Another train car leaked chemicals in Ohio. Here’s what to know about it.

Officials were working Wednesday to assess the scene the day after a chemical leak from a railcar prompted the evacuation of hundreds of residents in southwestern Ohio.

It was unclear what had started the incident, but by Wednesday morning, the railcar was “no longer venting,” John Keffer, the general manager of Central Railroad of Indiana, which owns the railway, said during a news conference. “We are still working with the professionals through this process to ensure the safety of the community as well as the emergency responders.”

On Tuesday, plumes of white vapor shot out from a pressure release valve of a railcar, leaking styrene - a toxic chemical - that saturated the area in a pungent odor midday. Firefighters in Whitewater Township were notified at 12:46 p.m. about an unusual odor and determined the leak originated from railcar located west of Cincinnati, Mike Siefke, chief of the Little Miami Joint Fire and Rescue District, said at a news conference Tuesday.

Officials from the railroad said they are unsure what started the incident.

“We are working as safely as possible to mitigate this as quickly as possible. We understand the inconvenience,” said Tom Ciuba, a media spokesperson from Central Railroad of Indiana.

Styrene is a toxic, flammable liquid mostly used in plastics and rubber manufacturing. In high concentrations, it can disrupt the nervous systems of those exposed to it, causing fatigue and issues with concentration and balance, according to the Centers for Disease Control and Prevention’s Agency for Toxic Substances and Disease Registry. Officials in California list styrene as a carcinogen.

Emergency services at the scene began to cool down the railcar by blasting it with water, Siefke said. Cooling efforts continued throughout Tuesday night, Ciuba told The Washington Post. Videos showed fire trucks hosing the train car as the railcar sat on the tracks. The evacuation order covers a half-mile radius from the leak location, including parts Hooven and Cleves, which have a combined population of more than 3,800 people. Hamilton County Emergency Management said that within the half-mile radius, there is still potential for exposure to toxic chemicals and potential for an explosion.

Residents outside the affected area in Cleve were asked to shelter in place to avoid exposure to the styrene.

Last year, a disastrous train derailment sent a massive plume of vinyl chloride billowing over East Palestine, Ohio - 300 miles away on the other side of the state. The incident in Cleve is the latest in a string of accidents tied to mass transportation and potential environmental health disasters.

Liz Hitchcock, the director of Toxic-Free Future’s federal policy program, an environmental health research and advocacy group, said accidents involving the transportation of cancer-causing chemicals in plastics are “a ticking time bomb.”

“The event in Ohio is the latest devastating result of using cancer-causing styrene to make plastic for everything from packaging to electronics,” Hitchcock said. “Just like the East Palestine vinyl chloride disaster, toxic chemical accidents are a serious threat to the people who live along the rail lines and near the manufacturing facilities.”

The solution, she said, is to stop making and transporting cancer-causing chemicals used to produce plastic products in large quantities

Siefke said there have been no reported injuries. Ciuba said air quality reports have not indicated cause for concern.

For longtime residents of southwestern Ohio, the incident may bring back memories of a railcar styrene leak in Cincinnati in August 2005, which took more than two days to resolve and prompted a class-action lawsuit.

Siefke said the process for mitigating the risks of Tuesday’s railcar leak could take some time. “We’re really at the mercy of the product and the material, and we’re taking it very methodically,” he said.