Organochlorines Reduce Bone Density in Polar Bears

Exposure to organochlorine chemicals is linked to reduced bone mineral density among polar bears from East Greenland, according to a study published today in the December issue of the peer-reviewed journal Environmental Health Perspectives (EHP). In the study of 139 polar bear skulls, researchers compared 41 samples collected between 1892 and 1932 with 98 samples collected between 1961 and 2002. Bone mineral density in the skulls collected before 1932—considered “pre-pollution” by the researchers—was significantly higher than that in skulls sampled in the “post-pollution” period after 1961, when scientists first began seeing organochlorines and other persistent organic pollutants (POPs) in the fat of polar bears.

The researchers also analyzed a subset of 58 skulls collected between 1999 and 2002 to examine the organochlorine body burden in relation to bone mineral density. In this group, exposure to PCB compounds and to chlordane (a now-banned insecticide) both correlated with low bone mineral density among younger bears. In adult males, concentrations of dieldrin (another banned insecticide) and total DDT residues also correlated with low density.

POPs resist breakdown, store easily in fat, and bioaccumulate through the food chain. Once widely used in agriculture and industry, several types have been classified as probably or possibly carcinogenic to humans, and there are now restrictions or bans related to their application. However, these highly toxic chemicals are very stable over time, and they remain widely present in the environment, where they still pose a serious health threat.

“Polar bears from East Greenland, Svalbard, and the Kara Sea carry higher loads of organochlorines than do polar bears elsewhere in the Arctic due to the different atmospheric transport routes,” the study authors write. “The strong correlative relationships [between bone mineral density and exposure] suggest that disruption of the bone mineral composition in East Greenland polar bears may have been caused by organochlorine exposure.”

Given the results of previous studies on organochlorines, the findings are not surprising, according to Dr. Jim Burkhart, science editor for EHP. “Previous research suggests DDT and PCBs reduce bone density in humans and in Baltic grey seals. Now they appear to have a similar impact on polar bears,” he said.

The lead author of this study was Christian Sonne of the National Environmental Research Institute in Roskilde, Denmark. Other authors included Rune Dietz, Erik W. Born, Frank F. Riget, Maja Kirkegaard, Lars Hyldstrup, Robert J. Letcher, and Derek C.G. Muir. The article is available free of charge at http://ehp.niehs.nih.gov/members/2004/7293/7293.html.

EHP is published by the National Institute of Environmental Health Sciences, part of the U.S. Department of Health and Human Services. EHP is an Open Access journal. More information is available online at http://www.ehponline.org/.