NEW YORK (CBSNewYork) — An unsettling and mysterious smell on the L train is provoking rider complaints, concerning transit unions, and forcing Metropolitan Transportation Authority workers to don masks meant for terror attacks.

The MTA says the problem is fixed following CBS2’s initial report on the odor sickening straphangers on Tuesday, but a slew of commuter tweets tell a different tale.

The transit union released a picture of an MTA agent wearing a mask over her head meant to protect her during a chemical attack. In this case, it was to ward off the oil or gas-smelling fumes along the L train.

“It smells like that rubbery, burning smell,” one person said. “It just smells awful.”

The smell was especially strong at the Graham Avenue station in Williamsburg on Tuesday. The station was evacuated, but later reopened. Then on Wednesday, commuters once again complained of headaches and sore throats.

“The smell on the L train is now disgusting,” one person tweeted. “I thought this was already cleaned up?”

“Great can’t take the L train,” tweeted another. “Apparently it smells like toxic fumes and making people light headed and vomit.”

The cause seems to have been determined — sort of.

“We don’t really totally know what it is but it appears to be some kind of oil, that seeped up through the ground,” MTA Chief Safety Officer Patrick Warren said. “It’s not clear exactly where it came from so that’s what it is.”

The MTA deployed heavy duty fans to help circulate the air away from the stations. City and state agencies agree the air is safe, a point emphasized by Warren in an above-ground press conference on Wednesday.

So are people really safe, even if people are feeling sick?

“It is absolutely safe for our passengers and workers to be down here,” Warren insisted.

Transport Workers Union Local 100 says at least four employees have been hospitalized, including two on Wednesday who were taken to the hospital by ambulance.

“From headaches to can’t breathe,” union representative Lynwood Whichard said.

TWU says some members even brought their own paper masks to protect themselves. The MTA allowed others to use the heavy duty ones, but they’re in limited supply.

“There is only one per booth, so it doesn’t have enough to accommodate any additional employees,” Whichard said.

Despite reassurances the air underground is safe, commuters remain doubtful.

Along with the fans, the MTA is using absorbent material to soak up the odor causing oils. The agency still can’t say for sure where the fumes are originating from, but encourage anyone with health concerns to go see their doctor.