An air quality advisory will remain in effect for the Bay Area through at least Monday, as smoke from wildfires burning in far Northern California drifts into the region.

The Bay Area Air District initially issued an advisory on Thursday, prompted by fires burning in the northern Sacramento Valley. On Saturday, officials extended the advisory for two more days.

"Smoke from the Butler and Green wildfires is still moving down the Sacramento Valley and is expected to continue to impact air quality in portions of the Bay Area on Sunday and Monday. Conditions can change rapidly and the amount and location of smoke at ground level is hard to predict," the agency said in a statement.

According to officials, areas expected to have the most impacts are higher elevations in parts of the North Bay and East Bay.

The Butler Fire, which merged with another fire and has since been renamed the Orleans Complex Fire, has been burning since July 3 in Siskiyou and Del Norte counties. According to the U.S. Forest Service, the fire has burned more than 5,843 acres and is 0% contained as of Saturday.

Containment of the Orleans Complex is not expected until July 26. Officials said the cause of the fire is under investigation.

Meanwhile, the Green Fire was started by lightning on July 1 and is burning in the Shasta-Trinity National Forest about 13 miles east of the town of Lakehead in Shasta County. As of Saturday, the fire has burned 8,395 acres with 0% containment, the Forest Service said.

Air quality officials said exposure to wildfire smoke is unhealthy even for short periods of time, due to fine particulate matter and other harmful pollutants. The elderly, children and people with respiratory illnesses are particularly vulnerable.

If exposed to smoke, people are urged to go indoors if possible and if temperatures allow. The use of indoor air filtration or going into a location with filtered air, such as a library or mall, can also help reduce smoke exposure, officials said.

The public can check for real-time smoke pollution levels by checking the EPA's Fire and Smoke map by visiting https://fire.airnow.gov/.