Two explosions were reported overnight at the chemical plant in the town of Crosby and officials have ordered residents within 1.5 miles (three kilometers) of the facility to evacuate.
"The bottom line is that we do what's called plume modeling," FEMA Administrator Brock Long said at a press briefing. "And that's what we base a lot of the evacuations on.
"And so by all means, yes, the plume is incredibly dangerous."
The plant operated by Arkema Inc produces organic peroxides, which have a number of commercial uses, including plastics, pharmaceuticals and construction materials.
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"There has been intermittent smoke, please stay clear of the area," it said.
The Harris County Sheriff's Office said one deputy had been taken to the hospital after inhaling fumes from the plant, with nine others drove themselves there as a "precaution."
In a later tweet, the sheriff's office said the smoke inhaled by the 10 deputies near the plant in Crosby was believed to be non-toxic.
Arkema released a statement saying "exposure to organic peroxides may cause eye, skin and/or respiratory irritation."
Crosby lies about 25 miles (40 kilometers) northeast of Houston.