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If you are suffering from allergies, tree and plant pollen may be the reason.


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Tree, Shrubs & Urban Allergies

Many people suffer from hay fever and asthma.

Allergies of all kinds, along with asthma, have been on the increase worldwide in recent decades, and costs associated with treating these conditions are on the increase. At least 50 million Americans suffer from pollen related allergies or asthma. A survey recently completed by the American College of Allergy, Asthma and Immunology found that the actual number of allergy sufferers may be twice as high as had been previously believed, or 100 million. According to this survey, 56% of individuals surveyed said they live in a household where at least one person has allergies. These trends are illustrated by the following information:

Hay fever is increasing. The most common form of allergy is seasonal allergic rhinitis or “hay fever” - the term used for allergies caused by airborne pollens and molds. In total, more than 35 million persons suffer from seasonal hay fever in the U.S., and 6 million of these are children. Hay fever is far more prevalent in urban than in rural areas. Research suggests that about 20% of persons with allergies either already have or will develop asthma.

Asthma is increasing. According to the U.S. Center for Disease Prevention and Control (the “CDPC”), reports of asthma in the U.S. population increased from 6.7 million in 1980 to 17.3 million in 1998, more than a two-fold increase. Approximately 85% of those with asthma also have pollen based allergies, which often serve as the “triggering event” in an asthma attack. Asthma is also far more prevalent in urban than in rural areas.

More children have asthma. The incidence of acute asthma attacks among children has increased 100% in the past decade, despite the development of medications that are highly effective if taken properly, according to the CDPC. Almost five million Americans with asthma are under age 18. Asthma is now the most common cause of hospitalization among American children, affecting a total of five million. Deaths among children with the condition rose by 78% from 1980 to 1993, according to a recent analysis.

These illnesses have enormously detrimental educational and economic effects.

Hay fever or chronic rhinitis is the leading cause of lost workdays in America, accounting for 9.3% of all missed days or 3.4 million days per year. Asthma is the leading cause of missed school days for U.S. children. Allergy related deaths, particularly from asthma, have almost doubled every decade for the past 40 years. Asthma treatment costs are approximately $12 billion annually, and hay fever treatment costs are about $6 billion per year. Currently, one out of every eleven visits to a doctor in America is for an allergy-related problem.

Prevention is the first principle of American health care for the new millennium.

Americans have become increasingly aware that modern medicine, while capable of many miracles, is not a cure-all. The American health care system is encouraging its patients to embrace more personal responsibility for their individual health, whether through exercise, smoking cessation, a better diet, or a general consciousness of the factors leading to disease and disability. An individual can reduce the likelihood of getting cancer by quitting smoking, or reduce the likelihood of contracting heart disease through exercise and diet. That same individual can reduce the likelihood of pollen-related allergic episodes by reducing the pollen in his or her immediate vicinity. And research indicates that the likelihood of developing pollen-related allergies in the first place is reduced when living in a less pollen-intensive micro-environment.

Allergies develop from repeated exposure to allergens, including plant pollen.

Allergies develop from repeated, often daily exposure to allergens, the substances that cause allergic reactions. Plant pollens are the most common allergen affecting human health. According to the American Academy of Allergy, Asthma and Immunology, avoidance of substances that cause allergic reactions, including plant pollen, is the key to allaying allergy symptoms.

In the case of plant or pollen allergies, the fewer allergenic plants in an individual's immediate vicinity, the less the chance of exposure, because the great majority of windborne pollen grains of all species are deposited quite close to their source. A pollen producing tree or shrub next to a home can create ten times more exposure than a tree or shrub one or more houses away. For most wind pollinated trees and shrubs, two thirds of all pollen is distributed within 60 feet of the tree, and 90% of the pollen falls within 90 feet of its source. For most grasses or other herbaceous plants, dispersal is still more concentrated, with most pollen falling within a few meters of the host plant.

For these reasons appropriate planting in the home garden and yard, on our boulevards and in our schoolyards and public parks, can have a major impact on the level of exposure to pollen, and on alleviating the symptoms of pollen allergies and asthma. A reduction in urban pollens can also have a prophylactic effect in reducing the number of persons likely to develop such allergies in the first place.

