The evacuation was for 210 homes in Whitewater Township, Ohio, 22 miles northwest of Cincinnati near the Kentucky state line. Residents outside of the evacuation radius are under shelter-in-place recommendations.

The chemical was styrene, a toxic, flammable gas used to make plastic, rubber, fiberglass, and other materials. The leak occurred around 1 p.m. at State Route 128 and U.S. Route 50, polluting the air and causing officials to test the air quality in the area.

When inhaled, styrene can cause headaches, nausea, disorientation, and respiratory irritation, and some long-term exposure can lead to organ damage. On Wednesday morning, Central Railroad of Indiana spokesman John Keffer said the railcar was no longer venting, and, according to local reports, the styrene is forming up and firming.

The threat of an explosion comes from the railcar’s rising temperature, prompting firefighters to continuously douse the tanker in an effort to cool it down. The tanker is still intact, as the leak was not caused by a derailment but rather by an open pressure release valve.

Public schools in the area shut down Tuesday and canceled instruction Wednesday, but the situation has stagnated as concerns escalated with evacuations. State Route 128 and U.S. Route 50 were also shut down in both directions around the site.