Environmental chemists have found more than 200 previously unidentified halogenated molecules in polar bear blood, suggesting that the animals’ exposure to such compounds has been underestimated (Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201809906). Halogenated contaminants have been showing up in polar bears since the 1970s. Bears sit atop a food chain that includes seals, fish, and other creatures, which makes them a useful sentinel species that can flag the presence of persistent organic pollutants in the environment. Exposure to the pollutants can impair the bears’ immune systems and hamper the development of cubs, says Jonathan W. Martin of Stockholm University, who led the study. “They don’t cause polar bears to die, but they add to the animal’s stress,” he says. Martin’s team studied blood samples gathered between 1985 and 2016 from polar bears in the Hudson Bay and Beaufort Sea regions of the Canadian Arctic. After removing proteins and lipids, the researchers added small pieces of polyethersulfone capillary tubes to absorb the contaminants, then used high-resolution mass spectrometry to identify the halogenated compounds. These included metabolites of polychlorinated biphenyls, perfluoroalkyl sulfonates (PFSAs), and other polychlorinated compounds. The analysis could not measure the absolute concentration of these compounds, but it did find a relative increase in PFSAs over time. Martin says that some parent compounds of these PFSAs are still in use, and with more evidence about their presence in the environment “there’s a potential to close off the tap.” The team is now doing targeted analyses to measure the concentrations of these compounds in polar bear blood and to look for them in other species.