A significant proportion of the threats to the Arctic environment come from outside the region, including POPs, ozone depleting substances and greenhouse gases (see definitions for ozone depleting substances and greenhouse gases as well). POPS are Persistent Organic Pollutants: toxic chemicals that are produced and consumed outside of the Arctic, yet disproportionately concentrate within the Arctic region. POPs are transported by wind, water, and food cycles making them transient across borders, continents, and ecosystems. Because they are resistant to environmental degradation, they persist for long periods of time in the environment and can accumulate and pass through the food chain. In 2001, 91 countries and the European Community agreed to reduce or eliminate the production of 12 key POPs in the Stockholm Convention. Some well known POPs include PCBs, DDT, and dioxins. While many developed countries have stopped or limited production, many still persist today, are unintentionally released, and are still being produced in developing countries.”While there has been a general decrease in trends for POPs appearing in air and biota over the last 20 or 30 years, the problem is resurfacing due to warming. The POPs that were once trapped under layers of snow and ice are now being released, increasing the risk of exposure to current and future human and animal populations in the Arctic. Exposure to POPs has significant human health effects, by impacting reproductive, developmental, behavioral and neurologic systems and amongst others.