For nearly two decades, federal authorities have directed pipeline operators to replace these leak-prone, cast-iron lines with pipes made of plastic and other modern materials. And many states, including New York, have embarked on programs to do just that.
But these efforts could take decades to complete, and in the meantime, weakened pipes could spring deadly leaks.
This possibility and the need for repairs have come into sharp focus in the aftermath of an explosion that tore apart two buildings in Manhattan on Wednesday morning, killing at least seven people and injuring dozens more.
"The human cost of inaction is clear," said Ydanis Rodriguez, a New York City councilman, in a statement to reporters. "If the necessary funding for these repairs and improvements is not granted by the federal and state governments, tragic occurrences such as today's may become more common in our city."
It's not yet clear what caused the leak that led to the explosion. Records show that the city approved a permit to replace 120 feet of gas piping last summer for a heating system at one of the collapsed buildings. The company that did the work, New York Heating Corp., did not return calls seeking comment.
Compared with a more pliable material like plastic, cast iron becomes brittle over time. A heavy blow can crack the pipes, and so can ground movements caused by alternating spells of warmth and cold.
Federal data on the integrity of gas mains show that serious leaks causing death, injury or major property damage stem from cast iron pipes four times more often than those made from other materials.
In January 2011, a leak in a cast-iron gas pipeline in Philadelphia led to a blast that killed a utility worker and injured five others. Fuel from another leaky cast iron gas main caused a major explosion in Allentown, Pa., a month later, killing five people, including a 16-year-old girl and a 4-month-old boy.