While i'm not exactly sure about the actual standards for emissions, one of the big reasons that they can somewhat safely use diesel is because of the grade of fuel they use.

American diesel is sulfur rich which equals acid rain.

Euro diesel is lower is sulfur and thus somewhat more eco-friendly.

This is part of the reason we in the states aren't getting some of the very popular (in europe) diesel cars which actually get amazing gas mileage considering how torquey they are. The motors can't handle the high sulfur content.

noel