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Organic compounds that are resistant to environmental degradation From Wikipedia, the free encyclopedia
Persistent organic pollutants (POPs) are organic compounds that are resistant to degradation through chemical, biological, and photolytic processes.[1] They are toxic and adversely affect human health and the environment around the world.[1] 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.
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.
Most POPs are pesticides or insecticides, and some are also solvents, pharmaceuticals, and industrial chemicals.[1] Although some POPs arise naturally (e.g. from volcanoes), most are man-made.[2] The "dirty dozen" POPs identified by the Stockholm Convention include aldrin, chlordane, dieldrin, endrin, heptachlor, HCB, mirex, toxaphene, PCBs, DDT, dioxins, and polychlorinated dibenzofurans. However, there have since been many new POPs added (e.g. PFOS).
POPs typically are halogenated organic compounds (see lists below) and as such exhibit high lipid solubility. For this reason, they bioaccumulate in fatty tissues.[3][4][5][6] 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.[1] 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.[7] Compounds that make up POPs are also classed as PBTs (persistent, bioaccumulative and toxic) or TOMPs (toxic organic micro pollutants).[8]
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.[9] 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.[10] 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.[11][12]
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.[1][13][14]
Bioaccumulation of POPs is typically associated with the compounds high lipid solubility and ability to accumulate in the fatty tissues of living organisms including human tissues for long periods of time.[13][15] 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.[16] 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.[9][17] This increase in concentration is called biomagnification, which is where organisms higher up in the food chain have a greater accumulation of POPs.[18] Bioaccumulation and long-range transport are the reason why POPs can accumulate in organisms like whales, even in remote areas like Antarctica.[19]
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 2024, there are 185 countries plus the European Union have ratified the Stockholm Convention.[20] 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. The initial meeting in 2001 made a preliminary list, termed the "dirty dozen", of chemicals that are classified as POPs.[21] As of 2024, 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.[20]
 | This section needs additional citations for verification. (July 2024) |
In May 1995, the UNEP Governing Council investigated POPs.[22] 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.[2][21][23]
Since 2001, this list has been expanded to include some polycyclic aromatic hydrocarbons (PAHs), brominated flame retardants, and other compounds. Additions to the initial 2001 Stockholm Convention list are the following POPs:[24][21]
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.[26] 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 bioaccumulate through the food chain. In general, POP serum levels increase with age and tend to be higher in females than males.[15]
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.[27]
The majority of POPs are known to disrupt normal functioning of the endocrine system. Low level exposure to POPs during critical developmental periods of fetus, newborn and child can have a lasting effect throughout their lifespan. A 2002 study[28] summarizes 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.[28]
The same study in 2002[28] 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.[2]
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 lower the birth weight and head circumference of the infants was, 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.[29] 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 birth weight, length, head circumference and chest circumference.[30][31]
Evaluation of the effects of POPs on health is very challenging in the laboratory setting. For example, for organisms exposed to a mixture of POPs, the effects are assumed to be additive.[35] Mixtures of POPs can in principle produce synergistic effects. With synergistic effects, the toxicity of each compound is enhanced (or depressed) by the presence of other compounds in the mixture. When put together, the effects can far exceed the approximated additive effects of the POP compound mixture.[7]
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 a major sources for human exposure via inhalation and ingestion.[36] Furthermore, significant indoor POP pollution must be a major route of human POP exposure, considering the modern trend in spending larger proportions of life indoors. Several studies have shown that indoor (air and dust) POP levels to exceed outdoor (air and soil) POP concentrations.[35]
In 2022, levels of at least four perfluoroalkyl acids (PFAAs) in rain water worldwide greatly exceeded the EPA's lifetime drinking water health advisories as well as comparable Danish, Dutch, and European Union safety standards, leading to the conclusion that "the global spread of these four PFAAs in the atmosphere has led to the planetary boundary for chemical pollution being exceeded".[37]
It had been thought that PFAAs would eventually end up in the oceans, where they would be diluted over decades, but a field study published in 2021 by researchers at Stockholm University found that they are often transferred from water to air when waves reach land, are a significant source of air pollution, and eventually get into rain. The researchers concluded that pollution may impact large areas.[38][39][40]
In 2024, a worldwide study of 45,000 groundwater samples found that 31% of samples contained levels of PFAS that were harmful to human health; these samples were taken from areas not near any obvious source of contamination.[41]
Soil is also contaminated and the chemicals have been found in remote areas such as Antarctica.[42] Soil contamination can result in higher levels of PFAs found in foods such as white rice, coffee, and animals reared on contaminated ground.[43][44][45]Current studies aimed at minimizing POPs in the environment are investigating their behavior in photocatalytic oxidation reactions.[46] POPs that are found in humans and in aquatic environments the most are the main subjects of these experiments. Aromatic and aliphatic degradation products have been identified in these reactions. Photochemical degradation is negligible compared to photocatalytic degradation.[2] A method of removal of POPs from marine environments that has been explored is adsorption. It occurs when an absorbable solute comes into contact with a solid with a porous surface structure. This technique was investigated by Mohamed Nageeb Rashed of Aswan University, Egypt.[47] Current efforts are more focused on banning the use and production of POPs worldwide rather than removal of POPs.[15]
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