Firefighters on Friday contained a four-alarm fire at a recycling center in Newark.

The blaze started at about 3:35 p.m. on the 6500 block of Smith Avenue. Police evacuated nearby businesses when responding to the fire atthe American Huanjia Group recycling plant. No injuries were reported.

"I see this big cloud and I think, 'Oh my God, is that a big rain cloud?'" said Ryan Duba of Newark. "And then, 'No.' I thought we got bombed or something."

Firefighters on Friday contained a large two-alarm fire at a recycling center in Newark.

Roughly 5,000 households in the area downwind — south of Central Ave and west of Interstate 880 to Auto Mall Parkway — were advised to shelter in place, according to Aisha Knowles of the Alameda County Fire Department.

Thick, black plumes of smoke were seen shooting into the sky while the fire burned. Bay Area residents commenting on the NBC Bay Area Facebook page said the smoke was visible even in San Jose and Palo Alto.

"It started off small and then started getting a lot bigger," said Matthew Martinez of Newark.

Ninety firefighters, some from Hayward, Piedmont, Livermore-Pleasanton and Fremont, contributed to the roughly two-and-a-half hour effort to douse the flames.

Firefighters respond to a two-alarm fire burning in Newark. (July 8, 2016)

Newark police urged residents to avoid nearby Cherry Street between Mowry and Central avenues.

Lisa Fasano with the Bay Area Air Quality Management District said health inspectors were dispatched to the scene to check on air quality.

Fasano advised residents who see smoke in their neighborhood and have a history of respiratory issues to stay inside and close their windows. They also asked people to turn off their air conditioners.

Although officials believe any toxins were destroyed by the flames, they plan to monitor the air overnight.

"No smoke is good to breathe so we always take that precaution," said Jim Call, deputy chief of the Alameda County Fire Department.

Two-Alarm Fire at Newark Recycling Facility

The roaring inferno closed Smith Road for more than six hours and kept dock and rail workers from their jobs. Rail shipments will be delayed tonight, they said.

Dozens of neighbors came to watch the fire, but many were concerned about the smoke, coming from a site loaded with metal, plastics, and other recyclables.

Newark resident David Puentes said he was "kind of hoping it's not real poisonous because we noticed sitting down in our living room [that] it suddenly got dark out and just thought it was overcast."

Firefighters don't yet know what caused the fire. Crews continued mopping up the charred materials late Friday and said it promised to be a long night.

Bay City News contributed to this report.