Allergenic Pollen in the Southwest
What makes pollen allergenic?
Most pollen will become an allergen if a susceptible person is exposed to a sufficient quantity of it. Attractive, brightly colored flowers that are pollinated only by insects (e.g., roses) rarely cause allergy (except in florists). Wind-pollinated plants produce comparatively huge quantities of pollen that become airborne easily, and can travel 20 miles or more on a windy day. Therefore most of the pollen found in air samples is derived from plants pollinated by wind. Pollen from all grasses, many weeds and most of the common deciduous trees are disseminated by wind from unattractive, inconspicuous flowers.
Pollen in the Desert
Wind-pollinated plants are ubiquitous throughout the world, indicating that there really is no safe haven for a person who is sensitive to several types of pollen. However, differences in climate and soil composition explain the obvious differences in the range and type of flora seen in different regions in North America. In the arid southwest, the rarity of a hard freeze allows something to flower at any time of the year. The Sonoran desert extends though large areas of the southwest and has a diverse flora. Diversity of the flora has been increased further by the introduction into urban areas of a large number of species from other regions of North America and the world.
Regional differences in plant prevalence and flowering seasons
Bermuda grass, an introduced species now prevalent in the southwest, has allergens that are unique among the grasses in North America. Grasses have a much longer flowering season in the southwest than in cooler areas, and some weed species flower both in the spring and the fall. One of the allergenic weeds in Southern Arizona, Triangle-leaf Bursage, is a Ragweed and flowers mainly in the spring, unlike the scourge of allergy sufferers in most other parts of the United States (Short and Tall Ragweed) which flowers in the fall. Fortunately, airborne Ragweed pollen counts in the southwest do not reach the enormous levels often recorded in the midwest and east. Allergens from Bermuda grass pollen and possibly from other types of pollen are carried in the wind as a fine particulate dust that can get into airways in the lung to provoke asthma.
Allergenic Plants in Arizona
In Arizona they include many species that are native to the region and many introduced species. Native allergenic plants include triangle leaf bursage (a species of Ambrosia), desert ragweed (Ambrosia dumosa), desert broom (Baccharis), Wing Scale (Atriplex canescens), Mesquite (Prosopis) and PaloVerde (Cercidium). A large number of introduced trees and shrubs such as Olive, White Mulberry, weeds such as Russian Thistle (Salsola) and Australian Saltbush, and introduced grasses including Bermuda grass (Cynodon dactylon) produce allergenic pollen. There is some evidence that horticulture accompanying increasing urbanization has caused increasing levels of certain types of atmospheric pollen in the past 40 years. More recently, it is thought that climate change from global warming may be responsible for increasing the amounts of airborne ragweed pollen, a type prevalent in most of the continental United States.