No employees were believed to have been inside the EQ Industrial Services plant when it was rocked by the blasts late Thursday. Officials said 41 people went to emergency rooms, most complaining of breathing problems, but nearly all had been released by midday.
A timely morning rainstorm helped scrub the air as firefighters cautiously approached the chemical fire. But the mayor warned that he would not give the all-clear until the fire was out.
"We're not going to have people return to their homes only to have another mishap," said Mayor Keith Weatherly.
The area near the plant smelled of "burnt rubber," reports Melissa Buscher for CBS affiliate WRAL-TV in Raleigh, and noticed smoldering debris and bent-up metal on a nearby street.
The plant handles a variety of industrial wastes, includes paints, solvents, pesticides and weed killers.
Because of the dangers in that mix, firefighters waited for daybreak to determine how to attack the blaze, and used a video camera to get a look at the burning building. Schools were closed, along with downtown Apex.
The cause of the blaze was not immediately known.
"Because of the many different types of waste that we bring in, it's very difficult to determine the cause of the fire," EQ spokesman Robert Doyle said.
About 25 employees work at the plant, but all had left the building by 7 p.m. Thursday, he said.
In March, the state fined EQ $32,000 for six violations at the plant, including failing to take steps to "minimize the possibility of a sudden or non-sudden release of hazardous waste ... which could threaten human health or the environment."
But the state said the company had passed a required inspection as recently as Sept. 28-29.
The mayor said it was unclear how many people fled, but several hundred took shelter at schools, while others probably stayed with friends or at hotels.
Beth Roach's family left their home about two miles from the plant around 3 a.m.
"The bad part was that we didn't really know what was going on," she said. "Ultimately, that's what made our decision to leave."
Cory Cataldo said he and his wife and two young sons were awakened around 1 a.m. by a knock at the door, and a man told them to evacuate because of a chemical fire.
"That's about all I needed to know," said Cataldo, who said his wife and sons have asthma. "My first concern was just to get everybody out."
Wake County officials said 41 people with minor injuries came to area emergency rooms, including 13 emergency workers. All but a handful were treated and released by midday Friday.
White House deputy press secretary Dana Perino said the Environmental Protection Agency has sent three of its personnel, eight EPA contractors and two toxic specialists to the scene as well as air-monitoring and emergency response equipment.