Persistent organic pollutants (POPs) are chemicals that are hard to break down through chemical, biological, or sunlight processes. These substances are harmful and can damage human health and the environment worldwide. Because they can travel through wind and water, many POPs created in one country can harm people and animals far away from where they are used or released.

In 2001, countries around the world discussed the effects of POPs on human and environmental health. They agreed to try to stop or greatly limit the production of these harmful chemicals through the Stockholm Convention on Persistent Organic Pollutants.

Most POPs are pesticides or insecticides. Some are also solvents, medicines, or industrial chemicals. While a few POPs occur naturally (such as from volcanoes), most are created by humans. The Stockholm Convention identified 12 key POPs, called the "dirty dozen," which include aldrin, chlordane, dieldrin, endrin, heptachlor, HCB, mirex, toxaphene, PCBs, DDT, dioxins, and polychlorinated dibenzofurans. Since then, many new POPs, such as PFOS, have also been added to the list.

Consequences of persistence

Persistent Organic Pollutants (POPs) are often chemicals that contain certain elements, such as chlorine, and dissolve easily in fats. Because of this, these chemicals build up in the fatty tissues of living things. These chemicals are also very stable, meaning they do not break down easily when exposed to water or sunlight. The more chlorine or other elements a chemical has, the more likely it is to stay in the environment for a long time. These chemicals harm the environment in two ways: they can travel long distances from where they were first used, and they can build up in the bodies of animals and humans to dangerous levels. These chemicals are also called PBTs (Persistent, Bioaccumulative, and Toxic) or TOMPs (Toxic Organic Micro Pollutants).

POPs can turn into gases when the temperature is right and rise from soil, plants, and water into the air. Once in the air, they resist breaking down and can travel far before falling back to Earth. This causes these chemicals to build up in places far from where they were first used, such as Antarctica and the Arctic, where they were never introduced. POPs can exist as gases in the air or stick to tiny particles in the air. Whether they are in gas form or attached to particles affects how far they can travel. When attached to particles, they are protected from breaking down by sunlight or chemical reactions in the air.

POPs do not dissolve easily in water but can stick to solid particles and dissolve in oils, fats, and fuels. These chemicals are hard to break down in the environment because they are very stable. Because they can travel long distances, POPs can pollute the environment in places where they were never used, and they will stay in these areas for many years even after rules are put in place to stop their use.

POPs build up in the fatty tissues of living things because they dissolve easily in fats. These chemicals stay in the body for a long time and are not easily removed. As POPs move up the food chain, their concentration increases because they are not fully broken down by animals. The way animals’ digestive systems work and the fact that POPs are hard to break down and do not mix with water make them likely to build up in animals. This process, called biomagnification, means that animals higher up in the food chain, such as whales, can have much higher levels of POPs in their bodies. This is why POPs can be found in animals living in remote areas like Antarctica, even though these chemicals were never used there.

Stockholm Convention on Persistent Organic Pollutants

The Stockholm Convention was created and started to be used by the United Nations Environment Programme (UNEP) on May 22, 2001. UNEP decided that regulations for persistent organic pollutants (POPs) needed to be addressed globally. The goal of the agreement is to protect human health and the environment from POPs. As of 2024, 185 countries and the European Union have ratified the Stockholm Convention. The convention and its participants recognize that POPs can cause harm to humans and the environment. They also recognize that POPs can travel long distances and build up in the environment and in living organisms. The convention studies and evaluates whether certain chemicals developed through advances in technology and science can be classified as POPs. The first meeting in 2001 created a list called the "dirty dozen" of chemicals classified as POPs. As of 2024, the United States has signed the Stockholm Convention but has not ratified it. A few other countries have not ratified the convention, but most countries in the world have done so.

In May 1995, the UNEP Governing Council looked into POPs. Initially, the Convention recognized only twelve POPs for their harmful effects on human health and the environment. It banned these chemicals globally and required its participants to take steps to eliminate or reduce their release into the environment.

