8.7 Persistent Organic Pollutants (POPs)

• Persistent: it resists being broken down by sunlight, water or microbes, so it can last for years to decades in soil, sediment and organisms.
• Organic: it is carbon-based. Most POPs are synthetic, meaning made by people, and many contain chlorine.
• Fat-soluble: it dissolves in fats and oils rather than water. Organisms can't easily flush it out in urine, so it is stored in fatty tissue and builds up over time (topic 8.8).
• Toxic: it can cause cancer, disrupt hormones (topic 8.3), harm the immune and nervous systems, or cause reproductive problems.

DDT is an insecticide used widely after World War II. It was banned for most uses in the United States in 1972, but it is still used in some countries to control the mosquitoes that spread malaria.

PCBs (polychlorinated biphenyls) were used as insulating fluids in electrical transformers and in other industrial products. US production was banned in 1979, but PCBs remain in river sediments, such as stretches of New York's Hudson River, where factories discharged them for decades.

Dioxins are unwanted byproducts formed when chlorine-containing materials are burned, such as in some waste incineration.

Other examples include several older pesticides like chlordane and dieldrin.

POPs can travel thousands of kilometers on wind and ocean currents. Many evaporate in warm places, travel in the air, and condense back out in colder places, sometimes repeating this several times, a pattern nicknamed the grasshopper effect. The net result is that POPs collect in cold polar regions far from where they were used. Arctic animals such as polar bears and seals, and Indigenous peoples whose traditional diets rely on fatty marine animals, carry some of the highest POP levels in the world even though these chemicals were never used there.

Because POPs cross borders, controlling them takes international cooperation. The Stockholm Convention, adopted in 2001, is a global treaty that bans or restricts the production and use of a list of POPs, while allowing limited DDT use for malaria control.

Most human exposure to POPs comes from food, especially fatty fish, meat and dairy products, because the chemicals are stored in animal fat. Mothers can pass POPs to babies during pregnancy and through breast milk, though health agencies still recommend breastfeeding because its benefits outweigh the risk. Exposure also comes from contaminated sites, such as old industrial areas and river sediments, and from eating fish caught downstream of them, which is why many rivers have fish consumption advisories.