Residents are encouraged to drive less and use public transit when air is especially bad, although transit will not be free today.
"We are asking Bay Area residents to take appropriate action (today) to protect their health and preserve our air quality," the air district's Executive Officer Jack Broadbent said in a statement.
Motor exhaust, industrial emissions, gasoline vapors and household chemicals contain volatile organic compounds and nitrogen oxides that when combined with oxygen in the heat can form ground-level ozone.
Breathing ozone can cause throat irritation, congestion and chest pain. Ozone can also trigger asthma, inflame the lining of the lungs and worsen bronchitis and emphysema, according to air quality officials.
Long-term exposure to ozone can reduce lung function.
There is no free transit today because the air district and the Metropolitan Transportation Commission are sponsoring one free transit day this year on June 19, to allow new riders the opportunity to plan transit into their commutes.