How Does Pollution Affect Polar Bears?

Pollution harms polar bears from the inside out, disrupting their hormones, weakening their immune systems, shrinking their reproductive organs, and thinning their bones. Despite living thousands of kilometers from any factory or farm, polar bears carry some of the highest concentrations of industrial chemicals ever measured in wildlife, a consequence of their position at the very top of the Arctic marine food web. The chemicals involved range from decades-old banned pesticides that still circulate in ocean currents to newer industrial compounds like per- and polyfluoroalkyl substances (PFAS) that are only beginning to be understood. Research spanning more than three decades shows that East Greenland and Svalbard bears, in particular, have been continuously exposed to pollutant levels above thresholds associated with immune, reproductive, and cancer risk.

How Industrial Chemicals Reach the Arctic

Polar bears do not live near pollution sources, yet they accumulate extraordinary chemical burdens because of a process called biomagnification. Persistent organic pollutants (POPs) released in industrialized regions evaporate, ride atmospheric currents northward, condense in cold Arctic air, and settle into the ocean. Once in the water, tiny organisms absorb these chemicals. Small fish eat the organisms, seals eat the fish, and polar bears eat the seals. At each step, the fat-soluble chemicals concentrate further because they are stored in body fat rather than excreted. By the time a polar bear eats a ringed seal, the pollutant load in its meal is orders of magnitude higher than what was originally in the water.

A study of 42 polar bears along Alaska’s Beaufort Sea coast used stable isotope signatures to trace how diet drives contamination levels. Bears feeding at higher positions in the food web had significantly higher concentrations of the most persistent PCB compounds and oxychlordane. The models explained up to 93 percent of the variation in certain organochlorine concentrations in female bears, with sex, age, and trophic position all playing a role.¹ In short, the more marine prey a bear eats and the higher up the food chain that prey sits, the more contaminated the bear becomes.

The Range of Chemicals Involved

The pollution problem for polar bears is not about a single substance. Their tissues contain a complex cocktail of chemical classes, each with its own sources and health effects.

PCBs and organochlorine pesticides: These legacy pollutants, including DDT-related compounds, chlordanes, and dieldrin, were widely used in agriculture and industry from the mid-twentieth century onward. Many have been banned for decades, yet they persist in the environment. A circumpolar survey of adult polar bears found geometric mean PCB concentrations ranging from about 2,800 to over 24,000 micrograms per kilogram of lipid weight, depending on the region.²
PFAS: These “forever chemicals,” used in nonstick coatings and firefighting foams, accumulate primarily in the liver and blood rather than in fat. A circumpolar study detected PFOS (the most common PFAS compound) in polar bear livers at concentrations as high as 2,730 nanograms per gram wet weight in the South Hudson Bay population.³
Brominated flame retardants: Polybrominated diphenyl ethers (PBDEs), used in electronics and furniture, have been found in polar bear fat across the Arctic. Only four congeners were consistently detected, with BDE-47 making up 65 to 82 percent of the total.⁴
Mercury: Released largely by coal burning and gold mining, mercury enters Arctic marine food webs and biomagnifies in a similar fashion to organic pollutants. Total mercury in polar bear hair has been linked to food web length, with bears in the southern Beaufort Sea carrying roughly three and a half times the mercury levels of bears in western Hudson Bay.⁵

PFAS distribution within the body follows a different pattern than the fat-soluble legacy chemicals. In East Greenland bears, PFAS concentrations were highest in the liver, followed by blood, brain, muscle, and adipose tissue. Shorter-chain PFAS compounds tended to concentrate in the liver, while longer-chain compounds accumulated more in the brain and fat.⁶ The fact that different chemicals park themselves in different organs means that a single blood sample may understate total exposure across the whole body.

Thyroid and Stress Hormone Disruption

One of the best-documented health effects of pollution in polar bears is disruption of the thyroid system. Thyroid hormones regulate metabolism, growth, and thermoregulation, all critical for a large Arctic predator. Multiple studies have found that higher PCB levels in polar bear blood are associated with lower levels of circulating thyroid hormones. In Svalbard, PCBs showed negative correlations with free thyroxine (T4) and free triiodothyronine (T3) in females with cubs, and the relationship was strong enough to explain a substantial share of the variation in hormone levels.⁷ Female bears appeared more sensitive than males, with PCBs affecting five thyroid hormone measures in females compared to two in males.

