Am Wochenende hab ich Besuch gehabt, dem es nicht so gut gegangen ist. Im Lonely Planet Reisefuehrer ist die Antwort gefunden, warum.
"Mexico City}s air pollution may aggravate allergies, asthma, hypertension, or heart, lung or breathing problems. If you have any such problems to a significant extent, you should consult a doctor before visiting Mexico City.
The altitude, dryness and pollution can cause a sore throat, runny nose or eyes, light-headedness, insomnia, slight headaches, or shortness of breath in anyone, especially at first."