On September 11, 2001, terrorist attacks created a grave disaster that included the destruction of the World Trade Center in New York. The day after 9/11, epidemiologists were asked to assess the environment around Ground Zero for potential hazards that might put those engaged in rescue and recovery at risk of harm. Beside the dust, what toxins might be in the air? Was the air quality safe or should rescue workers wear canister respirators or particle masks? What other protections might be necessary in the days following the disaster?