Guadalajara is the second largest city in Mexico, second only to Mexico City. It sits on a highland plateau at an altitude almost identical to that of Denver, making it Mexico’s “Mile High City.” As such, it is known for sunny days, cool nights, and a spring-like climate. You would think that a mile high city would be known for its clean, refreshing air. I used to think that about Denver until I learned differently. Like Denver, Guadalajara has an air pollution problem.