Officials with the Bay Area Air Quality Management District issued the third consecutive Spare the Air alert in the San Francisco Bay Area for today.
Commuters are urged to find alternatives to driving alone to improve today's air quality.
BAAQMD officials are asking commuters to consider taking transit, carpooling or working from home.
"Unhealthy air quality for a third consecutive day underscores the need for reducing traffic on Bay Area roads," BAAQMD Executive Director Jack Broadbent said in a statement.
Spare the Air alerts are issued when smog is forecast to reach unhealthy levels.
Smog can cause throat irritation, congestion, chest pain, trigger asthma, inflame the lining of the lungs and worsen bronchitis and emphysema.
It's particularly harmful to young children, seniors and people with respiratory and heart conditions.
Long-term exposure to smog can reduce the functioning of someone's lungs.
Today, outdoor exercise should be done only in the morning when there is less smog in the air.