Although the infamous ragweed is responsible for the majority of cases, seasonal allergic rhinitis can also be caused by a wide variety of tree, grass, and weed pollens as well as by mold spores. Naturally, symptoms will be at their worst during the time of year that the particular plant to which you are allergic is pollinating.
Pollination occurs at different times in different parts of the country, but the following can serve as a general guideline for seasonal allergies. Springtime allergic rhinitis (February to May) may be caused by hypersensitivity to the pollens from ash, birch, cypress, elm, maple, oak, poplar, sycamore, and walnut trees. Midsummer allergies (April to mid-June) usually result from pollinating grasses, such as orchard grass, redtop, rye, timothy, sweet vernal grass, fescue, Johnson grass, and various bluegrasses, which are the most problematic allergen in the West. Finally, end-of-summer and early autumn allergies (mid-August to October) are mostly due to ragweed pollens and to the pollens of several closely related weeds: cosmos, golden glows, golden-rods, and zinnias.
Like hay fever, rose fever is another misnomer. In general, brightly colored and fragrant flowers such as roses do not rely on the wind to disperse their pollens. Instead, the generally sticky pollens they produce attach to birds, bees, and insects, which is how they are transferred from plant to plant. For that reason, bright and fragrant flowering plants are seldom the cause of seasonal allergic rhinitis.
Molds abound in most places in the world except for deserts. They are particularly common at the shores of seas and lakes. Outdoors, they may be found on decaying vegetation and wood, fresh-cut grass, piles of leaves, and compost heaps. They may also be found on a variety of vegetables, including corn, tomatoes, and sweet potatoes. The bulk of outdoor mold allergies are linked to two main spore producers: Alternaria and Cladosporium (Hormodendrum). Depending on where you live, the height of the mold season may be anywhere from April to November.
Interestingly, the allergy to Christmas trees that many people suffer from may be caused by both molds and pollens. Both kinds of allergens are attached to the trees and remain there after the trees have been cut and stored. Indoor heat then releases the offending allergens, which accounts for the several-week-long December hay fever "season" experienced in many homes.
The weather may also play an important role in allergic rhinitis. In general, most people with seasonal allergic rhinitis caused by pollens fare better on cool, windless, muggy or rainy days because cool weather reduces pollination, and dampness makes pollen less aerodynamically efficient. By contrast, since warmth encourages pollination, hot, sunny, and especially windy days, with their ability to spew clouds of pollens or spores into the air, can be a curse for seasonal allergy sufferers.
The ability of molds to be dispersed by air currents in response to changes in temperature, humidity, and wind conditions is fairly similar to that seen with pollens. Consequently, mold suffers also do better on cool or rainy days. Unfortunately, molds may survive outdoors far longer than pollens, even during freezing weather. Troublesome spores may abound in stored hay, grain, and straw, causing year-round distress to mold-allergic farmers and other grain workers. For this reason we usually do not refer to a "mold season" the way we do to a "ragweed season."
Finally, there is one difficult-to-control environmental condition in which no one fares well, particularly allergy sufferers: air pollution. The combination of elevated levels of air pollution and high levels of molds and pollens spells misery for allergy sufferers.
