Heavy smoke from wildfires in Sonoma and Napa counties prompted a smoke advisory throughout the Bay Area, with air quality readings in some areas well above the level considered “very unhealthy,” the region’s air board said.

The Bay Area Air Quality Management District said a monitor in Napa recorded an air quality index of 442 Monday morning, with a high of 292 in San Rafael. Air quality indexes of 201 to 300 are considered “very unhealthy,” said Kristine Roselius, a district spokeswoman.

Late Monday morning, Napa was at 159 and San Rafael at 164, she said.

The district has no monitor in Santa Rosa.

Air quality above 100 is considered “unhealthy for sensitive groups,” and from 151 to 200 is unhealthy.

Roselius said she could not recall another time in which air quality indexes were so high for such a sustained period.

Particulate matter in smoke can irritate the eyes and airways, which can cause coughing, a scratchy throat and irritated sinuses, the district said. Smoke causes a greater risk to the elderly, children and people with respiratory illnesses.

“If it smells smoky or looks smoky, it’s a good time to stay indoors and especially to avoid exercise,” Roselius said.

Setting air conditioning to recirculate air rather than pull in outside air also is recommended, she said.

Breezy conditions, with north winds gusting more than 30 mph, continued through Monday morning, but were expected to ease overnight and shift to a flow from the south, National Weather Service meteorologist Roger Gass said.

Temperatures in the Sonoma and Napa county fire area hit the mid- to upper-80s Monday and were expected to cool by about 10 degrees Tuesday, he said.

Humidity was in the teens Monday but was expected to rise to 60 to 85 percent overnight, especially in North Bay valleys, while the hills remain drier.

Smoke will linger in the air in the two counties, especially to the west of the fires, Gass said.