That would be the air shaft—the slender opening between tenements that developers built to satisfy an 1879 requirement mandating a window facing outdoors in every room.
These shafts did provide a bit of air and light. Unfortunately, they also functioned as dumps, with tenants tossing their waste down the air shaft, rendering them funnels of filth and disease.
Just how disgusting was it? This passage conveys it well. It’s from A Tree Grows in Brooklyn, Betty’s Smith’s account (based on her own childhood) of a young girl growing up in a Williamsburg slum:
“The airshaft was a horrible invention. Even with the windows tightly sealed, it served as a sounding box and you could hear everybody’s business. Rats scurried around the bottom. There was always the danger of fire. A match absently tossed into the airshaft by a drunken teamster set the house afire in a moment.
“There were vile things cluttering up the bottom. Since the bottoms couldn’t be reached by man (the windows being too small to admit the passage of a body), it served as a fearful repository for things that people wanted to put out of their lives. Rusted razor blades and bloody clothes were the most innocent items.
“Once Francie looked down into the airshaft. She thought of what the priest said about Purgatory and figured it must be like the airshaft bottom only on a larger scale.”
Ephemeral New York 