Mexico City sits in a valley surrounded by mountains. Smog frommanufacturing and chemical plants as well as vehicle exhaust fumesget trapped by the surrounding mountains due to lack of anypowerful-enough winds. This makes up the layer of gloom that sitsover the valley due to a "cook-pot effect".This smog can cause burning in the eyes and throat of visitorsnot used to it within a few days, especially in those withsensitivities to pollutants.