Smoke from wildfires in Northern California and Oregon arrived in the Bay Area Thursday, reducing visibility in some places and increasing concentrations of soot as the region braces for several days of intense heat and smog.

“The smoke has been moving toward us for several days and now it’s here,” said Kristine Roselius, a spokeswoman for the Bay Area Air Quality Management District.

The smoke was thickest in the East Bay and North Bay on Thursday, but the smoke also was drifting into San Mateo and Santa Clara counties in the afternoon, the air district reported.

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The highest soot concentrations in the Bay Area were measured midday at Concord and Livermore, where the smoke was so thick as to be labeled unhealthy for the general population to breathe.The air improved slightly there over the afternoon to a level where it was considered unhealthy for sensitive individuals to breathe.

In response, the air district issued a smoke advisory urging people who smell the smoke to limit time outdoors, close doors and windows, and set air conditioners and car vents to recirculate.

The smoke prompted many East Bay residents to call local fire local departments to ask if the skies were dirty from local fires, but were told it was mostly from wildfires far away.

“These are unusually high readings for the Bay Area,” said Daniel Alrick, an air district meteorologist. “But the readings can change with shifts in the plume and wind direction. We advise the public to check air quality readings before deciding whether to pursue outdoor activities.”

In addition to the smoke problems, Roselius said unhealthy smog concentrations are expected to reduce visibility in Bay Area skies as well this week.

The wildlfire smoke contains gases that turn into smog, which irritates lungs and eyes, as well as tiny soot particles that can lodge deep in the lungs and escalate the risks of asthma attacks and strokes.

“If you smell the smoke,” Alrick said, “you’re likely to be affected by it.”

The smoke is drifting in from a series of wildfires along the Oregon and California border, and more recently from a wildfire near Oroville.

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How thick the smog gets and where it drifts will depend on wind and weather conditions that can change daily, officials said, but added it’s possible the smoke will stick around at least through Saturday.

Also Thursday, the air quality district issued a Spare the Air alert for unhealthy smog levels predicted on Friday for the Bay Area. This is the second day in a row with an alert.

“Extreme temperatures, like those we are expecting this weekend, require caution,” said Jack Broadbent, the air district executive officer. “To protect your health, avoid exercising or outdoor activities during the hottest part of the day and follow the advice of local health officials to find cooling centers in your area.”