APEX, N.C. ó As many as 17,000 people were urged to flee homes on the outskirts of Raleigh early Friday as flames shot from a burning hazardous waste plant and a chlorine cloud rose high over the area.

No employees were believed to have been inside the EQ Industrial Services plant when the fire started late Thursday and a series of explosions began rocking the property.

Eighteen people, many of them law enforcement officers, were taken to emergency rooms with respiratory problems, hospital officials said.

EQ Industrial Services handles a wide array of industrial waste, from paints to solvents, and houses chemicals such as chlorine, pesticides, herbicides, sulfur and fertilizer.

Because of the potential dangers in that mix, firefighters waited for daybreak to determine how to attack the blaze, officials said. Friday morning, area schools and downtown Apex were closed, and police blocked off streets into the area as the plant continued to burn.

"You can't put foam or water on it," Mayor Keith Weatherly said. "That just exacerbates it."

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