Look at the graphic below that shows there is no end to the extent to which they will push manufactureres - their stated goal was to make the exhaust from a diesel engine clean enough you could breathe it. The allowed leveles of NOx (oxides of nitrogen) and Particulate Matter (what made the black smoke) have been progressively pushed down year after year, in cahoots with the EU standards. There is never a point at which they were willing to say "OK this is good enough". Instead every Class 8 truck now must carry a reservois of pee (DEF aka urea) that is injected into the exhaust stream. We pay for this insanity.
Nobody likes pollution but there has to be a balance, and the EPA on it's own is incapable of finding it.
