Once released to the environment, some chemicals can travel around the globe, transported freely across national boundaries by winds and ocean currents, rivers and streams, ending up in even the most remote environments. They can also remain in the environment for a long time and build up in the food chain to levels that are harmful to human and ecosystem health.

Some of these chemicals—pesticides like DDT and lindane, as well as mercury and dioxins and furans, and flame retardants like polybrominated diphenyl ethers (PBDEs)—have been identified by Canada, Mexico and the United States as chemicals of mutual concern because of the risks they pose to North America’s citizens and ecosystems. Many of these same substances are also the subjects of international agreements that seek to reduce chemical risks worldwide.