The EPA Green Vehicle Guide shows two ratings for pollution. Smog Forming and Greenhouse Gas.
The smart has a Greehouse Gas score of 9 with 10 being best. But an Air Pollution score of 7 with 10 being best.
It is surprising that the pollution score is higher than many other cars, but still very low compared to most cars on the road.
Check out the website and see how well the smart does overall.
BTW - smart is listed under Mercedes-Benz