Fall Comes to the Desert

It’s finally starting to feel like fall.  Nighttime and early morning temperatures have dropped below 90 degrees, the monsoon humidity and risk of monster dust storms is on the decline, kids are back in school, and club sports are in full swing.   We are also seeing early signs of the Arizona fall allergy season.   Unlike the Midwest and the South, where fall can bring some of the worst allergy misery of the year, the fall allergy season in the desert can be hit or miss depending on the amount of monsoon rain during the summer.   This year, things are shaping up to be a real hit.  Higher than normal rain fall has produced a bumper crop of allergenic weeds including Russian Thistle (tumble weed), Careless Weed, and Ragweed.  In many parts of the country, ragweed pollen defines the fall allergy season.  In the Phoenix area, ragweed often plays a minor role in the fall allergy season because it is too hot and dry.  On the other hand, ragweed is a major cause of springtime allergy symptoms, particularly when the valley gets sufficient winter rain. This year may be an exception, and residents of Phoenix who usually have allergy problems during the spring may find themselves suffering as much itching, sneezing, and wheezing this September and October as they usually do in March and April.

Bermuda grass, although pollinating throughout the summer, becomes a more serious allergy problem in the fall as children start practicing and playing on bermuda grass sport fields.  Landscapers and homeowners also begin “scalping” and dethatching there bermuda grass lawns in preparation for planting winter rye grass. Scalping a lawn (mowing the grass very close to ground) sends a cloud of bermuda grass dust high into the dry fall air where it can be carried throughout the neighborhood.

All this increase in atmospheric pollen and particulate production is compounded by the phenomenon of temperature inversion.  In the fall, as the nighttime temperatures begin to drop, a layer of cooler air becomes trapped by a layer of warmer air above. Particulates, including dust and pollen, become trapped in this cool air mass close to the ground,  right at nose and mouth level.

Patients with asthma often have a harder time keeping under control during the fall.  This is not only the result of increased fall pollen and air pollution exposure but also because of the fall cold and flu season.  Viral upper respiratory infections are the number one cause of asthma attacks, particularly in school age children.