Medical Editor: William C. Shiel Jr., MD, FACP, FACR
Seasonal allergic rhinitis and asthma are caused by pollen allergens from trees, grasses, and weeds. Springtime brings an abundance of tree and grass pollens, which are typically small and light, enabling them to be windborne for long periods. These characteristics also give them easy access to the upper and lower respiratory tracts. Repeated exposure to pollen in sensitive individuals increases the likelihood of an allergic reaction. The result: itchy, watery eyes, stuffy sneezy itchy drippy nose, and even asthma.
Pollen counts measure the amount of each pollen species and are represented as the number of pollen grains per cubic meter of air (grain/m3). The counts are affected by local weather conditions: lower on rainy days and higher on hot humid days. Pollen counts are also highest in the morning and evening hours. Most pollen sensitive individuals start having symptoms when pollen counts are greater that 50-grain/ m3. Generally, in North America, tree pollens peak in April and May, while grass pollens are highest from May through August. Weed pollens predominate in the fall months. In areas such as Southern California, seasons are longer and less well delineated than in other areas such as in New England or the South Eastern United States.
Pollen from deciduous trees (e.g. Birch, Elm, Oak, Alder, Olive) is more allergenic than that from conifers (e.g. Cedars and Cypress). Although pine pollen is abundant, it is heavier, falls to the ground quicker, and is therefore less likely to enter the respiratory tract or eye membranes. Pollen from fruit trees is usually not allergenic.
Grass pollens are the most abundant worldwide. They also show "cross reactivity" amongst themselves. This means that if you are allergic to one type of grass pollen, you are usually allergic to them all. Bermuda grass is the exception and does not show this cross reactivity with other grasses.
While on the subject of cross reactivity, allergy sufferers should
be aware of the possible cross sensitivity between plants and certain foods, particularly
fruits. Examples include birch tree pollen and apples, rubber (latex) and avocado, ragweed and
cantaloupe to name a few. When pollen sensitive individuals ingest these foods,
itching swelling and tingling of the mouth and throat may occur, as the body
thinks that it is eating pollens. These reactions are generally (but not always)
mild and rarely cause anaphylactic shock,
which can occur from other foods, such as peanut and fish.
For additional Allergy and Asthma information, please visit the Allergy and Asthma Centers.