At a summit in Exeter last week, residents and EPA officials met to discuss a class of industrial chemicals known as PFAS. It was the first of several sessions addressing concerns about these toxic substances, which have been linked to cancer, among other health problems. In New Hampshire, these chemicals have been found at high levels at several sites, including Pease International Tradeport and the Saint Gobain plastics factory in Merrimack, prompting concern among nearby communities. State