Delegates from 122 nations reached agreement yesterday on a treaty to ban or reduce the use of 12 persistent organic pollutants (POPs), chemicals such as PCBs and pesticides that have been linked to cancer, birth defects, and genetic abnormalities in humans and wildlife. The ban, which must be ratified by 50 countries to become legally binding, will require some industries to find new ways to make their products without using POPs or creating POPs as a byproduct. Twenty-five developing nations will be allowed to continue to use the pesticide DDT to combat malaria until better alternatives are found. Greenpeace and the World Wildlife Fund praised the treaty as an important first step toward ending toxic pollution.