Persistent organic pollutant

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Persistent organic pollutants (POPs) are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes.[1] Because of this, they have been observed to persist in the environment, to be capable of long-range transport, bioaccumulate in human and animal tissue, biomagnify in food chains,[1] and to have potential significant impacts on human health and the environment.

Many POPs are currently or were in the past used as pesticides. Others are used in industrial processes and in the production of a range of goods such as solvents, polyvinyl chloride, and pharmaceuticals.[1] There are a few natural sources of POPs, such as volcanic activity and vegetational fires,[2] but most POPs are created by humans in industrial processes, either intentionally or as byproducts.[1]

Public concern about contamination by POPs exists, because several have been identified as hormone disruptors which can alter normal function of endocrine and reproductive systems in humans and wildlife. There are many risks and effects of having these chemicals in our environment and none of them are a benefit to the Earth. After these pollutants are put into the environment, they are able to stay in the system for decades causing problems such as cancer, birth defects, learning disabilities, immunological, behavioral, neurological and reproductive discrepancies in human and other animal species.

The effect of POPs on human and environmental health was discussed, with intention to eliminate or severely restrict their production, by the international community at the Stockholm Convention on Persistent Organic Pollutants in 2001.

Compounds[edit]

Following the Second World War, scientists started to recognize that certain organic chemicals, deriving from both natural and anthropogenic origin are able to persist in the environment for exceptionally long periods of time by resisting photolytic, chemical, and biological degradation. The compounds characterized as POPs were observed to travel long-ranges in the air, and water before settling in sediment and soil. POP's were found to accumulate in food chains to levels that could result in serious harm to the environment, wildlife, and human health.[3][page needed] The groups of compounds that make up POPs are also classed as PBTs (Persistent, Bioaccumulative and Toxic) or TOMPs (Toxic Organic Micro Pollutants).

In May 1995, the United Nations Environment Programme Governing Council decided to investigate POPs, beginning with a list of the following twelve POPs, known as the 'dirty dozen':[4] aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and toxaphene.[1]

The 2001 Stockholm Convention’s Dirty Dozen are:[2][5][not in citation given] [6][full citation needed]

Since 2001, this list has generally been accepted to include such substances as carcinogenic polycyclic aromatic hydrocarbons (PAHs), certain brominated flame retardants, and some organometallic compounds such as tributyltin (TBT).[citation needed] Other additions to the initial 2001 Stockholm Convention list are as follows:[7][8]

Chemical properties[edit]

POPs are characterized by low water solubility, high lipid solubility, semi-volatility, high molecular mass, and stability. This allows for bioaccumulation in fatty tissues of living organisms and slow metabolism, which confers the compound’s persistence and accumulation in food chains. Halogenation with chlorine, bromine, and fluorine and aromatic rings resists against hydrolysis; the greater number of chlorine substitutions/functional groups, the greater resistance to biological and photolytic degradation is associated with POPs stability and persistence.

Long-range transport[edit]

POPs enter the gas phase under certain environmental temperatures and volatize from soils, vegetation, and water bodies 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 POP's have never been introduced such as the Antarctica, and the Arctic circle. POPs can be present as vapors in the atmosphere or bound to the surface of solid particles. 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.[9][10][11]

Bioaccumulation[edit]

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 biomagnification of POPs highlights another characteristic of the compounds, to increase in chemical concentration with increasing trophic level in food webs, against the thermodynamic gradient. The natural capacity for animals gastrointestinal tract concentrate ingested chemicals, along with poorly metabolized and hydrophobic nature of POPs makes such compounds highly susceptible to biomagnification.[10][11][12][13][14]

Additive and synergistic effects[edit]

It is hard to predict specific effects and the potential for health problems in experiments and studies performed in labs on POPs. Laboratory experiments most often test individual compounds. In the environment, however, organisms are exposed to a mixture of multiple POP compounds at the same time. It is difficult to test mixtures of different POPs in controlled experiments due to the cost and the lack of resources. To approximate the toxicity of a mixture of compounds to an organism in nature, the effects are assumed to be additive. That is, the effects of each individual compound are added together to approximate the overall magnitude of effects of the mixture as a whole. Sometimes, however, this is not alway the case. Mixtures of POPs, as with many other chemical mixtures, can sometimes produce synergistic effects. With synergistic effects, the toxicity of each compound is enhanced by the other compounds in the mixture. When put together, the effects can far exceed the approximated additive effects of the POP compound mixture. Thus, mixtures of POPs in nature can be more toxic to organisms than predicted in laboratory experiments.

Health effects[edit]

POP exposure may cause developmental defects, chronic illnesses, and death. Some are carcinogens per IARC, possibly including breast cancer.[1] Many POPs are capable of endocrine disruption, within the reproductive system, the central nervous system or the immune system. People and animals are exposed to POPs mostly through their diet, occupationally, or while growing in the womb.[1] For humans not exposed to POPs through accidental or occupational means, over 90% of exposure comes from animal product foods due to bioaccumulation in fat tissues and biomagnification up the food chain.In general, POP serum levels increase with age and tend to be higher in females than males.[12]

Studies have investigated the correlation between low level exposure of POPs and various diseases. In order to assess disease risk due to POPs in a particular location, government agencies may produce a human health risk assessment which takes into account the pollutants' bioavailability and their dose-response relationships.[15]

Endocrine disruption[edit]

