The three-alarm fire was burning on River Avenue just south of the 10 Freeway at a pallet yard where 55-gallon drums were also being consumed by flames.
The exact nature of the materials on fire was not immediately known, but a hazardous materials team was responding to the scene,
By around 3:30 p.m. firefighters had extinguished most of the visible flames but smoldering materials continued to emit thick clouds of smoke.
The cause of the fire remains under investigation. No major injuries were reported but one firefighter was treated for heat-related illness.
Local residents and people who worked in the area were astonished at the sight.
"I just saw the cloud, the big black smoke covered the blue skies," said Henry Su of Rosemead. "You couldn't' really see the sky anymore. it was pretty alarming."