Air quality in a Northern California city has turned unhealthy as firefighters continued for a third day on Saturday to contain a large blaze at a medical equipment warehouse.

Authorities in Tracy, a city of around 100,000 residents, urged people sensitive to smoke to stay indoors as thick smoke spread from the Medline warehouse fire, which has been burning since Thursday. Air monitoring stations in the southern part of the city recorded pollution levels in the “unhealthy” range.

Officials said smoky conditions are likely to continue over the next few days as crews work to fully extinguish the fire inside the massive facility, according to South San Joaquin County Fire Authority Fire Chief Randall Bradley.

Investigators, including the local fire marshal, are examining the cause of the blaze. Fire officials are also coordinating with company representatives and structural engineers to assess damage to the building.

Plans are being made to allow employees to safely retrieve their vehicles from the site. Authorities have urged the public to stay away from the area and warned against touching or moving debris.

Medline, the company operating the warehouse, is a major supplier of medical and surgical products, including gloves, masks and surgical instruments.

The fire broke out at the nearly 1 million-square-foot warehouse, located about 55 miles (88.5 kilometres) east of San Francisco, sending embers across a wide area.

Firefighters said their efforts were hampered by malfunctioning sprinklers and hydrants with little or no water flow. They believe the problem lies within the facility’s internal fire-suppression system, not the city’s water supply.

Officials added that an external inspection of the sprinkler system in January had found no problems.