Today is the third straight weekday Spare the Air day, when Bay Area smog levels reach such unhealthy levels that asthmatics stay indoors and transit agencies let people ride for free (today is the final time they will do so before the federal grant money to reimburse them runs out). After only one such day in 2005, the Bay Area Air Quality Management District has already declared three, and more are surely on the way.
The conventional wisdom is that the cause of all of this is heat and low winds. And while those are both pretty crucial, the main ingredient in smog is car exhaust, meaning the main culprits are drivers.
By the way, since VTA does not seem to have enough fare inspectors to cover the light rail system, today isn't exactly the last day to ride transit for three, just the last day to do so ethically. This makes for an interesting ethical question -- is it better to ride transit without paying than it is to drive?