Smoky air conditions are expected through Sunday, according to the Bay Area Air Quality Management District, and residents are encouraged to stay inside with the windows closed and set air conditioners to recirculate until the conditions subside.
District officials issued a late Spare the Air alerts for Thursday and Friday because smoke from wildfires, which was over the Pacific Ocean, is blowing into the area.
The alerts are the ninth and 10th of summer Spare the Air season.
Residents of the Bay Area are urged to avoid exercising outdoors because of "unhealthy" air, officials said.
Forecasters had expected the smoky conditions to subside, but that forecast changed because the marine layer, a shallow cool layer of air near the ground, has been deeper than anticipated.
Smoky air at higher altitudes expanded to the ground level due to that marine layer and strong onshore winds, according to district officials.
These conditions particularly impact children, the elderly and anyone with respiratory problems, so anyone in those groups should take precautions to avoid exposure.
In general, smoky conditions can cause irritation of the eyes, throat and sinuses. Coughing and wheezing can also occur, especially for anyone with asthma or emphysema.
Real-time air quality readings from throughout the Bay Area are available online, courtesy of the air district.
