OAKLAND, Calif. -- The fifth Spare the Air Day in the Bay Area for this year's summer season was announced for Friday since unhealthy air is expected in the region, according to the Bay Area Air Quality Management District.

Residents are encouraged to drive less and use public transit when air is especially bad but transit will not be free Friday.

The Metropolitan Transportation Commission sponsored one free transit day Thursday to allow riders the opportunity to plan transit into their commutes.

"We hope the many Bay Area residents who took advantage of their free transit opportunity on Thursday will continue to use public transportation on Friday to protect public health and reduce smog," Air District spokeswoman Lisa Fasano said in a prepared statement. "Making clean air choices such as driving less, carpooling and reducing energy use can help improve air quality and protect the climate."

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.