Research on East Greenland bears confirmed and extended these findings, showing that individual PCBs and their hydroxylated metabolites were negatively associated with T4 in both plasma and muscle tissue. Certain organochlorine pesticides and flame retardants also influenced the enzymes that activate and deactivate thyroid hormones in the liver, kidney, and muscle.⁸ The disruption is not limited to the thyroid axis. Analysis of hair cortisol in the same population found that the stress hormone system also appeared affected by contaminant mixtures, suggesting that pollutants may interfere with how polar bears respond to environmental challenges.⁹

For an animal that depends on carefully regulated metabolism to survive months-long fasts on the sea ice, even modest thyroid disruption could have cascading consequences for energy balance and survival.

Reproductive Harm

The reproductive system may be where pollution’s effects on polar bears are most alarming. In East Greenland, researchers examined reproductive organs from 55 male and 44 female bears and found significant inverse relationships between contaminant levels and the size of testes, the baculum (penis bone), and ovaries. Bears with higher concentrations of PCBs, DDT-related compounds, chlordanes, and flame retardants had smaller reproductive organs even after accounting for age. Bone mineral density in the baculum also decreased with rising contaminant loads.¹⁰ Smaller organs could mean reduced sperm counts, fewer viable eggs, and weaker mating success.

Hormonal links have also been documented. In Svalbard females with cubs, PCB concentrations explained about 27 percent of the variation in progesterone levels, a hormone essential for maintaining pregnancy.¹¹ Work on testosterone in East Greenland males was less clear-cut: while some individual organochlorines showed relationships with testosterone, overall models did not reach statistical significance, probably because factors like body condition and reproductive status mask the chemical signal in free-ranging animals.¹² The researchers noted this is a common challenge in wildlife toxicology: real-world confounders make it hard to isolate a pollutant’s effect, even when laboratory studies clearly show the compound disrupts the endocrine system.

Weakened Immunity and Long-Term Disease Risk

A functioning immune system is non-negotiable for wild animals that have no access to veterinary care. In Svalbard polar bears, levels of immunoglobulin G (IgG), a key antibody, were negatively correlated with PCB and hexachlorobenzene concentrations in blood plasma.¹³ Lower antibody levels suggest a reduced ability to fight off infections and parasites.

A broader risk assessment covering East Greenland bears from 1983 to 2013 calculated additive risk quotients for immune, reproductive, and carcinogenic effects. The risk quotients exceeded the threshold of concern in every single year of the study, meaning this population has been at continuous risk of contaminant-mediated health effects for over three decades. PCBs alone accounted for 71 to 99 percent of the total risk across all three categories.¹⁴ The finding that cancer risk is elevated is particularly striking because documenting cancer in wild populations is extremely difficult; the risk assessment is based on the known carcinogenic properties of the detected compounds at the concentrations measured.

Damage to Bones and the Brain

Organochlorines do not stop at soft tissues. Skull bone mineral density in East Greenland polar bears showed negative correlations with PCBs and chlordane in subadults, and with dieldrin and DDT compounds in adult males. Flame retardants also showed a borderline association.¹⁵ Weakened skulls in a species that depends on crushing seal skulls to feed could have direct survival consequences.

Mercury poses a particular threat to the brain. Even at surprisingly low total mercury levels in the brain stem, researchers found a significant negative relationship between mercury and the density of a key neural receptor involved in learning and memory. The effect was detected at concentrations that would not traditionally be flagged as toxic, raising the possibility that mercury causes subtle neurological damage well before clinical symptoms appear.¹⁶ Separately, liver and kidney mercury levels in some East Greenland bears exceeded the toxic threshold for terrestrial mammals, with 9 out of an examined group of kidney samples surpassing 30 micrograms per gram wet weight.¹⁷

Mercury exposure has even been linked to changes at the molecular level. An analysis of polar bear brains found an inverse association between mercury levels and global DNA methylation, an epigenetic marker that helps regulate gene expression. The trend was most apparent in males.¹⁸ While the sample size was small and the statistical significance was borderline, the finding raises concerns about whether pollutants could alter gene regulation in ways that ripple across generations.

Mothers Pass the Burden to Their Cubs

Polar bear mothers transfer contaminants to their offspring through two routes: across the placenta during pregnancy and through milk during the long nursing period. The transfer is highly efficient for fat-soluble compounds. A study comparing PCB levels in mother-cub pairs found that cubs carried roughly 2.7 times the PCB concentration of their mothers, a reflection of how aggressively these compounds move from maternal fat stores into nutrient-rich milk.¹⁹

Milk from Southern Beaufort Sea bears contained up to about 72 micrograms per kilogram of total mercury and up to 690 micrograms per kilogram of PCBs. Blood contaminant levels in dependent young aged one to five years were similar to those of adult females and actually higher than those of adult males, reflecting sustained exposure through nursing.²⁰ While the daily mercury intake through milk was estimated to fall below human tolerable daily intake thresholds, the PCB intake for cubs of the year exceeded those thresholds. The researchers noted, however, that calculated dioxin equivalents for PCBs in milk were still below physiological thresholds established for marine mammals.

