SVALBARD, Norway -- Already imperiled by melting ice and a brew of toxic chemicals, polar bears throughout the Arctic, particularly in remote dens near the North Pole, face an additional threat as flame retardants originating largely in the United States are building up in their bodies, according to an international team of wildlife scientists.
The flame retardants are one of the newest additions to hundreds of industrial compounds and pesticides carried to the Arctic by winds and ocean currents. Accumulating in the fatty tissues of animals, many chemicals grow more concentrated as larger creatures eat smaller ones, turning the Arctic's top predators and native people into some of the most contaminated living organisms on Earth.
In urban areas, particularly in North America, researchers already have shown that levels of flame retardants called polybrominated diphenyls, or PBDEs, are growing at a rapid pace in people and wildlife. Although they have been found in much lower concentrations in the Arctic, scientists say their toxic legacy will persist there for years because they are slow to break down, particularly in cold climates.
In polar bears, the effects are unknown. But in tests on laboratory animals, PBDEs disrupted thyroid and sex hormones and damaged developing brains, impairing motor skills and mental abilities, including memory and learning.
Scientists say that other industrial chemicals with properties similar to PBDEs are already weakening the bears' immune systems, altering their bone structure, skewing their sex hormones and perhaps causing small numbers of hermaphroditic bears.
What remains uncertain, however, is whether those physiological changes are killing bears or reducing their populations. Some specialists suspect that many cubs, which are contaminated by their mother's milk, are not surviving.
An even more immediate threat to the world's 20,000 to 25,000 polar bears is global climate change, which is melting their hunting grounds. Bears in Canada's western Hudson Bay -- the most well-researched population -- declined from 1,100 in 1995 to fewer than 950 in 2004.
In Alaska, wildlife biologists for the first time have documented that polar bears are drowning. Scientists predict that some populations could become extinct by the end of the century as more sea ice melts.
Derek Muir of Canada's National Water Research Institute, who led the new research, said the geographical patterns in contamination suggest that the East Coast of North America and northwestern Europe are the primary sources of the flame retardants.
The most highly contaminated bears are in eastern Greenland and Norway's Svalbard islands, where the chemicals are about 10 times more concentrated than in bears in Alaska and four times more than in Canada, according to new research published in December in the journal Environmental Science and Technology.
The team of scientists from Canada, Norway, Denmark and Alaska, who tested 139 bears captured in 10 locations, found that some brominated flame retardants magnify from prey to predator at an extraordinary rate. One compound was 71 times more concentrated in polar bears than in ringed seals, their major food source.
Manufacturing industries in the United States use large volumes of PBDEs in furniture, carpet padding, electronics and plastics. The most abundant PBDE in the bears comes from a compound called penta, used primarily in North America to make foam furniture cushions fire-resistant.
The only US manufacturer of penta and another PBDE called octa ended their production in 2004 after the compounds began building up in human breast milk and Europe and California banned their use. Yet stockpiles remain, as well as products that contain penta, octa and other flame retardants, so the chemicals are still hitchhiking to the Arctic on northbound winds.
Michael Ikonomou of Canada's Institute of Ocean Sciences reported some good news last year. Although the flame retardants had been doubling every four to five years in Arctic ringed seals from 1981 to 2000, they have stabilized as the bans on the two compounds go into effect. Muir said he expects ''a quick downturn" in the polar bear levels as factories stop using stockpiles. But he warned that other industrial chemicals are turning up in Arctic creatures.
In addition to the brominated flame retardants, perfluorinated compounds used to manufacture Teflon and formerly used in Scotchgard have been detected in polar bears in Greenland, Canada and Alaska, and a PBDE compound called deca, used in large volumes in electronics, is in the blood of Svalbard bears.
The new study also detected another flame retardant used in building materials and household furnishings, called HBCD, or hexabromocyclododecane, in Arctic bears. Chemists had thought it had a low potential to migrate long distances but now believe it is spreading globally.