8.7 - Persistent Organic Pollutants (POPs)

Learning Objectives

I. What POPs Are

• Definition: Persistent organic pollutants (POPs) are synthetic (human-made), carbon-based chemicals that don’t easily break down in the environment. Once they’re released, they can stay in soil, water, sediments, and living things for years or even decades.

• What “organic” means here: In chemistry, organic just means the molecule is built on a carbon skeleton. It doesn’t mean natural or safe, the way it does on a food label.

• Key examples:
• DDT: A broad spectrum insecticide. The military first used it widely during World War II to control the insects that spread malaria and typhus, and it was later sprayed heavily on crops. The EPA canceled its remaining U.S. uses in 1972 because it persists in the environment, builds up in fatty tissue, and harms wildlife.
• PCBs (polychlorinated biphenyls): Industrial chemicals made of carbon, hydrogen, and chlorine. They don’t burn, they’re chemically stable, and they insulate electricity, so they were used in electrical equipment like transformers and capacitors, and in paints, plastics, and carbonless copy paper. The U.S. banned their manufacture in 1979, but equipment made before the ban can still contain them.
• Dioxins and furans: Unlike DDT and PCBs, nobody makes these on purpose. They’re unintentional byproducts of burning (like trash burning and incineration) and of some industrial processes.

II. Why POPs Persist

• Stable molecules: Many POPs, including DDT, PCBs, and dioxins, are organochlorines, meaning carbon-based molecules with chlorine atoms attached. The bonds between carbon and chlorine are very stable, and because these molecules are synthetic, few decomposers have the enzymes to break them down quickly. Sunlight, water, and microbes break them down only very slowly.

• Half-life example: DDT’s half-life in soil is about 2 to 15 years. With a 15 year half-life, after 60 years (four half-lives) about $\left(\frac{1}{2}\right)^4 = \frac{1}{16}$, or roughly 6%, of the original DDT is still there.

• Sediments as reservoirs: POPs stick to soil particles and settle into the sediments at the bottoms of rivers, lakes, and oceans. Those sediments become long-term reservoirs that can release POPs back into the water when they’re stirred up by floods, storms, or dredging.
• Example (Hudson River PCBs): From 1947 to 1977, two General Electric capacitor plants in New York discharged an estimated 1.3 million pounds of PCBs into the Hudson River. The PCBs stayed in the river sediments for decades, so the EPA required dredging of a 40 mile stretch of the upper river, which didn’t start until 2009. The main risk to people is eating contaminated fish.

III. Fat Solubility and Toxicity

• Fat soluble, not water soluble: POPs dissolve easily in fats and oils but barely dissolve in water.

• Stored in fatty tissue: When an organism takes in POPs (by eating, drinking, or breathing them, or absorbing them through gills or skin), its body can’t flush them out in urine, which is mostly water. Instead, they’re stored in fatty tissues like body fat and blubber.

• Why that makes them toxic: Since POPs aren’t excreted, even small, repeated doses add up over time. Stored POPs can also be released back into the blood later, for example when an animal burns through its fat reserves during starvation. Mothers can pass stored POPs to their young through egg yolks and breast milk.

• Link to 8.8: Fat solubility is the reason POPs bioaccumulate (build up in one organism over its lifetime) and biomagnify (reach higher concentrations at each higher trophic level). Those two processes are covered in 8.8.

IV. Long Distance Travel

• By air: Many POPs can evaporate from warm soil and water, travel through the atmosphere as a gas or attached to dust particles, and then get redeposited onto land and water by rain, snow, or settling particles.

• Grasshopper effect (global distillation): POPs can repeat this evaporate, travel, and redeposit cycle many times, “hopping” across the globe. Warm temperatures push them into the air, and cold temperatures make them condense and fall back out. Over time, this moves them toward the poles and high mountains, where it’s too cold for them to evaporate again, so they get trapped there.

• By water: Ocean currents and rivers carry POPs that are dissolved in the water or attached to sediment particles. Migratory animals, like fish and birds, can also carry POPs in their fat from one ecosystem to another.

• Case study (the Arctic): The Arctic is far from where most POPs were made or used, but POPs show up in its air, snow, and food webs. Seals, whales, and polar bears carry high levels in their fat, and Inuit communities in Greenland and Canada, whose traditional diets include marine mammals, have some of the highest POP exposures measured in people.

V. Effects on Ecosystems and Humans

• Wildlife: POPs have been linked to population declines, disease, and abnormalities in fish, birds, and mammals. Common effects include reproductive failure, developmental deformities, and weakened immune systems. Many POPs, including DDT and PCBs, are also endocrine disruptors (see 8.3), which explains a lot of their reproductive and developmental effects.

• Top predators hit hardest: Because POPs build up in fat and concentrate up the food chain, top carnivores carry the highest levels.
• Birds of prey: DDE, a breakdown product of DDT, thinned the eggshells of birds like bald eagles, ospreys, and peregrine falcons. The thin eggs often cracked under the weight of the parent sitting on them, so few chicks hatched and populations crashed (more in 8.8).
• Killer whales: PCBs were banned in most of the world decades ago, but a 2018 study estimated that their effects on reproduction and immune function threaten the long-term survival of more than half of the world’s killer whale populations.

• Humans: People are exposed to POPs mainly through food, especially fatty fish, meat, and dairy. Long-term exposure is linked to cancer and to reproductive, developmental, nervous system, and immune system problems.

VI. Reducing POPs (Solution)

• National bans: The U.S. canceled DDT’s remaining uses in 1972 and banned PCB manufacturing in 1979. Since POPs persist, their levels in the environment drop slowly, over decades, after a ban.

• Stockholm Convention: POPs cross borders, so countries have to act together. The Stockholm Convention (adopted in 2001, in force since 2004) is an international treaty that requires its member countries to eliminate or restrict the production and use of listed POPs and to reduce unintentional releases, like dioxins. It started with 12 chemicals, nicknamed the “dirty dozen,” and more have been added since.

• Tradeoff (DDT and malaria): The convention still allows DDT for disease vector control, mainly spraying the inside walls of homes to kill the mosquitoes that carry malaria, in places where effective and affordable alternatives aren’t available yet. It’s a good example of weighing environmental harm against human health benefits.

Big picture: Persistent organic pollutants (POPs), like DDT and PCBs, are synthetic, carbon-based chemicals that resist breakdown, so they last for years to decades in soil, sediments, and living things. Because they’re fat soluble, organisms store them in fatty tissue instead of excreting them, which lets them build up to toxic levels and sets up the bioaccumulation and biomagnification covered in 8.8. Wind and water carry POPs long distances before redepositing them, so they harm wildlife and people even in places like the Arctic, far from where they were used. That’s why national bans and the international Stockholm Convention matter.