The link goes to the most populated cities wherein you can find Mexico city at #10.
List of cities proper by population - Wikipedia, the free encyclopedia
Seoul is actually a bit more populated, and it's not all that dirty. It is polluted, but it's not all that bad. Having a high population doesn't automatically give it the worst air pollution.
That being said, if they legalize it, maybe I should take a second look at some of those TOEFL jobs in Mexico if they pass this. It's going on to two years I've been living in cities where MJ is virtually non-existent - at least at the street level. I smelt a grow room in Korea once, but what do I do, go door to door, "Hey, do you have an MJ grow-room going on here or what?" It had to be on the second or third floor - ground floor would've been easy for me to find. In any case, would be nice to have the world's most popular antidepressant available to me again, especially if it's legal.