Out of sight of most New Yorkers is a sprawling underground labyrinth of about 7,500 miles of sewers, part of the city’s vast and, in many cases, aging subterranean infrastructure. Besides old age, the sewers, which are essential to the health of the city, are under assault from a nemesis above ground: grease.

Across the city, the remains of deep-fried this or pan-fried that are being carelessly and improperly poured down kitchen drains and other plumbing outlets. The grease often ends up sticking to other debris in the sewer system, until it hardens and blocks pipes, like clogged arteries.

Carter Strickland, the city’s environmental commissioner, said the cardiovascular analogy was overused — but accurate. “Grease clings to the surface and it builds on itself over time,” Mr. Strickland said. “The sewage cannot get through, and bad things happen.”

Bad things like sewage backing up into sinks and bathtubs, or onto city streets.

“At any one time, it’s such a big network,” Mr. Strickland said, “you’re going to have an issue.”

In fact, 62 percent of the 15,000 sewer backup complaints the city’s Environmental Protection Department logged last year were caused by buildups of fat, oil and grease, a combination known in the waste industry as FOG.

That percentage is comparable to the grease scourge faced by sewer systems elsewhere, but the challenge in New York is unique because of the sheer size of the system and the approximately 24,000 food establishments in the city that rely on grease.

“One thing these eateries have in common,” said Eric A. Goldstein, New York City environmental director at the Natural Resources Defense Council, “is that they use a large amount of grease and food oils. If we don’t find ways to recycle these food oils, it’s not surprising that we still have these problems.”

The city has now gone on the offensive, embarking on an aggressive multipronged campaign to make residents and businesses aware of the threat posed by grease, not only in creating foul messes, but also in requiring expenditures: Clearing backups caused by grease cost the city an estimated $4.65 million last year.

The Environmental Protection Department has increased regular sewer cleaning in grease hot spots in Queens, using trucks equipped with high-power vacuums or water hoses and a supply of degreasing agents.

The agency also uses cameras lowered into sewers to look for clogs. It has tripled the number of remote sewer-monitoring devices, which are placed inside manholes and send an alert if they detect rising wastewater.

And it has started an outreach program, Cease the Grease, to educate residents about the dos and don’ts of discarding food oils.

Queens has the dubious distinction of being the leader when it comes to the share of backups caused by grease — grease accounted for 74 percent of backups there last year. Staten Island was next at 55 percent, with Manhattan and the Bronx tied at 49 percent and Brooklyn at 46 percent.

Mr. Strickland said he had no definitive answer as to why Queens had such a big grease problem, “other than its neighborhoods are more settled, and it has a lot of single-family homes where people may be pouring grease down the drain.”

Buried below Queens is nearly 40 percent of the city’s sewer infrastructure, and the borough has historically been prone to sewer problems, particularly in flat and low-lying areas. Some neighborhoods have smaller mains, sometimes limiting capacity.

In the city, every business that generates fat, oil and grease must have grease interceptors to block the substances from reaching sewer lines, according to city sewer regulations. A trap is connected to a sink by pipes and separates grease from wastewater. In the interceptor, fat, oil and grease float to the top, where they build up until they can be removed, while the grease-free wastewater continues through the interceptor and into the sewer system. 

City environmental inspectors check the equipment, making sure it is properly sized, installed, maintained and operated. Business owners can be fined up to $10,000 a day for each violation.

Despite the agency’s efforts, grease clogs remain a recurring issue.

“It’s prolific. It’s a really big problem,” said Ted Vitanza, who owns a Mr. Rooter plumbing service franchise in Queens and has been a licensed plumber for more than 30 years. He said he had seen wedges of grease reduce an eight-inch pipe to, in effect, a four-inch pipe. “There are far too many people who don’t understand it’s not a good thing to take a frying pan, after you’re done making chicken cutlets, and pour the grease down the drain,” Mr. Vitanza said. “It’s mostly innocent people, people who aren’t educated to these facts.”

The Environmental Protection Department has held more than 30 grease awareness workshops in over different neighborhoods and mounted a yearlong campaign at the Baruch Houses in Lower Manhattan.

The development, with more than 2,100 families in 17 buildings, is one of the New York City Housing Authority’s largest projects, and it has had repeated sewer backups over the years. Those sewer lines empty into a main under nearby Delancey Street, which also has recurring backups. Grease buildup has been the main cause.

The surrounding Lower East Side neighborhood has shifted over the years, with small dry goods shops and clothing stores giving way to bars, restaurants and hotels, the kind of commercial tenants that steadily produce grease.

Disposing of grease properly is simple, city officials said. Pour the hot liquid into a coffee can, tuna can or any other sealed container that will not burn or melt. Once the liquid has cooled, the container can be discarded with the rest of the garbage. Dishes should be cleared of oil, grease and food scraps with paper towels before they are washed.

Like many residents who came to a sparsely attended grease workshop at the Baruch Houses, Valerie Morgan, 46, has her own way of getting rid of grease. Ms. Morgan, a mother of five, pours her used grease into a coffee can. After she reuses the oil a few times, she lets it cool, then stuffs the can with paper towels, puts the can in a plastic bag, and tosses it in the incinerator.

But she said she had never considered how interconnected all the city’s pipes were. She recalled a clog in her kitchen sink that resulted in rust-colored water, dirt and clumps of hair spitting into her bathtub.

“It’s something that we have to think about,” Ms. Morgan said, “so we don’t have complications.”

“And,” she added, “having to go through the headache of having to wait for people to come fix it.”