The California Air Resources Board has directed refineries to begin producing and selling winter-blend gasoline.
Governor Jerry Brown requested the early switch because of the recent gas price spikes.
The blend is usually not sold until October 31st because it's more volatile and its quick evaporation can harm air quality during hot weather.
But the Board's Dave Clegern says that's unlikely now.
"I don't think there is going to be too dramatic an impact on air quality because the temperatures have begun to cool in the nick of time you might say and the cooler temperatures that we're seeing now should help blunt any air quality impact.
Clegern says the state has only made an early transition after Hurricane Katrina and a few years ago after a refinery fire.
He says it's likely to increase fuel supply by 8 to 10 percent, which could lower fuel prices.