There’s no nice way to say it. Delco smells.

For months, a mysterious fuel-like stench has plagued the 200-square-mile county, aggravating residents and confounding government officials. It emerges at unpredictable times, travels between towns, then evaporates as quickly as it arrived, disappearing for days or weeks before returning.

Some compare the elusive odor to gasoline; others liken it to sulfur. No one knows where it’s coming from.

“That’s where the whodunit factor comes in,” said Tim Boyce, director of emergency services for Delaware County. “When you’re dealing with fuel spills, smoke, pipeline incidents, there’s usually some type of starting point. But when it’s in the air, it’s hard.”

Boyce’s department is working with members of the state Department of Environmental Protection and other state and local officials to investigate the odor, which blew through Chester last week. Weather stations have been installed in Chester, and as soon as Tuesday, Boyce’s department plans to roll out more technology to help in the search — paid for with grants and county funds — such as handheld testing meters that can measure chemicals or other ingredients present with the odor.

The county and state also will launch monitors to establish a baseline in the area for airborne contaminants, a step that could help determine whether the smell is harmful. And Boyce’s department is using Weatherlink, an app that collects real-time data on wind, that may help them identify the direction the odor is traveling and even where it’s headed next.

“The challenge we’re up against when you’re on the ground level is, the wind can be blowing in different directions, depending on where you are,” Boyce said. “By the time someone gets to where the report came from, it might not smell anymore.”

The smell has largely been reported southwest of Philadelphia International Airport, north of Marcus Hook, and in the communities along I-95, including Chester, Aston, Middletown Township, and Brookhaven. But it’s also been sniffed as far away as New Castle County, Del.

Chester Fire Commissioner William C. Rigby said his department has fielded dozens of reports since the summer, and many more over several days last fall when the stench was most pronounced. In October, administrators at Chester Community Charter School evacuated hundreds of students when the overpowering odor seeped into the ventilation system.

“At first we were really the only ones dealing with this, but now it’s been filtering into the rest of the county,” said Rigby.

The DEP, meanwhile, is looking at local industrial facilities and pipelines in the area in an effort to eliminate possible suspects. A list of potential culprits has included areas in Gloucester County, which sits directly across the river. Further confusing matters, the mystery smell emerged one day last November at the same time as an unrelated leak from a Sunoco pipeline in Middletown.

“A lot of the folks who are working on this have never seen anything quite like it,” said Virginia Cain, community relations coordinator for the state DEP. “Something that comes and goes, seemingly at random. Usually with a case like this, we’d normally get repeated complaints from the same group of people. But seeing it in such a widespread area makes you wonder: Is it coming from a facility? Or something that’s moving?”

Though she’s a Delaware County resident, Cain has never smelled the odor.

“But I want to,” she added.

Boyce said that some residents of Norwood, a borough with a population of under 6,000 that sits a few miles from the airport, have worried that the smell could be related to their town landfill. Boyce believes that’s unlikely, due to the petroleum-tinged nature of the odor.

One late afternoon last week in Norwood, the air smelled like a typical winter day, without even a hint of petroleum.

Several residents said they’d never smelled the odor. Others said they didn’t think it was worth worrying about.

“I’ve heard people say it’s like an oily smell,” said Dawn Hamilton, a clerk at Davis’ Trading Post on Winona Avenue. “But if you live in this area, you get used to stuff like that. This isn’t the country, it’s the city. I’ll take the petroleum smell over cows.”

Ridley Park resident Bill Smith said the odor in his town had been “pungent” at times over the fall. He said the scent reminded him of fuel, but didn’t irritate his throat, and in any case, he hasn’t smelled it for a few months.

“As long as it’s not going to hurt anyone, who cares?” he asked.

Boyce believes it’s only a matter of time before investigators zero in on the culprit. If it turns out a company is behind the stench, the case may end up with the Attorney General’s Office.

Boyce encouraged residents to keep the reports coming: If you smell something, say something.

“‘Don’t worry about it’ is not an answer that people should have to accept,” Boyce said. “No one should be accepting of gas fumes rolling through the county.”