They do not have their cars inspected for emissions before driving them into the US, ---I think you know this too.
In case you don't know:
http://www.texasep.org/html/air/air_2std_brdrair.html
A problem specific to the area is the large number of vehicles waiting to enter or exit the United States from Mexico. The El Paso-Ciudad Juárez border crossing is one of the busiest crossings along the border. Each vehicle crossing the border, about 40 percent of which have Mexican license plates, must wait at least ten minutes at the border while its paperwork is processed. Vehicles at idle produce higher emissions. In addition, Mexican vehicles, because they are older and have less stringent maintenance and emission controls, tend to be higher emitters.