SAN FRANCISCO — When the emergency bell sounds at Fire Station 1 here, firefighters pull on boots and backpacks, swing into Engine 1 and hurtle out the door in almost a single motion, a blast of red lights and caterwauling sirens. More often than not, there is no fire.

Instead, the calls that ring in this and nearby fire stations tend to go like this: Male, apparently homeless, sprawled unconscious on a train platform. Male, prone on a street corner pushing a needle into his arm.

In a measure of just how much homelessness has become an all-encompassing problem here, this city has the busiest fire engine in America — yet just over 1.5 percent of its runs last year involved fires.

“We are here to help anybody — we have this infrastructure, and it’s always been here to help the city and its people,” said Lt. Tom Johannessen, an officer on the crew for Engine 1, which is based in the South of Market Street neighborhood and includes in its catchment the gritty Tenderloin neighborhood. “We are your best point for help, where it helps people in a car crash or a medical call. But where it gets to be picking up the same homeless person twice a day, it gets to be strained.”