California emissions standards are stricter and all aftermarket add-on performance (even non-performance) products have to have be CARB (California Air Resources Board) approved. It will be assigned an approval number which , by law, you should keep in you vehicle or stick it on the aftermarket part. That said, there are also about a half dozen other states that have adopted CARB emission standards so those states probably have the same rules regarding the aftermarket parts. Of course many people wil run whatever they can get by with since there is very little enforcement to deter them.