Part of it has to do with the mountains surrounding the city, I think it keeps the pollution in like Denver. I don’t know what the laws and regulations are like now, but I can tell you when my husband lived there, over 20 years ago they made an effort to alleviate some of the pollution by forbidding you from driving your car one day a week. I think they put a sticker on your license plate, or something like that, to indicate what day it could not be on the road. The wealthier Mexicans just bought another car to get around it. it will be interesting to see if people have some more current information.