The fire broke out about 5:30 p.m. Wednesday on the 2700 block of Pittsburg-Antioch Highway, an industrial area north of Highway 4 and south of the Dow Chemical Plant.
Investigators say a candle in a tent ignited the blaze in a homeless encampment along old railroad tracks near the road.
Driven by strong winds, the fire engulfed the tent, spread to nearby brush and into a junkyard and garden business. It sent out billows of smoke, which eventually led the county to issue a health advisory that lasted 10 hours into Thursday morning.
"You could see the smoke from very far away," said Robert Marshall, Contra Costa County Fire District fire marshal.
The smoke shot up into the air and drifted downwind over the Sacramento-San Joaquin Delta, possibly as far east as Isleton, some 20 miles away, according to Marshall.
"It was a lot of very thick, black smoke that was choking the air," Marshall said.
While the smoke lasted for hours and prompted health concerns, Sawyer said it did not affect residential areas.
The fire burned tires, motor oil and plastic among other products, according to county officials.
County health crews took readings at 15 locations near the fire, using hand-held air monitors. Some of that data will be sent to the Bay Area Air Quality Management District, which is investigating the fire to determine if any air violations occurred, according to district spokesman Ralph Borrmann.
Marshall said the fire is the latest in a series of similar blazes ignited near homeless encampments along railroad tracks between Pittsburg and Brentwood.
Crews have likely responded to scores of fires along that corridor -- most of them small -- in the last year or so, according to Marshall.
"We do get a lot of fires in the right-of-way either from cooking fires that people see and they report, to fires that escape like this one. It is something that happens quite frequently," he recalled.
Sawyer said smoke from Wednesday night's blaze most likely affected people living in some of the nearby encampments.