• Aldrin is an insecticide used in soils to kill termites, grasshoppers, Western corn rootworm, and others. It also harms birds, fish, and humans. People are mainly exposed through dairy products and animal meats.
• Chlordane is an insecticide used to control termites and on agricultural crops. It harms birds, including mallard ducks, bobwhite quail, and pink shrimp. It stays in soil for about one year. It may affect the human immune system and is considered a possible human carcinogen. Air pollution is the main way humans are exposed.
• Dieldrin is a pesticide used to control termites, textile pests, insect-borne diseases, and insects in agricultural soils. It has a half-life of about five years. It is highly toxic to fish and aquatic animals, such as frogs, which can develop spinal deformities after exposure. It is linked to Parkinson’s disease, breast cancer, and classified as harmful to the immune system, nervous system, and endocrine system. Dieldrin has been found in air, water, soil, fish, birds, and mammals. People are mainly exposed through food.
• Endrin is an insecticide sprayed on crop leaves to control rodents. Animals can break down endrin, so it does not build up in fatty tissues. However, it stays in soil for up to 12 years. It is highly toxic to aquatic animals and humans as a neurotoxin. People are mainly exposed through food.
• Heptachlor is a pesticide used to kill soil insects, termites, cotton insects, grasshoppers, crop pests, and mosquitoes that carry malaria. Even at low doses, it harms wild bird populations, such as Canada geese and American kestrels. High doses are lethal, and low doses cause behavioral changes and reduced reproduction. It is considered a possible human carcinogen. People are mainly exposed through food.
• Hexachlorobenzene (HCB) was first used between 1945 and 1959 to treat seeds by killing fungi on food crops. Eating HCB-treated seeds can cause skin lesions, colic, weakness, and a metabolic disorder called porphyria turcica, which can be deadly. Mothers who pass HCB to their infants through the placenta or breast milk may have limited reproductive success, including infant death. People are mainly exposed through food.
• Mirex is an insecticide used against ants, termites, or as a flame retardant in plastics, rubber, and electrical goods. It is one of the most stable pesticides, staying in the environment for up to 10 years. It is toxic to plants, fish, and crustaceans and may cause cancer in humans. People are mainly exposed through animal meat, fish, and wild game.
• Toxaphene is an insecticide used on cotton, cereal, grain, fruits, nuts, vegetables, and for tick and mite control in livestock. Its widespread use in the United States and its persistence in soil for up to 12 years have left residues in the environment. It is highly toxic to fish, causing weight loss and reduced egg viability. People are mainly exposed through food. While direct exposure to toxaphene is not highly toxic to humans, it is considered

Health effects

Exposure to POPs can lead to developmental problems, long-term health issues, and even death. Some POPs are classified as carcinogens by IARC, which may include breast cancer. Many POPs can interfere with the endocrine system, which includes the reproductive system, central nervous system, and immune system. People and animals are most often exposed to POPs through their diet, occupational activities, or during fetal development. For humans not exposed through work or accidents, more than 90% of POP exposure comes from eating animal products. This is because POPs build up in fat tissues and move up the food chain through bioaccumulation. Generally, POP levels in the blood increase with age and are often higher in females than in males.

Studies have looked at how low levels of POP exposure are linked to different diseases. To evaluate health risks from POPs in a specific area, government agencies may create a human health risk assessment. This assessment considers how easily the pollutants can be absorbed by the body and how their effects change with different doses.

Most POPs are known to disrupt the normal function of the endocrine system. Low-level exposure to POPs during important developmental stages, such as in the fetus, newborn, or child, can have long-lasting effects throughout a person's life. A 2002 study reviewed data on how POPs disrupt the endocrine system and cause health problems during critical developmental periods. The study examined whether long-term, low-level exposure to POPs affects the endocrine system and development in different species. It found that exposure during critical developmental times can cause permanent changes in an organism's development. Exposure during non-critical times may not lead to noticeable health problems later in life. In wildlife, critical developmental periods include being in the womb, inside an egg, and during reproductive times. In humans, the critical period is during fetal development.