The majority of POPs are known to disrupt normal functioning of the endocrine system, for example all of the dirty dozen are endocrine disruptors. Low level exposure to POPs during critical developmental periods of fetus, newborn and child can have a lasting effect throughout its lifespan. A 2002 study[citation needed] synthesizes data on endocrine disruption and health complications from exposure to POPs during critical developmental stages in an organism's lifespan. The study aimed to answer the question whether or not chronic, low level exposure to POPs can have a health impact on the endocrine system and development of organisms from different species. The study found that exposure of POPs during a critical developmental time frame can produce a permanent changes in the organisms path of development. Exposure of POPs during non-critical developmental time frames may not lead to detectable diseases and health complications later in their life. In wildlife, the critical development time frames are in utero, in ovo, and during reproductive periods. In humans, the critical development timeframe is during fetal development.

Reproductive system[edit]

The same study in 2002[citation needed] with evidence of a link from POPs to endocrine disruption also linked low dose exposure of POPs to reproductive health effects. The study stated that POP exposure can lead to negative health effects especially in the male reproductive system, such as decreased sperm quality and quantity, altered sex ratio and early puberty onset. For females exposed to POPs, altered reproductive tissues and pregnancy outcomes as well as endometriosis have been reported.

Exposure during pregnancy[edit]

POP exposure during pregnancy is of particular concern to the developing fetus.

Transport across the placenta[edit]

A study about the transfer of POPs (14 organochlorine pesticides, 7 polychlorinated biphenyls and 14 polybromodiphenyl ether (PBDEs)) from Spanish mothers to their unborn fetus found that POP concentrations in serum from the mother were higher than from the umbilical cord and 50 placentas.[16] Because transfer of the POPs from mother to fetus did not correspond with passive lipid-associated diffusion, authors suggested that POPs are actively transported across the placenta.[16]

Gestational weight gain and newborn head circumference[edit]

A Greek study from 2014 investigated the link between maternal weight gain during pregnancy, their PCB-exposure level and PCB level in their newborn infants, their birth weight, gestational age, and head circumference. The birth weight and head circumference of the infants was the lower, the higher POP levels during prenatal development had been, but only if mothers had either excessive or inadequate weight gain during pregnancy. No correlation between POP exposure and gestational age was found.[17] A 2013 case-control study conducted 2009 in Indian mothers and their offspring showed prenatal exposure of two types of organochlorine pesticides (HCH, DDT and DDE) impaired the growth of the fetus, reduced the birthweight, length, head circumference and chest circumference.[18][19]

Cardiovascular disease and cancer[edit]

POPs are lipophilic environmental toxins. They are often found in lipoproteins of organisms. A study published in 2014[citation needed] found an association between the concentration of POPs in lipoproteins and the occurrence of cardiovascular disease and various cancers in human beings. The higher the concentration of POPs found in lipoproteins, the higher the occurrence of cardiovascular disease and cancer. Highly chlorinated polychlorinated biphenyls are specifically found in high concentrations in lipoproteins. Cardiovascular disease is shown to be more associated with higher concentrations of POPs in high density lipoproteins and cancer is shown to be more associated with higher concentrations of POPs in low density lipoproteins and very low density lipoproteins.[20]

Obesity[edit]

There have been many recent studies assessing the connection between serum POP levels in individuals and instances of obesity. A study released in 2011[citation needed] found correlations between different POPs and obesity occurrence in individuals tested. The statistically significant findings from the study show that there is actually a negative correlation between various PCB congener serum levels and obesity in individuals tested. The study also showed a positive correlation between beta-hexachlorocyclohexane and various dioxin serum levels and obesity in individuals tested. Obesity was determined using the Body Mass Index (BMI). One proposed explanation in the study is that PCBs are very lipophilic, therefore they are easily stored and captured in the fat deposits in human beings. Obese individuals have higher amounts of fat deposits in their body, and thus more PCBs could be captured in the fat deposits leading to less PCBs circulating in blood serum. The study provides evidence demonstrating that the correlation between POP serum levels and obesity occurrence is more complicated than previously expected. The same study also noted a strong positive correlation between serum POP levels and age in all individuals in the experiment.[21]

Diabetes[edit]

A study from an article published in 2006[citation needed] revealed a positive correlation between POP serum levels and type II diabetes in individuals, after other[which?] variables were adjusted for. The correlation proved stronger in younger, Mexican-American, and obese individuals. Individuals exposed to low doses of POPs throughout their lifetime had a higher chance for developing diabetes than individuals exposed to high concentrations of POPs for a short amount of time.[22]

Stockholm Convention on Persistent Organic Pollutants[edit]

State parties to the 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 2014, the United States of America 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.

POPs in urban areas and indoor environments[edit]

Traditionally it was thought that human exposure to POPs occurred primarily through food, however indoor pollution patterns that characterize certain POPs have challenged this notion. Recent studies of indoor dust and air have implicated indoor environments as sources for human exposure via inhalation and ingestion. Furthermore significant indoor POP pollution seems to be the only solution to the route of exposure in countries where the trend is a high proportion of indoor life. Various studies have shown that in general indoor (air and dust) POP levels exceed outdoor (air and soil) POP concentrations. Human exposure to POPs due to indoor environment contamination is a new development in POP research, yet highly significant route of inquiry.[3][23]

See also[edit]

References[edit]

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