Commuters will ride free today on nearly all of the Bay Area's transit systems as a combination of high pressure, heat and light winds threaten to produce unhealthy air quality.

"The overarching theme is for people to get out of their cars and use transit," said Luna Salaver, spokeswoman for the Bay Area Air Quality Management District, which is running the Spare the Air program.

"We're hoping that people will take advantage of the free commutes. Once they find out how easy it is to use transit, hopefully after the three (free) days are used, they'll continue to use transit whenever we call a Spare the Air Day," she said.

The day was declared because a confluence of factors — hot air, little wind and high pressure — that help car exhaust and other pollutants such as evaporating fuel at gas stations, paints and household chemicals such as hairspray create ground-level ozone. Vehicle exhaust accounts for about 45 percent of those emissions, Salaver said.

Air district officials predict the air quality index will reach 106 in the eastern parts of the East Bay, a large region that includes eastern Alameda County, all of Contra Costa County and southern Solano County. The unhealthy threshold is 101, Salaver said.

The expanded $7.5 million program reimburses 25 area transit agencies, including BART, Muni, AC Transit, SamTrans, WHEELS, and Caltrain, for the entire operating day. The air agency says it is the largest free transit smog-fighting program in the nation.

Funding for the program comes from the federal Congestion Mitigation and Air Quality Improvement program, the Transportation Fund for Clean Air and state air district funding.

The 2005 program funded only the morning commute for two days, but it was only invoked on one day, July 26, to stave off smog.

This year's program, which runs from June 1 through Oct. 13, has enough funding for three entire days.

For Corinne Waller, an adminstrative assistant, free BART will be a strong incentive to show up in Fremont before station parking is usually gobbled up at 7:30 a.m. Her commute from her home in San Jose to downtown Oakland is already half transit, but a lack of parking prompts her to drive about half of her workdays.

"I'll just have to get to the station a little early," she said as she waited for her car at an Oakland parking garage.

On the other hand, accountant Lee Wilson said the program wouldn't help his commute from Montclair in the hills to downtown Oakland. "I'm not real close to a bus and I don't have definite hours, so I'll just drive like I usually do."

On BART, which collects more than $900,000 in fares on a typical weekday, bleary-eyed commuters whoinsist on inserting their cards into fare gates will find the slots covered up, said BART spokesman Linton Johnson.

The program started four years ago when the Livermore-Amador Valley Transit Authority, or WHEELS, started offering free rides on bad air days, followed by BART in 2004 and 20 area transit agencies in 2005.

Spare the Air Day dos

- Leave car/SUV at home

- Ride free public transit

- If you must drive, carpool

- Call 511 or log onto 511.org to get transit information.

- Combine necessary errands into one trip.

- Avoid strenuous activities outdoors during the heat of the day. If you must exercise or do manual labor, try to do it in the early morning or after sunset.

Spare the Air Day don'ts

- Pump gasoline during the morning or afternoon. Wait until after 6 p.m.

- Use gasoline-powered equipment such as lawn mowers, leaf blowers and trimmers. Electric appliances are OK.

- Use any products that give off fumes containing volatile organic compounds (VOCs), such as paint, hairspray, cleaning fluids. Such chemicals evaporate easily and contribute to the formation of smog.

Today's free rides

- Muni; ACE ; AC Transit; Alameda-Oakland Ferry; Alameda-Harbor Bay Ferry; BART; Benicia Breeze ; Caltrain ; Cloverdale Transit; County Connection; Dumbarton Express; Fairfield/Suisun Transit; Golden Gate Transit and Ferries; Livermore-Amador Valley Transit Authority (WHEELS); Petaluma Transit; Rio Vista Breeze; SamTrans; Santa Rosa CityBus; Sonoma County Transit; Tri Delta Transit; Union City Transit; Vacaville City Coach; Napa VINE; VTA; WestCat.

For more information, log on to http://www.sparetheair.org.

Contact Erik Nelson at enelson@angnewspapers.com and read his Capricious Commuter blog at http://www.ibabuzz.com/transportation.