Mark R. Sneller, Ph.D* Jacob L. Pinnas, M.D.
When there is growth in a community, it leads to a decrease in natural vegetation. Therefore, this process of urbanization can lead to a decrease in the level of aeroallergens such as occurs which natural forests are replaced by roads and buildings. However, city development can also result in an increase in the level of local aeroallergens when ornamental vegetation is added for landscape purposes for the comfort, convenience, and beautification of the urban environment.
The latter is precisely what has happened in the desert Southwest. This includes such cities as Phoenix, Tucson, Las Vegas, El Paso, and Albuquerque where the natural environment has been augmented by the planting of ornamental trees and grasses that also happen to produce allergenic pollen which is airborne. These trees include mulberry, olive, sycamore, ash, elm, poplar, mesquite, and cypress, as well as ornamental juniper.
It has been shown that the urban increase in aeroallergens appears to parallel the increase in human population. In the Tucson basin, allergenic tree pollen has increased more than twenty-fold over the past several decades. This led to the creation of a countrywide ordinance in 1984 (1984-29). This ordinance banned the sale and planting of olive and mulberry trees and ordered domestic Bermuda grass to be cut before it can pollinate.
For most of the cities mentioned, regulatory actions have already been put in place or are in the planning process. These regulatory actions are tailored to the specific plants and problems of that city. When successfully implemented, these bans forbid the planting of certain allergenic trees in newly developing areas of the city and forbid their planting in the existing city as well.
It is important to realize that the substitutes for banned trees may or may not be as allergenic. For example, South African sumac is a popular shade tree whose pollen may be less allergenic or less airborne when compared to many other trees. However, oak and the stately London plane trees produce extremely high levels of pollen that easily become airborne and be carried long distances. The pollen from these trees can be moderately or strongly allergenic.
In addition to the introduction of allergenic grasses and trees, the disturbance of natural land can lead to the spread of very allergenic weedy species in the urban environment. These species include various native and non-native ragweeds and numerous noxious weedy species belonging to the goosefoot and pigweed families such as Russian thistle, careless weed, and saltbush.
Date collected over many years also reveals an increase in mold associated with the increase in vegetation. For example, Alternaria, the most common allergenic mold, has increased almost tenfold over the past twenty years. This mold thrives on decaying plant material.
*Dr. Sneller is owner/director of Aero-Allergen Research, Tucson, Arizona.The monitoring of aeroallergens and public education are necessary to understand the impact of local vegetation has upon human health. In view of the high incidence of respiratory allergies and asthma in Arizona, twice the national average, exposure to a growing number of allergens may be a questionable tradeoff for shade.
Posted in: Urban Environment