Another forecast of unhealthy air prompted the air district to issue the health advisory for the seventh time this season.
The air district has also issued a health advisory due to wildfire smoke.
"The current air quality is unprecedented," Air District Executive Officer Jack Broadbent said in a statement. "We historically have experienced days of high smog, but the combination of ozone and smoke in the air is creating uniquely unhealthy conditions."
Officials are warning residents that vigorous outdoor exercise should only be done in the early morning hours when ozone concentrations are lower, and are reminding residents that ozone pollution is especially harmful for young children, seniors and people with respiratory and heart conditions.
Bay Area transit agencies will not offer free transportation Wednesday.
