The EGR reduces the NOx Emissions only. Nitrogen and Oxygen like each other and bind at High temps and make NO1, NO2, NO7 yatta yatta yatta. The EPA swares that NOx is more harmful than Carbon Dioxide (CO2) and Carbon Monoxide (CO) particulate matter. The EGR actually causes the engine to make more of all. Even though CO and CO2 kill thousands of people every year. And CO2 is supposedly causing this "Global Warming" Apocalypse that Al Gore turned into a Virus.
But back on track. The EPA allows no trade off of emissions because they are lazy, stupid and corrupt. Thus we have the EGR valve that almost levels the playing field when considering buying a car or truck and deciding whether to go Gas or Diesel.