Over the last two decades, thanks to the implementation of four consecutive comprehensive programmes called ProAire (Box 4.1) there has been a dramatic reduction in local air pollution (Figure 4.5). Since 2003, the levels of SO2 and CO have not been above the limit established by the Mexican Official Norm (NOM) of 0.130 ppm average per 24 hours and 11 ppm average per 8 hours respectively. The levels of lead (Pb) have been below the accepted limits since the component was removed from fuels in 1997.