A chemical gas leak from a rail tanker near Cincinnati prompted fears of an explosion and evacuation orders for people within at least a half-mile radius of the incident.

Roughly 210 households in Whitewater Township, about 22 miles west-northwest of Cincinnati in an area near the city airport and the Kentucky state line, were under evacuation orders, officials said Tuesday night. Other residents were under shelter-in-place recommendations, they said at a news conference.

Colorless, odorless gas spewing from the tanker at State Route 128 and U.S. Route 50 was reported to first responders shortly after 1 p.m., Chief Mike Siefke of the Little Miami Joint Fire and Rescue District said at a pair of news conferences.

Authorities determined the chemical is styrene, he said. It's used in the production of plastic, rubber, fiberglass and other structural materials.

The chemical can irritate the respiratory system, cause headaches and disorient those who breathe it, according to the Centers for Disease Control and Prevention. Long-term exposure has been associated with some forms of cancer, the CDC says, but it's not known as a direct killer.

The threat for the community is that the rail car tank has been heating up and will explode if it continues, Siefke said. Firefighters were dousing the container with water in an attempt to reverse its temperature rise, he said.

“This will be a long, long event,” Siefke said.

Environmental officials were taking parts-per-million measurements in the community to determine the leak's impact, he said.

Some residents may have sought treatment for unknown ailments, but the number of patients and the exact nature of their potential injuries was unconfirmed, he said.

Area public schools, the Three Rivers Local School District, shut down instruction early Tuesday and canceled instruction and all activities scheduled for Wednesday, according to the district website.

It wasn't yet clear who owns the rail car or its cargo, officials said Tuesday night. A spokesperson for Central Railroad of Indiana said in a statement that it was cooperating with first responders.

State Route 128 and U.S. Route 50, which takes motorists from coast to coast, were shut down in both directions near the site, the state Transportation Department said.

Whitewater Township and Hamilton County officials said the situation is likely to remain static overnight as they await input from environmental agencies.

"We're waiting for the partners that are mitigating this to come up with a strategy," Hamilton County Communications Director Andrew Knapp said.