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Heavy smoke from the Alisal Fire near Santa Barbara was moving southeast, prompting an air quality advisory for the region through Thursday afternoon.

Areas of Los Angeles, Orange, Riverside and San Bernardino counties could experience poor air quality due to smoke from the wildfire, the South Coast Air Quality Management District warned.

“The smoke will move further inland, and elevated AQI levels are expected in the entire South Coast Air Basin on Wednesday evening throughout Thursday afternoon,” AQMD officials said in an alert.

Smokey conditions are expected along the Santa Barbara south coast and in coastal Ventura and L.A. county areas by Thursday afternoon, according to the National Weather Service.

L.A. and Orange counties were both expected to see air quality levels that are unhealthy for sensitive groups.

Breathing in fine particulate matter can lead to heart attacks, asthma aggravation, decreased lung function, coughing or difficulty breathing. It may also lead to premature death in people with heart or lung disease.

Residents in affected areas were advised to close all windows and doors, and run their air conditioners or air purifiers.

As of Thursday morning, the Alisal Fire had burned 16,801 acres and was just 5% contained after breaking out Monday on a ridge south of Solvang, along the south coast of Santa Barbara County.

More than 1,300 firefighters were battling the blaze Thursday, planning to make water drops from aircraft all day to support crews defending structures on the ground.

Shifting winds pose a challenge for firefighters as they work to contain the fire threatening more than 100 homes, ranches and other buildings.