How do European cars compare to American cars as far as emissions standards?

Can you give me some good research sites to look at?|||I can't give you any sites to go to, but I can tell you what I know as a technician. American cars run much, much cleaner than import vehicles. They hold themselves to higher standards than imported vehicles which often have to be heavily modified in order to be sold in the US.|||normally European standards are accepted worldwide as euro 3 and euro 4 engines. However, both American and European cars do not make much difference as far as emission levels are concerned.