Officials identified the plant involved as INEOS.
Houston TranStar cameras captured a large explosion at the plant, located at 3503 Pasadena Freeway, at about 12:07 p.m.
Cameras capture large explosion and massive fire burning at Pasadena plant along Highway 225
In an afternoon update, Harris County Commissioner Adrian Garcia described the incident as a "bleve explosion" involving over-the-road tanker trucks, not rail cars.
"When you have a liquid petroleum or a liquid propane inside a tank, the fire that occurred outside can heat the tank to the point that the inside is boiling and the pressure is so great that it overcomes the release mechanisms, the safety valves, and the container holding it explodes," Harris County Fire Marshal's Office Chief Investigator Mitchell Weston said when asked to describe a bleve explosion.
A tanker reportedly filled with liquid petroleum gas was transferring the chemical to a storage tank through a hose.
Garcia said as a result of the process, something went wrong, and the explosion occurred.
The fire marshal's office will investigate the original source of the fire.
"At this time, we really don't know," Capt. Chad Shaw with the HCFMO said. "Any time you have a transfer of chemical like that, if there's any type of vapor, it can find an ignition source anywhere."
WATCH: Officials give update after reported explosions caused large fire at INEOS plant in Pasadena
Officials give update after reported explosions caused large fire at INEOS plant in Pasadena
One person was taken to the hospital, according to Harris County Sheriff Ed Gonzalez. At last check, Garcia said that person was stable.
There's no word on the individual's identity or the nature of any injuries. Everyone at the plant was accounted for, officials said.
The Houston Fire Department also tweeted that it performed "mutual aid" to crews at the scene.
Beltway 8 reopened at Highway 225 after it was closed out of precaution. The Sam Houston Tollway Ship Channel bridge has also reopened in both directions.
SkyEye video showed several people standing in an open space nearby as the flames burned and what appeared to be smoking tanks with burn marks surrounding the facility.
In a written statement, Harris County Judge Lina Hidalgo said no pollution was reported in the water and there do not appear to be any remaining hazardous material leaks.
According to the company's website, INEOS produces a number of products relating to oil and gas, chemicals, and polymers.
No impacts to the surrounding area were reported, according to officials. Hazmat crews responded to the scene and monitored the area.