Tesoro's oil refinery in Carson, California. A proposed integration project linking Tesoro's Wilmington and Carson refineries will see storage capacity at the combined plant increased by 3.4 million barrels. (Photo: Daniel Ross)

Tesoro Corp. claims the proposed merger of two adjacent refineries in Southern California's Long Beach area to create the largest oil refinery on the West Coast will substantially reduce emissions. But critics say the project's overall environmental, safety and economic impacts have been worryingly underestimated.