Firefighters die because they run into dangerous places, and they die because they stay.

Chief Ronald R. Spadafora stayed — for nine months. His assignment was ground zero, the patch of steel and debris that a federal labor official in 2001 called “potentially the most dangerous workplace in the United States.”

The job of making sure no one got hurt fell to him.

In fall 2001, he watched rescue workers trade their dust masks for half-face respirators with triple-combo cartridges for organic vapor and acid gas.

In the winter, he made a plan for extracting 8,000 pounds of chemical coolant from underground tanks, the substance so toxic that, at high enough concentrations, it could cut glass.