Evacuation centers were set up at the Dayton Convention Center and the University of Dayton Arena, about 20 miles northeast of Miamisburg.
Bruce Wagman, an officer at nearby Wright-Patterson Air Force Base and an emergency-room-qualified nurse, was working as a volunteer at the downtown convention center.
"They've got 2,500 people rolling in here," Wagman said. "A lot of these people need eye irrigation; many of these people have chemical burns on various parts of their bodies, mostly their arms and legs."
The Miami Valley Regional Transit Authority used at least a dozen buses to evacuate nursing homes and transport the handicapped.
Shoppers and workers at the busy Dayton Mall were ordered to leave as the fumes neared.
"They gave an out-and-out order, and security guards and sheriff's deputies started pushing everyone toward the exits," said Wagman, who was at the mall at the time.
The Ohio State Patrol said that Interstate 75 was open to through traffic only, and that motorists were not allowed to use exits into the communities being evacuated.
There were no signs of panic, although the roads leading out of the towns were jammed with cars.
Spokesmen at six hospitals said that none of the 78 people treated required hospitalization. Most were treated for shortness of breath, irritated eyes and sore throats.
Franks speculated that the phosphorus had been smoldering all day, even though firefighters had been flooding it with 5,000 gallons of water a minute, and that the tank car finally became too hot.
At mid-evening, John Weithofer, Miamisburg's assistant city manager, said firefighters had decided to let the fire burn itself out while continuing to flood it with water.
"When it will burn itself out, I have no earthly idea," he said.
The explosion occurred shortly after 6 p.m. as firefighters tried to put out the phosphorus fire, which had reignited in the tanker. In addition, a nearby tank car carrying sulfur was burning, officials said.
Earlier in the day, officials had said they feared the fire would spread to the car carrying the sulfur, which can form highly toxic compounds in combination with phosphorus.
No firefighters were injured in the explosion, said Elsie Griffin, a dispatcher for the Jefferson Township Fire Department.
Because fumes engulfed the wreckage, firefighters were pulled back from the scene, and unmanned pumpers were being used to drench the area.
On Tuesday, about 17,500 people were evacuated from their homes after the train derailed and the phosphorus caught fire. More than 250 people were treated for nausea, irritated eyes and other symptoms. None was seriously injured.
Phosphorus, a highly corrosive chemical, is used to make rat poison, detergent, food chemicals and weapons.
A spokesman for CSX Corp., parent firm of the Baltimore & Ohio, said the company did not know why the train derailed.
"The train was going 45 miles an hour and that's the speed for this track," said John Fields, manager of casualty prevention for CSX.
Dale Hawk, division manager for CSX transportation, said that the four crew members were tested for drug and alcohol usage - standard procedure after an accident - and that all had tested negative.