According to AIRNow, which monitors the Air Quality Index, conditions in Vacaville were hazardous in the morning.
The air quality in Davis, Rio Vista, Vacaville and Woodland was forecasted to be unhealthy on Wednesday.
The rest of the region, from Sacramento down to Modesto, was expected to be moderate to unhealthy for sensitive groups. Sensitive groups include children, the elderly and people with lung disease.
Officials cautioned the public to limit outdoor activity.
A slight increase in air quality is expected Thursday.