A voluntary evacuation notice was issued Saturday morning for people living in 44 homes near a railroad tanker car that released a hazardous chemical the night before, sickening 23 people.

The notice, which affected homes within 1,500 feet of the railroad car, was issued because the pressure inside the car, carrying styrene, could be increasing, said Jamie Turner, director of the Delaware Emergency Management Agency.

"We don't expect anything to happen, but we want to err on the side of safety," Turner said.

No chemicals have been released since around midnight, Turner said. The cause of the release remained unknown.

The car is parked at the Dow Reichhold Specialty Latex plant, which was evacuated Friday evening. Twenty-three people were treated for flulike symptoms and released.

Residents within a mile of the leak were urged to remain in their homes Saturday morning with their doors and windows shut and the air conditioning off, said Rosanne Pack, an agency spokeswoman. On Friday night, people living within five miles were asked to stay indoors.

Victoria Brown, 44, who lives within about a mile of the plant with her mother and two brothers, said they spent the night in cars parked outside Dover High School, where the American Red Cross has set up a shelter.

"They just had the little floor mats," Brown said. She said she was told cots would be brought to the shelter.

The internal temperature of the car is rising and is related to a natural chemical process called polymerization, Turner said.

The evacuation notice was issued as a precaution because the pressure inside the car could be increasing, as well, Turner said. The pressure release valve on the car apparently is not working.

"There's no way to inhibit or halt the process," Turner said.

Several roads were closed after the release, and a section of state Route 13 remained closed Saturday morning, according to the Delaware Department of Transportation.

Reichhold site manager Michael Galbus said the company would make a statement Saturday.

Styrene is used by chemical companies that make plastics, synthetic rubber, resins and insulators, according to the Environmental Protection Agency's Web site. Styrene vapor irritates the eyes, the nose, and the throat and can adversely affect the human nervous system.