Key Points
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Sources of airborne allergens vary from microscopic to large fauna.
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Outdoor aeroallergenic particles include intact pollen grains or spores, as well as cell fragments and submicronic particles.
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Preferred samplers measure a calculable volume of air to determine the density of aeroallergens and have capture efficacy of particles down to 3–5 micron.
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Meteorological parameters of temperature and precipitation have the greatest impact on timing and intensity of airborne pollen concentration.
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Increased atmospheric temperature and CO 2 concentration act to increase allergenic plant biomass and pollen production.
Aerobiology is the science of airborne emanations of organic or biologic origin. Aeroallergens may be dispersed on a variety of particle sizes and come from various sources and settings (see Table 20-1 ). The origin may be: microscopic such as bacteria or protozoa; at the limits of visual detection such as dust mites; or easily seen such as mushrooms, bracket fungi or animals such as cats, dogs or horses. The airborne particle may be: a cell, intact pollen grain or cytoplasmic component thereof; fungal spore or mycelial fragment; or protein adhering to epidermal scales or dust particles, or dissolved in water droplets. Outdoor sources are frequently of plant origin, while animal allergens are greater problems indoors; fungal spores may be troublesome both indoors and outdoors. Once entrained into airstreams, aeroallergens may be deposited on conjunctival or nasal membranes, or inhaled into the lungs.
| Allergen Source | Particle Type |
|---|---|
| Bacteria | Cells, fragments, metabolites |
| Thermophilic actinomycetes | Spores, metabolites |
| Algae | Cells, fragments, metabolites |
| Protozoa | Metabolites |
| Fungi | Spores, mycelial fragments, metabolites |
| Ferns and mosses | Spores |
| Grasses, weeds and trees | Pollens, cytoplasmic particles |
| Arthropods | Feces, saliva, body parts |
| Birds | Feces, serum proteins, epidermal debris |
| Mammals | Dander, saliva, urine |
General Principles of Allergen Aerobiology
Pollen
Vascular plants propagate through extension, trunk or root shoots, rhizomes or stolons, or by seed. Sexual reproduction is accomplished by transport of the male gamete, the pollen or spore, to the female gamete, the ovary. Pollen dispersal mechanisms are via the wind, anemophily or with a vector such as an insect, entomophily. Insect pollinated plants are less frequently inducers of hay fever. Some amphiphilous plants utilize both mechanisms.
Fungi
Fungi comprise one of the seven kingdoms of living organisms, more closely related to the animal kingdom than to the plant kingdom. Fungi are eukaryotic organisms with chromosomes within membrane-bound nuclei, dividing through mitosis. Fungi have chitin-containing cell walls, a polysaccharide found also in insect exoskeletons. Fungi may be unicellular, syncytial (many nuclei not separated into different cells) and multicellular (nuclei separated by septa). Complex life cycles have multiple life stages, with both sexual and asexual reproduction. ‘Holomorph’ refers to the fungus throughout its entire life cycle, with ‘anamorph’ referring to the asexual reproductive stage and ‘teleomorph’ to the sexual reproductive stage. Sometimes the alternate life stage is not known, with only the anamorph or the teleomorph identified. Anamorphs without a known teleomorph stage are frequently classified as Deuteromycota, or Fungi Imperfecta: an artificial taxon, a paraphyletic group united only by asexual propagation.
Animals
While primarily indoors, animal sources may be significant outdoor allergens as well. Heavy hatches of caddis flies or mayflies, or miller moth infestations have been reported to induce allergic symptoms. Occupational exposures to tussock moths in pine trees may bother lumberjacks, and sewer flies municipal sanitation workers. Outdoor horse dander allergen can be sampled down wind of stables.
Submicronic Allergenic Particles
The finding that ragweed hay fever symptoms may persist after intact pollen is no longer seen spurred studies demonstrating airborne allergens in submicronic particles. Airborne birch antigenic activity has been demonstrated in particles smaller than 2.4 micron. Cytoplasmic starch granules are prominent in grass (Poaceae) and dock ( Rumex , Polygonaceae) pollen. Grass starch granules have heavy concentrations of groups 1, 5 and 13 allergens. While the force of storm-driven raindrops may disrupt grass pollen grains, releasing large amounts of respirable allergen-laden starch granules, Schäppi and colleagues demonstrated that a moisture-drying cycle will result in starch granules emanating through the grass pollen aperture.
Characteristics of Wind-Pollinated Plants
Although wind pollination may appear to be a simpler process than vector-facilitated pollination, it is the later evolutionary mechanism. Its characteristics are summarized in Box 20-1 . Anemophilous plants have incomplete flowers, with separate male and female structures. The male pollen-producing flowers are exposed to the wind. On trees, dangling structures called catkins have hundreds of small individual flowers. On weeds or grasses the inflorescences are thrust up into the air from the higher portions of the plant. Female flowers are lower, at the axils of leaves or at stem junctions. Attractants such as color of petals and sepals, fragrance, or nectar are absent. The pollen grains tend to be small and dry, with reduced ornamentation to minimize turbulence, and with little sticky resin (pollenkitt).
