It´s usually dusty from February on. Don´t wear contact lenses definitely in March or early April. If you must, then be prepared with some light gogles. Students groups, organized tours and horses sometimes rise up dust in the moment you less expect. In these months you´ll see lots of tourist wearing surgical masks, not because of diseases, but to protect themselves from the dust. They are very cheap and you can find them in any pharmacy. Ask for “cubrebocas contra polvo”, usually one or two pesos each.