Mexico City
Mexico City is an exciting, bustling urban jungle about 2235 m (7,300 feet) above sea level.
So the headache that plagued me most of yesterday could have easily been mild altitud sickness. Best recommendation: twice your normal water in take… and maybe a Tylenol 
Of course, the increased noise and air pollution are also headache inducing. Not much to do about those. A visit to the city cannot include staying indoors during the sunny most gorgeous mountain wealther to avoid the smog. Maybe for residents.