Air pollution does not consist entirely of man-made substances. Many pollutants are released directly from natural sources, including plants, and some pose as much of a health hazard as man-made substances. These "natural" pollutants include pollen and mold spores. They are also know as aeroallergens because they are airborne substances that cause an allergic reaction. Some of the reactions can include itchy, watery eyes, sneezing, and wheezing.