity for long-range transport, pop environmental contamination is extensive, even in areas where pops have never been used, and will remain in these environments years after restrictions implemented due to their resistance to degradation. bioaccumulation bioaccumulation of pops is typically associated with the compounds high lipid solubility and ability to accumulate in the fatty tissues of living organisms for long periods of time. persistent chemicals tend to have higher concentrations and are eliminated more slowly. dietary accumulation or bioaccumulation is another hallmark characteristic of pops, as pops move up the food chain, they increase in concentration as they are processed and metabolized in certain tissues of organisms. the natural capacity for animals gastrointestinal tract to concentrate ingested chemicals, along with poorly metabolized and hydrophobic nature of pops, makes such compounds highly susceptible to bioaccumulation. thus pops not only persist in the environment, but also as they are taken in by animals they bioaccumulate, increasing their concentration and toxicity in the environment. this increase in concentration is called biomagnification, which is where organisms higher up in the food chain have a greater accumulation of pops. bioaccumulation and long-range transport are the reason why pops can accumulate in organisms like whales, even in remote areas like antarctica. stockholm convention on persistent organic pollutants state parties to the stockholm convention on persistent organic pollutants main article: stockholm convention on persistent organic pollutants the stockholm convention was adopted and put into practice by the united nations environment programme (unep) on may 22, 2001. the unep decided that pop regulation needed to be addressed globally for the future. the purpose statement of the agreement is "to protect human health and the environment from persistent organic pollutants." as of 2014, there are 179 countries in compliance with the stockholm convention. the convention and its participants have recognized the potential human and environmental toxicity of pops. they recognize that pops have the potential for long range transport and bioaccumulation and biomagnification. the convention seeks to study and then judge whether or not a number of chemicals that have been developed with advances in technology and science can be categorized as pops or not. the initial meeting in 2001 made a preliminary list, termed the "dirty dozen", of chemicals that are classified as pops. as of 2022, the united states has signed the stockholm convention but has not ratified it. there are a handful of other countries that have not ratified the convention but most countries in the world have ratified the convention. compounds on the stockholm convention list in may 1995, the unep governing council investigated pops. initially the convention recognized only twelve pops for their adverse effects on human health and the environment, placing a global ban on these particularly harmful and toxic compounds and requiring its parties to take measures to eliminate or reduce the release of pops in the environment. aldrin, an insecticide used in soils to kill termites, grasshoppers, western corn rootworm, and others, is also known to kill birds, fish, and humans. humans are primarily exposed to aldrin through dairy products and animal meats. chlordane, an insecticide used to control termites and on a range of agricultural crops, is known to be lethal in various species of birds, including mallard ducks, bobwhite quail, and pink shrimp; it is a chemical that remains in the soil with a reported half-life of one year. chlordane has been postulated to affect the human immune system and is classified as a possible human carcinogen. chlordane air pollution is believed the primary route of human exposure. dieldrin, a pesticide used to control termites, textile pests, insect-borne diseases and insects living in agricultural soils. in soil and insects, aldrin can be oxidized, resulting in rapid conversion to dieldrin. dieldrin's half-life is approximately five years. dieldrin is highly toxic to fish and other aquatic animals, particularly frogs, whose embryos can develop spinal deformities after exposure to low levels. dieldrin has been linked to parkinson's disease, breast cancer, and classified as immunotoxic, neurotoxic, with endocrine disrupting capacity. dieldrin residues have been found in air, water, soil, fish, birds, and mammals. human exposure to dieldrin primarily derives from food. endrin, an insecticide sprayed on the leaves of crops, and used to control rodents. animals can metabolize endrin, so fatty tissue accumulation is not an issue, however the chemical has a long half-life in soil for up to 12 years. endrin is highly toxic to aquatic animals and humans as a neurotoxin. human exposure results primarily through food. heptachlor, a pesticide primarily used to kill soil insects and termites, along with cotton insects, grasshoppers, other crop pests, and malaria-carrying mosquitoes. heptachlor, even at very low doses has been associated with the decline of several wild bird populations – canada geese and american kestrels. in laboratory tests have shown high-dose heptachlor as lethal, with adverse behavioral changes and reduced reproductive success at low-doses, and is classified as a possible human carcinogen. human exposure primarily results from food. hexachlorobenzene (hcb) was first introduced in 1945–59 to treat seeds because it can kill fungi on food crops. hcb-treated seed grain consumption is associated with photosensitive skin lesions, colic, debilitation, and a metabolic disorder called porphyria turcica, which can be lethal. mothers who pass hcb to their infants through the placenta and breast milk had limited reproductive success including infant death. human exposure is primarily from food. mirex, an insecticide used against ants and termites or as a flame retardant in plastics, rubber, and electrical goods. mirex is one of the most stable and persistent pesticides, with a half-life of up to 10 years. mirex is toxic to several plant, fish and crustacean species, with suggested carcinogenic capacity in humans. humans are exposed primarily through animal meat, fish, and wild game. toxaphene, an insecticide used on cotton, cereal, grain, fruits, nuts, and vegetables, as well as for tick and mite control in livestock. widespread toxaphene use in the us and chemical persistence, with a half-life of up to 12 years in soil, results in residual toxaphene in the environment. toxaphene is highly toxic to fish, inducing dramatic weight loss and reduced egg viability. human exposure primarily results from food. while human toxicity to direct toxaphene exposure is low, the compound is classified as a possible human carcinogen. polychlorinated biphenyls (pcbs), used as heat exchange fluids, in electrical transformers, and capacitors, and as additives in paint, carbonless copy paper, and plastics. persistence varies with degree of halogenation, an estimated half-life of 10 years. pcbs are toxic to fish at high doses, and associated with spawning failure at low doses. human exposure occurs through food, and is associated with reproductive failure and immune suppression. immediate effects of pcb exposure include pigmentation of nails and mucous membranes and swelling of the eyelids, along with fatigue, nausea, and vomiting. effects are transgenerational, as the chemical can persist in a mother's body for up to 7 years, resulting in developmental delays and behavioral problems in her children. food contamination has led to large scale pcb exposure. dichlorodiphenyltrichloroethane (ddt) is probably the most infamous pop. it was widely used as insecticide during wwii to protect against malaria and typhus. after the war, ddt was used as an agricultural insecticide. in 1962, the american biologist rachel carson published silent spring, describing the impact of ddt spraying on the us environment and human health. ddt's persistence in the soil for up to 10–15 years after application has resulted in widespread and persistent ddt residues throughout the world including the arctic, even though it has been banned or severely restricted in most of the world. ddt is toxic to many organisms including birds where it is detrimental to reproduction due to eggshell thinning. ddt can be detected in foods from all over the world and food-borne ddt remains the greatest source of