Some plants produce much more pollen than others.

Pollen – the allergen that triggers many allergy and asthma incidents – is produced only by the “male” parts of a plant. Plants vary widely in their capacity to produce and disperse pollen and thus, to trigger or exacerbate allergies and asthma. Many plants (characterized by botanists as “perfectly flowered”) contain the “male” parts of the plant within the “female” flower. They often generate a heavy, sticky pollen, which is not normally carried by air or wind; such plants are generally pollinated by insects or birds.

However, a significant number of plants, including many grasses, trees and shrubs, are “monoecious” or “dioecious.” A dioecious plant has either male or female flowers, while a monoecious plant has both male flowers and separate female flowers. These plants are most significant in pollen allergies, because they create far more pollen than do “perfectly flowered” species, and because their pollen is almost always windborne. From a plant allergy perspective, the worst plants are dioecious males, which will bear only pollen and no fruit or seed. The best plants are dioecious females, since they bear no pollen. In addition to the sexuality of the plant type, other factors that contribute to a plant’s capacity to exacerbate allergies and asthma are:

• The duration of blooming. Certain trees, shrubs and grasses may produce allergenic pollen for as little as 2 3 days per year, while others bloom intermittently throughout the year and may release pollen for six to seven months. In areas of the U.S. with particularly long growing seasons, some plants will come into flower multiple times during the year.

• The weight of the pollen. Some pollen grains are heavy and will not fall far from the tree, while others are quite light and float easily in the wind. Light, wind blown pollen is more likely to cause allergy or asthma problems.

• The moisture level of the pollen. Sticky, moist pollen often clings to an element close to its source, while dry pollen floats and then lands and sticks on any available moist surface, such as mucus membranes.

By understanding the sexual and other characteristics of a plant type, it is possible to predict whether that plant is allergy-friendly or is likely to trigger or exacerbate allergies and asthma. And knowing whether a particular yard, public space or community contains mostly pollen-intensive plants, or more allergy-friendly plants will allow horticulturists to better understand how allergenic an area may be.

Historical purchasing preferences exacerbate the problem.

Over a period of time, the types of plants selected by homeowners, landscapers and government agencies have changed significantly – and the changes have made the plant allergy problem substantially worse. For reasons of convenience, more and more shrubs, trees and other plants are selected for their “litter-free” characteristics – that is, they are male plant types and generate little or no seeds or fruits. In 1949 the USDA first recommended that American cities begin planting male tree and shrub cultivars on our streets and in our public parks. Over the past fifty years that advise has become firmly rooted in the psyche of American ornamental horticulture. American horticulture has become strongly oriented to such “clean” plants and has developed thousands of seedless varieties. Additionally, many American cities have plant ordinances that ban the planting of seed, fruit or nut producing cultivars on our streets or in our public spaces. As a result, modern urban landscapes have evolved to consist predominantly of trees, shrubs, grasses and ground covers that are:

• Dioecious male cultivars selected because they produce no fruit or seed; or
• Monoecious cultivars selected, at least in substantial part, because they minimize female sexual characteristics (fruit and seed).

These plant choices, of course, incidentally maximize the most significant male sexual characteristic: pollen production. Thus, in order to avoid the problem of cleaning up fruit and seeds from female trees we have – unthinkingly – altered the natural mix of male and female plants in such a way that pollen loads have increased, and correspondingly, so have allergies and asthma. Pollen is the invisible “litter” in urban America. We have increasingly clean sidewalks and, from a pure health perspective, dirty air.

An example of typical urban planting can be found in Minneapolis, Minnesota. In 1999, the Minneapolis Parks and Recreation Board (responsible for all street tree and public park plantings) purchased and planted about 3,500 new trees. Of these, only 23 – or less than 1% – were dioecious female trees that bear no pollen and cause no allergy. Approximately 800 of the new trees were dioecious male trees that produce substantial amounts of airborne pollen. Many of the remaining trees planted in Minneapolis in 1999 were monoecious trees bred, at least in part, to minimize female sexual characteristics. Such monoecious cultivars tend to express substantial amounts of pollen. The result: when these trees reach sexual maturity, they will increase both the number and the severity of allergy attacks suffered by Minneapolis residents and visitors to the city.

