CO and VOC combine with heat and sunlight to form ground level ozone which is detrimental to people with respiratory problems. More CO = more ground level ozone.

Dump the gassers. Go to diesel (no CO).

POINT: both types of engines have problems. Diesel has high NOx output. Gasoline has high CO output. Both throw out tons of soot. Like the last poster said, it's a tradeoff.