"Granting California's request for waiver authority under section 209 of the Clean Air Act will in turn allow all other states to adopt the same program under section 177 of the Act," the letter says. "Fourteen other states have adopted California's standards, or are in the process of adopting them. Another four are moving toward adopting the California standards. All together, those 19 states represent more than 152,000,000 Americans – a majority of the U.S. population."