The Clean Air Act (CAA) is the broad name given to the United States' air quality laws, which are administered by the Environmental Protection Agency (EPA). Originally passed in 1963, the CAA has been amended numerous times. The CAA prevents states from imposing their own emissions regulations for new vehicles, with an exception that states which had imposed vehicle emissions regulations prior to March 30, 1966 may apply for a waiver to enforce their own emissions regulations contingent on the regulations being more stringent than the national regulations, along with several other conditions.

As the only state meeting the date requirement, California is uniquely positioned to adopts its own emissions regulations. Additionally, the CAA permits other states to adopt, without federal review, emissions regulations imposed by California which have been granted a waiver by the EPA.

Despite recent legal challenges, California has been granted numerous waivers from the EPA, allowing it to have the most stringent emissions regulations in the country. In 2022, California proposed a new rule which would mandate that 100% of new light vehicle sales in the state be zero emission vehicles (ZEVs, which includes all-electric, plug-in hybrid, and hydrogen fuel cell vehicles) by 2035. The proposed rules are still pending a waiver from the EPA.