A stench like rotting garbage on a hot day is wafting from some elevators in the city’s subway system.

For years, subway riders have passed by elevators in stations at Union Square, Borough Hall, Penn Station, Grand Central, and Yankee Stadium, as well as a handful of others, only to be assaulted by an odor passengers have described with phrases like decaying corpse and dirty diaper.

The stench comes from the biodegradable oils used by New York City Transit to lubricate the hydraulic lifts in 98 elevators across the city’s 468 stations.

The smell has elicited scores of complaints, New York City Transit employees said, but riders and the workers who clean the stations say the problem has not been adequately addressed.

“It’s there all the time, the same smell,” a subway rider, Brian Mannain, 45, said of the odor at the Yankee Stadium station in the Bronx. “If you’ve ever smelled a dead body, that’s what it smells like. The whole platform reeks. You just want to run to get out of there.”

Even for those who grin and bear it, the smell is a mystery, a kind of collateral damage associated with riding the subway in the city.

It’s a case of good intentions gone wrong.

The transit authority has been using the oils for more than five years to avoid the toxic effects of petroleum-based lubricants and the high costs associated with cleaning up spills under environmental health standards, those who manufacture biodegradable oils said.

Ira Pierce, the president of Green Oil Company, a manufacturer of a biodegradable oil used by New York City Transit for several years, said the costs of cleaning toxic oil spills and leaks can run between $15,000 and $20,000.

Mr. Pierce said his company’s lubricant comes from canola oil and is 100% biodegradable. He said the oil, in its natural state, smells like french fries. It is supposed to break down quickly – about 70% within the first month – if it leaks, he said.

“It should biodegrade rapidly,” he said. “If it smells, it won’t smell for long.”

That has not been the experience of New York City Transit, which discontinued using the canola-based oil several years ago because it broke down in high heat and emitted a rancid odor.

The transit authority then switched to a synthetic oil made by Tribology Tech-Lube that is 90% biodegradable. At the Union Square station, however, complaints are made each day about the smell. Cleaners at the station, who felt they had been blamed for the odor, have become unusually outspoken about the way the transit authority has dealt with the problem.

The general superintendent of elevators and escalators for the MTA, Herman Hausmann, said the stench in certain subway stations may be attributed to residual amounts of the canola-based oil in the hydraulic systems of elevators.

“Everybody complains. All you have to do is look at their face,” a transit worker who cleans the Union Square station, Johnny Millan, said. “An hour after we clean, the smell comes back.”

Mr. Millan said he was frustrated that the transit authority had not addressed such an obvious problem.

“I’m tired of this,” he said. “I’m just a little guy working on the floor, and they’re telling me nobody can find a solution?”

Last Friday, Mr. Hausmann’s crew added an “odor mask,” or perfume, into the oil to keep the stink out. A check of the elevator after the odor mask was applied showed that the smell lingered. It’s not clear if the odor mask will work long term.

When a leak occurred a year ago in the elevator pistons at the Yankee Stadium station on the D line, cleaning crews used a three-part cleaner to attack the odor, complete with a degreaser and perfume. Riders, however, continue to gag at the smell.

At other stations, like Borough Hall and 207th Street on the Broadway line, heaters that keep the oils viscous in winter were left on over the summer, releasing the oil’s stench like a fry kitchen on a hot summer day. Mr. Hausmann said he has fixed the problem, which was exacerbated by the heat of summer, but riders say the smell persists.

“It makes me sick,” one rider, Tam Matthews, 24, said late last week after transit officials said they fixed the problem. “I was holding my nose the whole time I was going down the elevator.”

Another rider, Gail Lewis, 50, said she could not describe the smell without crossing the bounds of good taste.

“Oh, please, I can’t tell you,” she said. “It’s disgusting.”

A scientist who manufactures a similar lubricant made from soy beans grown by farmers in Iowa said the smell could be caused by conditions that make the oils decompose. Water or urine turn the oil into sludge.

“Oil will become rancid as it oxidizes,” the president and Chief Executive Officer of Environmental Lubricants Manufacturers Incorporated, Lou Honary, said. “When it biodegrades, it smells like rotten food. A landfill could be compared to some odor like that.”

Mr. Hausmann said the elevators are designed to contain leaks from pistons inside the hydraulic lift so oil does not drip into the floor of the elevator pit and decompose.

When pressed for a reason why the elevator at Union Square emitted such a noxious odor despite having neither a leak nor any residue from the canola oil lubricant in its hydraulic lines, Mr. Hausmann said the smell was nothing out of the ordinary.

“I’ve been there 15 times,” he said. “But that’s the natural smell of the oil.” At any rate, he said, the cool weather will tone down the scent.

To riders, though, the smell – and their reaction to it – may be a little too natural.

“I’m going to throw up,” Azul Marcenaro said with her hand over her mouth as she fled up the nonworking escalators at the station.