As the one-year anniversary of the attacks on the World Trade Center and the Pentagon nears, some people are trying to assess the impact of the tragedy on the environment. In New York Harbor, biologists are studying the effects on aquatic life of the smoke and building fragments that drifted into the Hudson River. The debris had high levels of dioxins, PCBs, and metals. Meanwhile, New York City residents are still concerned about the fallout from what was, among other things, the worst environmental disaster in the history of the city. Most people say the city’s air quality is back to normal, or “at least normal for New York,” as Louise Leavitt of the American Lung Association state chapter put it. But downtown residents still worry about the lingering ash and dust, some of which is asbestos-tainted. In June, a poll by an independent health research group found that lower Manhattan residents were more concerned about air quality than about another terrorist attack.