Lived across the water and my family lived in the city during 9/11. Everyone (news, first responders, agency officials) was saying avoid ground zero unless you have respiratory equipment. That didn't stop heros from running in maskless to help people, or stop people from running through the dust to get to safety. But as you moved uptown and to other boroughs, the particle density dropped significantly, even though the wind was sending dust and debris over brooklyn/staten island. It was immediately well known that people were going to get sick from the fallout

If I can remember later, I'll come back and post a NASA satellite image showing the particle density that I saved from that morning