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Incomplete flowers (spatially separate male and female)
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Male flowers exposed to wind
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Petals and sepals insignificant or absent
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Absent attractants (color, aroma, nectar)
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Pollen grains small and dry, reduced ornamentation
Wind-pollinated trees produce extraordinary amounts of pollen. Erdtman reported a single birch catkin produced about 6 million pollen grains, and an alder catkin 4.5 million. An English oak catkin released 1.25 million grains. By tabulating the number of catkins on such trees, Erdtman calculated the pollen produced in a single year. A birch tree released over 5.5 billion grains over a single year, an alder 7.2 billion and an oak 0.6 billion. Spruce, like birch, produced about 5.5 billion grains in a year. Cereal rye grass produced 4.25 million pollen grains per inflorescence.
Eighty years ago, August Thommen set out five principles necessary for a plant to be an important inducer of pollinosis ( Box 20-2 ). Thommen’s Postulates have remained correct, with some caveats. That the pollen must contain an excitant of hay fever is self-evident, and such are proteins or glycoproteins that are easily elutable or coat the surface of expelled respirable cytoplasmic particles. While the majority of pollinosis inducers are wind pollinated, some primarily insect-pollinated plants will release sufficient airborne pollen to cause sensitization in the proper setting. A single point source could lead to sensitization, such as a tree or shrub situated at a bedroom window. Although most pollen grains come to rest within meters of their source, grains may be transported for hundreds of miles.
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Pollen must contain excitant of hay fever
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Pollen must be anemophilous
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Pollen must be produced in sufficiently large amounts
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Pollen must be buoyant to carry long distances
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Plant must be widely and abundantly distributed
Floristic Zones
The distribution of individual plant species is dependent on a multitude of factors. Foremost is ‘climate’: average high and low temperatures, ambient humidity and average precipitation. Soil factors such as mineral content, pH, and density also impact on plant adaptation and selection. Certain plants are cosmopolitan, adapting to diverse circumstances; others are limited to a niche, adapting to extremes of moisture or temperature. The range of native indigenous species may be limited by niche selectivity. The extent an introduced plant will spread is determined by its adaptability, its aggressiveness and the length of time from introduction. Which plants may be found in different locations may be deduced from several sources. Numerous gardening texts contain ‘hardiness zone’ maps defined by the United States Department of Agriculture (USDA). There are ten climatic zones in the North American continent based on the average annual minimum temperature: beginning with zone 1 at −50°F, and progressing by about 10° increments to zone 10 at 30–40°F. These isotherms generally define the northern limits of species, determined by ability to survive the winter cold. Exceptions may occur in protected sites with extraneous sources of heat. The USDA maps also consider other factors like rainfall or maximum temperature. A more exact 24-climate zone classification system has been described for the western half of the USA, determined by the interplay of six factors: latitude, elevation, Pacific Ocean influence, continental air mass influence, mountains and hills, and local terrain. However, 24 zones are cumbersome to consider. Solomon popularized ten floristic zones which are a cross between the USDA hardiness zones and additional factors, and offer a useful compromise. While the zone boundaries are purposely ill-defined, they are descriptive of the territories they encompass. The zones are: Northern Forest, Eastern Agricultural, Southeastern Coastal Plain, Florida Subtropical, Central Plains, Rocky Mountain, Arid Southwest, Great Basin, Northwest Coastal, and California Lowland. A useful reference giving the distribution maps of many native allergenic plants is Airborne and Allergenic Pollen of North America . However, numbers of introduced major allergenic plants do not have distribution maps.
Characterized Allergens
Numerous allergens have now been characterized, and a list of those that have been fully sequenced is maintained and updated on-line by the International Union of Immunological Societies (IUIS). Allergen nomenclature, by convention, is the first three letters of the genus and the first letter of the species, followed by a number; e.g. the major allergen of short ragweed is Amb a 1, initially known as Antigen E. The number may signify importance or chronology of discovery. Allergens may be renumbered to conform to the function and number of a related allergen. Variations in the molecular weight or charge of an allergen due to amino acid substitutions or glycosylation are called isoallergens and are designated by a decimal point followed by four digits (e.g. Phl p 5.0102 and Phl p 5.0201).
Aeroallergen Sampling
In order to assess the type and intensity of the aeroallergen exposure, it is necessary to monitor the environment. Table 20-2 lists the types of samplers that are useful in assessing outdoor and indoor air. The earliest samplers relied on gravity. The Durham sampler is a greased microscope slide mounted horizontally on a stand, with a roof or rain shield above. Petrie dishes containing the appropriate agar medium have been used indoors for mold studies, with the advantage that the growth medium allows identification of viable spores from the distinctive colony characteristics. Disadvantages of gravimetric samplers are that they can only be quantified in terms of surface area (cm 2 ) and do not give an estimate of particle burden in a volume of air. Capture is skewed to larger particles, with smaller mold spores underrepresented as air currents may carry them over the top of the surface. Gravimetric samplers are no longer considered adequate for meaningful study.
| Type | Example | Comment |
|---|---|---|
| Gravimetric | Durham Petrie dish | Large particles overrepresented Particles per surface area (p/cm 2 ) |
| Volumetric | Particles per air volume (p/m 3 ) | |
| Impaction | ||
| intermittent rotary | Rotorod | Easily overloaded |
| suction drum | Burkard | Wind orientation necessary |
| cascade | Anderson | Indoor or outdoor use |
| Filtration | Accu-Vol | Microscopic or immunoassay |
| Personal sampler | Clinically relevant exposure | |
| Automatic counter | NTT-Shinyei KH300 | Misreading likely Lack of sensitivity |
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