Respiratory allergies are caused by pollen, mold or dust that circulates in the air. Allergies are quite common, especially during the spring when flowers and other plants start to bloom. However, it is virtually impossible to eliminate the various plants and trees in one's city that cause respiratory allergies. The only practical solution is to move to less humid climates where fewer pollinating plants grow. The state of Arizona in the United States has become a popular retirement place for elderly people (many of whom suffer from allergies). There are fewer cases of pneumonia and other winter-related illnesses. Entire retirement communities have sprung up in Arizona due to the dry climate.