People living in the San Joaquin Valley are accustomed to high ozone levels, but air quality is even worse than usual this week.
"We have smoke coming in from various fires around the Sierra, especially the Rough Fire, which is the largest fire in the state, creating particulate pollution," says Anthony Presto, with the San Joaquin Valley Air Pollution Control District.
Presto also says the almond and walnut harvests and the drought, have also reduced air quality.
"Conditions are extra dry because of the drought throughout California, so we have dust in the air, we have a lot of ozone in the air and we have fine particulate matter from smoke," says Presto.
He says people living throughout the eight-county region are being asked to reduce driving.
Presto says the air is "unhealthy for sensitive groups" - which include children, the elderly and people with respiratory illness or heart disease.