Environment and allergic disease              Return to menu

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.  (See Tables for details of individual plants that may cause allergy in Maricopa, Pinal and Pima Counties, Arizona).

References:

Benson L, Darrow RA: Trees and Shrubs of the Southwestern Deserts.  The University of Arizona Press, Tucson, 3rd Edition, 1981.
Kearney TH, Peebles RH et al: Arizona Flora. The University of California Press, Berkeley, 2nd Edition, 1960.
Parker KF:  An Illustrated Guide to Arizona Weeds.  The University of Arizona Press, Tucson, 1972
Turner RM, Bowers JE, Burgess TL:  Sonoran Desert Plants.  An Ecological Atlas.  The University of Arizona Press, Tucson, 1995.
Lewis WH, VinayP, Zenger VE:  Airborne and Allergenic Pollen of North America.   The Johns Hopkins University Press, Baltimore, 1983.

For additional information, see links in  Environment and Allergic disease and links to other internet sites.

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Disclaimer:   This site is for educational purposes only.  Any information that you have found in this web site is not intended to replace medical care or advice given to you by your own physicians. You should consider consulting your local medical library and other web sites for additional information. 

Comments and suggestions welcome!   Email: schumach@u.arizona.edu
Content Owner:  Michael J. Schumacher, MB, FRACP, The University of Arizona

Updated 3/2012