California's Air Resource Board says that the imminent opening of the state's freeways and ports of entry to older, diesel-fueled trucks from Mexico could dramatically increase toxic pollutants, a new source of smog equal to 2.2 million additional cars on the road, according to San Diego’s Union Tribune. The Bush administration is expected to erase the existing border-truck policy, which had limited most Mexican truckers to a 20-mile zone within California. The air board's report said an announcement repealing the limit is “imminent,” according to the Union Tribune. Regulators estimate that daily truck crossings from Mexico will increase from 3,500 to 17,500, spewing another 50 tons of smog. A quarter of those trucks were on the road before 1980, and as many as nine of every 10 were built before 1993, according to the air board report. Opening California's borders to more trucks from Mexico is part of the North American Free Trade Agreement, signed by former President Clinton a dozen years ago. Mexico has promised to require the use of cleaner-burning, low-sulfur diesel fuel starting in 2007 in the border regions and to extend the standard countrywide by 2009. The United States will implement a similar fuel standard this year.