Seasonal allergies affect millions of Americans each year, causing symptoms that include sneezing, runny nose, nasal congestion, and itchy, watery eyes. Pollen is the most common outdoor allergen responsible for triggering these symptoms. Both the number and severity of allergy cases have been growing significantly over the last few decades, with this growth occurring primarily in urban environments, as opposed to rural areas. Many theories exist to explain this increase in prevalence, but only one focuses on the question as to how pollen could be worse in cities when they harbor fewer plants.
This theory, put forward by Thomas Ogren, blames the preferential planting of male trees in city landscapes for the rise in pollen and allergies. Males are the pollen producers, so it only makes sense that more males would raise the pollen count, leading to more allergy problems. Awareness and acceptance of this concept is just starting to catch on, but it holds much promise for alleviating the allergy burden.
Pollination and tree gender
At this point, a little detail on how trees reproduce will allow for a greater understanding and appreciation of Ogren’s theory.
Essentially, the male structure of a plant is responsible for producing pollen, while the female segment is built to capture it. The pollen must eventually find its way from the male, to a female of the same species. It tends to be light and dry, which helps it to achieve that goal. The design of the female reproductive part includes a moist, sticky trap for pollen, called a stigma. Furthermore, the female can actually draw pollen to it because airborne pollen carries a positive electrical charge, and the female flower has a negative charge. Given the chance, female plants act as natural air filters.
The problem is that these filters have been removed by modern landscape practices, resulting in air that’s full of pollen with no females around to trap it. Unfortunately for allergy sufferers, the mucous membranes lining our eyes, nose and throat now catch this overabundance of pollen.
How males came to outnumber females
Female trees produce seeds, seedpods and fruits, which can create a bit of a mess when dropped onto city streets and sidewalks. Beginning around 1950, the USDA recommended the selection of male trees over females. A lower maintenance, “litter-free” landscaping mentality was the driving force behind this suggestion.
Such male favoritism coincided with the spread of Dutch elm disease. In the past, many streets were lined with elm trees. As the disease wiped them out, modern landscape design replaced the elms with males of different species. This led to an abundance of males, while female numbers dwindled. According to Ogren, the demise of elms across the country can be correlated to the spread of allergy.
Before these changes took place, the tree population in cities and towns was reflective of the natural environment, with males and females being roughly equal in number. There was a natural balance between pollen producers and pollen collectors. That balance has been disturbed.
Close to home
Critics of Ogren’s theory argue that discriminatory planting cannot be blamed for the increased pollen levels. They claim that local pollen production is of little concern since pollen can be carried in from outside sources. The problem with this argument is that it fails to acknowledge the importance of pollen concentration, as well as its rather limited mobility.
Pollen count specifically refers to the average number of pollen grains present in a given volume of air. Though some amount may reach a distant location, its concentration will be greatly diminished. To illustrate, consider that an allergenic tree in your yard would result in ten times the exposure to pollen, as compared to the same tree growing just a block away. Even the most airy pollen tends to reach no more than half a mile beyond its source, the bulk of it being deposited within a 100-foot radius.
The number of grains present in every breath of air is the key factor, and this number grows higher as one approaches the source. Thus, individual homeowners can take certain measures to reduce the pollen yield of their lawns and gardens. The next step is to improve the allergy forecast of entire cities.
The politics of pollen
Greater awareness of the link between urban planting practices and human health has brought the beginnings of what Ogren calls “the politics of pollen.” So far, five major U.S. cities have enacted local ordinances to regulate planting methods and control pollen.
Ogren has developed a useful tool to inform such allergy-friendly landscaping. It is the first plant-allergy ranking system, called OPALS, which offers a scale for characterizing allergenicity, based on specific plant type. The system assigns a number between 1 and 10, with 1 being the least likely to elicit an allergic response and 10 being most likely. The scale is currently being used by researchers to make allergy projections for major urban areas.
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