A study of Svalbard mother-cub pairs found that pollutant exposure was associated with changes in fat tissue gene expression in both mothers and cubs. In mothers, over 5,000 genes were correlated (positively or negatively) with cumulative POP exposure, and in cubs, over 4,200 genes showed similar associations. More than 700 of those genes overlapped between mothers and cubs, suggesting shared molecular responses to the same chemical burden.²¹ This means cubs are not just receiving chemicals from their mothers; their tissues are actively responding to those chemicals from a very early age.

Why Some Populations Are Hit Harder Than Others

Not all polar bear populations carry the same chemical burden. Geography matters enormously. Bears from Svalbard, East Greenland, and certain Canadian Arctic islands consistently show the highest PCB concentrations, while Alaskan and western Hudson Bay bears tend to be less contaminated. The pattern is driven partly by atmospheric and oceanic transport pathways that deliver more pollutants to the European Arctic, and partly by differences in diet. Bears in different regions eat different proportions of prey species, and those prey vary in their own contamination levels.²² Diet signatures explained 18 to 21 percent of variation in PCB levels and 14 to 15 percent of variation in flame retardant levels across subpopulations, with East Greenland and Svalbard signatures associated with higher levels.

Even within a single population, individual foraging strategy matters. In the Barents Sea, bears that spend more time offshore in the pelagic zone carry higher pollutant loads than coastal bears, because they eat more marine prey from higher trophic levels, have higher energy demands, and forage in marginal ice zones closer to pollutant transport pathways.²³ For PFAS specifically, eastern populations like South Hudson Bay and East Greenland had significantly higher PFOS levels than western populations like the Chukchi Sea, and PFOS concentrations increased with age at most locations.³

Are Bans and Regulations Working?

The Stockholm Convention on Persistent Organic Pollutants, which entered into force in 2004, aimed to eliminate or restrict the worst legacy chemicals. There is evidence that it has made a difference, though the picture is uneven. In Barents Sea polar bears monitored over multiple years, concentrations of PCBs, certain flame retardants, and oxychlordane declined linearly over time. Endosulfans and hexabromocyclododecane, regulated more recently, were detectable in Hudson Bay bears sampled in 2007–2008 but had dropped below detection limits by 2013–2014.²⁴

Not everything is improving, though. In the same Barents Sea population, DDE (a DDT breakdown product) and hexachlorobenzene declined until roughly 2009–2012 and then began rising again, likely driven by secondary emissions from environmental reservoirs like thawing permafrost or remobilized sediments. Changes in bear diet and body condition did not explain the trends, pointing to ongoing chemical releases as the main driver.²⁵ Meanwhile, newer chemicals like PFAS continue to accumulate, and in East Greenland, PFAS concentrations in polar bear livers reach levels two orders of magnitude above those in their ringed seal prey.²⁶

Plastic Waste and Microplastics

Beyond chemical contaminants dissolved in water and stored in prey fat, polar bears also encounter physical pollution. A study of stomach contents from 42 bears harvested in the Southern Beaufort Sea between 2010 and 2020 found user plastics in nearly 29 percent of stomachs, and non-plastic rubbish in about 12 percent. A third of the stomachs showed acute gastritis, and half of those cases occurred in animals that had ingested plastics.²⁷

Microplastics, the tiny fragments that break down from larger plastic items, tell a somewhat different story. Analysis of fecal samples from Canadian polar bears detected polypropylene, polyethylene, and polyethylene terephthalate particles in samples from eight bears, but at very low concentrations, averaging less than one particle per gram of dry feces. The researchers concluded that microplastic ingestion in Canadian bears appears to be low for now.²⁸ Whether microplastic levels will rise as Arctic sea ice retreats and shipping traffic increases remains an open question.

Mercury and the Gut Microbiome

A more recently discovered dimension of pollution’s impact involves the community of bacteria living in the polar bear gut. In bears from the Southern Beaufort Sea, bacterial diversity in the gut decreased as both seal consumption and hair mercury concentrations increased together. Bears with mercury levels above a certain threshold had different gut bacterial communities: more Firmicutes and Proteobacteria, fewer Bacteroidetes and Actinobacteria. Eighteen specific bacterial groups differed significantly between high- and low-mercury bears, with the majority being suppressed rather than enriched at higher mercury exposure.²⁹ The gut microbiome influences digestion, nutrient absorption, and immune function, so shifts in its composition add another layer to the cascade of pollution effects on polar bear health. This area of research is still in its early stages, but it suggests the full scope of how contaminants reshape polar bear biology may be broader than the organ-by-organ effects documented over the past three decades.