The same 2002 study also found a connection between low-dose POP exposure and reproductive health issues. It noted that POP exposure can cause negative health effects, especially in the male reproductive system, such as lower sperm quality and quantity, changes in the sex ratio of offspring, and earlier onset of puberty. In females, POP exposure has been linked to changes in reproductive tissues, pregnancy outcomes, and conditions like endometriosis.

A 2014 study in Greece looked at how maternal weight gain during pregnancy, levels of PCB exposure, and PCB levels in newborns relate to birth weight, gestational age, and head size. The study found that lower birth weight and head size in infants were linked to higher POP levels during prenatal development, but only when mothers had either too much or too little weight gain during pregnancy. No connection was found between POP exposure and gestational age. A 2013 study in India found that exposure to two types of organochlorine pesticides (HCH, DDT, and DDE) during pregnancy reduced fetal growth, birth weight, length, head size, and chest size in offspring.

Evaluating the health effects of POPs in laboratory settings is very difficult. For example, when organisms are exposed to a mixture of POPs, the effects are often assumed to be additive. Mixtures of POPs can sometimes create stronger effects together than they would individually. In some cases, the presence of other compounds in the mixture can increase (or decrease) the toxicity of each POP. When combined, these effects can be much greater than expected from the mixture alone.

In urban areas and indoor environments

Traditionally, it was believed that people were mainly exposed to POPs through food. However, recent studies of indoor dust and air have shown that indoor environments are a major source of exposure through breathing and eating. Spending more time indoors has increased the impact of indoor POP pollution. Studies have found that POP levels in indoor air and dust are often higher than those in outdoor air and soil.

In rainwater

In 2022, levels of at least four perfluoroalkyl acids (PFAAs) in rainwater worldwide were much higher than the EPA's lifetime drinking water health advisories and similar safety standards from Denmark, the Netherlands, and the European Union. Scientists concluded that the global spread of these four PFAAs in the atmosphere has gone beyond safe limits for chemical pollution. The most common PFAS found in the environment is Trifluoroacetic acid (TFA). It is found everywhere in the environment, especially in water ecosystems, where it remains and increases in concentration around the world.

It was once believed that PFAAs would eventually reach the oceans, where they would be spread out over many years. However, a 2021 study by researchers at Stockholm University found that PFAAs often move from water to air when waves reach land. This process contributes to air pollution and leads to PFAAs entering rain. The researchers said that this pollution could affect large areas. Soil is also contaminated, and these chemicals have been found in remote places like Antarctica. Contaminated soil can increase PFAS levels in foods such as white rice, coffee, and animals raised on polluted land. In 2024, a global study of 45,000 groundwater samples found that 31% of the samples had PFAS levels harmful to human health. These samples were taken from areas far from any obvious pollution source.

Contamination has also been found in water wells and other drinking water sources. This pollution is reported in the United States, United Kingdom, Germany, Japan, and Canada, but information about PFAS contamination in most developing countries is very limited. The lack of data about PFAS in developing countries, especially in Africa, is due to limited resources and less advanced technology.

Control and removal in the environment

Current research focused on reducing POPs in the environment is studying how these substances behave during photocatalytic oxidation reactions. POPs that are most commonly found in humans and in water environments are the main focus of these studies. Scientists have identified broken-down substances, such as aromatic and aliphatic compounds, in these reactions. Photochemical degradation has a very small effect compared to photocatalytic degradation. One method tested for removing POPs from ocean environments is adsorption, which happens when a substance that can be dissolved comes into contact with a solid material that has many tiny holes. This method was studied by Mohamed Nageeb Rashed from Aswan University in Egypt. Today, more attention is being given to stopping the use and production of POPs globally than to removing them from the environment.