Nine dangerous chemicals used in farming and industry will be added to a list of banned substances whose presence in the environment causes serious health risks, more than 160 government agreed on Saturday. The nine pesticides and industrial chemicals join 12 substances targeted for elimination under the 2001 Stockholm Convention on Persistent Organic Pollutants (POPs). Trade in some of the chemicals can amount to billions of dollars a year, but countries at the United Nations conference agreed they are so dangerous that alternatives must be found. “Just five years after this convention came into force, we will have nine new chemicals added to the list of those that the world community agrees we need to control and ultimately get rid of,” said Achim Steiner, executive director of the UN Environment Program (UNEP), which hosted the conference. Donald Cooper, executive secretary of the Stockholm Convention, set out why the banned substances were exceptionally dangerous: They cross boundaries and are found everywhere, from the Tropics to polar regions; they persist for long periods in the atmosphere, soil and water, and take years to degrade; they accumulate in bodies; they accumulate in food chains. The chemicals can damage reproduction, mental capacity and growth and cause cancer, he said. “In most cases the question is not simply how do we control them, but how we eliminate them,” he told a news briefing on Friday.