Dutch Elm Disease changed the face and the toxicity of the urban forest.

In another irony, the massive effort across this country to replace the millions of American Elms lost to Dutch Elm Disease has been an indirect cause in the proliferation of allergies. Prior to 1970, the predominant urban tree in the U.S. was the American Elm, which is allergenic, but produces a relatively modest amount of pollen. Dutch Elm Disease has killed most American Elm trees throughout the U.S. over the past 40 years. Unfortunately, most of the trees planted to replace the Elm produce far more pollen and are just as allergenic.

In the St. Paul-Minneapolis area alone, millions of American Elms died between 1972 and 1990. These Elms were replaced by trees with far greater allergenic impact upon the health of the cities' residents. The American Elm experience of Minnesota has been replicated in most urban communities across the country. Since trees will often take up to a decade or longer to reach full sexual maturity, the impact of their allergenic nature continues to increase with each passing year. As the trees get larger, the amount of pollen they create and distribute increases exponentially. The public health consequences of these horticultural decisions are becoming apparent in cities and suburbs throughout the country.

USDA analysis of the urban allergen problem.

In an analysis done by the U.S. Department of Agriculture (USDA) it was determined that the sexual nature of plants growing in urban settings is both disproportionately male and highly pollen intensive. In its urban forest analysis the USDA has utilized “OPALS” (Ogren Plant Allergy Scale TM), a plant/allergy scale ranging from 1 to l0, with “1” representing a pollen free plant, and “10” representing a male, allergenic, airborne pollen intensive plant. The USDA has found that every urban forest studied, including New York, Boston, Philadelphia, Baltimore, Jersey City and Atlanta, has ranked between a “6” and a “7,” and is thusly highly allergenic.

American urban communities are high in pollen and increasingly allergenic.

Data from the National Institutes on Health indicate that Americans living in urban areas are 20% more likely to suffer from pollen based allergies than are those living in rural areas. This would seem to be counter intuitive, because of the abundance of plant life and simple biomass found in rural communities. However, rural trees, shrubs and grasses reflect the natural sexual balance determined by native seedling growth, while urban plants are more often selected from asexually propagated clones that are disproportionately male and pollen intensive.

Recognition of the problem is increasing.

In recent years a number of American cities have begun to recognize the allergy related health problems such predominantly male plants are causing. Albuquerque, New Mexico, for example, has banned the planting of many allergenic male tree and shrub cultivars within city limits, and has mandated the labeling of allergenic plants sold in city nurseries. Tempe, Arizona, and Las Vegas, Nevada, have outlawed the planting of Olive and Mulberry trees (the commercial varieties of which are generally pollen-intensive and extremely allergenic) in an attempt to hold down allergies. Other cities in the Southwest have banned the planting of Bermuda grass in turf lawns because of its pollen load. I expect the trend of cities and states in enacting ordinances or laws related to the allergenic properties of plants to spread across the country over the next decade. The U.S. Environmental Protection Agency is studying the correlation between urban horticulture and the increase in urban allergies and asthma. And this growing recognition is causing a reappraisal of ornamental horticultural practices across Europe as well, particularly in Germany.

What can be done to help?

Plant allergy-friendly trees and shrubs. Ask breeders and nurseries to develop, propagate, and stock allergy-friendly cultivars. Change municipal ordinances to accept female cultivars and to discourage pollen-intensive, allergenic cultivars. Invite the public health elements of your community to the discussion regarding our urban forest. Plant beautiful female cultivars in your yards and gardens. Good health and beautiful landscapes can and must coexist in our communities.

Paul Ogren
Aristone Corporation
9920 Zilla Street NW, Suite 310
Coon Rapids, Minnesota 55433
Fax: (763) 757-0827

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