FUNGI

Fungi produce airborne spores and mycelial elements that are believed to contribute significantly to allergic disease throughout the world. These allergens are typically 3–30
m in diameter. With few exceptions, such as Alternaria in asthma, Aspergillus in allergic bronchopulmonary aspergillosis, and various fungi in allergic fungal sinusitis, the clinical importance of common fungi has been difficult to assess. Alternaria alternata (major allergen Alt a 1) species are common outdoor molds that have been associated with triggering respiratory arrest in patients with asthma.7 Cladosporium (major allergens Cla h 1, 2) is also a common outdoor mold species, and like Alternaria, it has a seasonal prevalence in the warmer months between spring and autumn. The first hard frost of late autumn decreases spore counts significantly until warm weather returns. In contrast, Aspergillus fumigatus (major allergen Asp f 1) and Penicillium citrinum (Pen c 13,18) species are common indoor molds and may provide allergenic triggers throughout the year. High spore counts in homes are associated with warm, humid environments and may be reduced by air conditioning in the summer, removal of mold in homes with contamination, preventing water damage, and dehumidification if needed.

ENVIRONMENTAL CHANGES 

Climate changes due to global warming are expected to increase temperatures by 1–2°C in this century. This will affect vegetation and will likely result in a higher allergic disease burden. The 2006 U.S. Department of Agriculture hardiness zone map showed a shift northward of floristic zones, which influence the type of native vegetation found in a region.8 This shift exemplifies the effect of global warming on the type of trees and other plants that can survive in a given latitude. In addition, studies have shown increased size and pollen production of ragweed with increased CO2. 9 This was especially seen in urban areas where CO2 levels and temperatures were higher than in rural areas.

DUST MITES 

Dust mites, particularly Dermatophagoides pteronyssinus (major allergen Der p 1) and Dermatophagoides farinae, ingest human epithelial scales and obtain water from the ambient water in the air. They produce feces that provide a perennial allergen source within homes. Dust mites are small (0.33 mm long), eight-legged animals that are present in pillows, mattresses on box springs, sofas, and carpets (shag much more than lownap carpets). They thrive in warm, humid conditions and, therefore, peak in the summer months in the United States. The typical allergen size is 1–10
m in diameter, and its ability to cause allergic respiratory disease is enhanced by intrinsic enzymic activity that penetrates the respiratory mucosal barrier and promotes inflammation

ANIMAL AEROALLERGENS 

Animals produce allergens in forms unique to each species. Dander (desquamated epithelium), saliva, urine, hair, and feathers are the major allergen sources. Cat allergen, most importantly Fel d 1, is found mainly in cat saliva but also in sebaceous glands in the skin and in urine of male cats. Allergen size can be 5
m, allowing cat allergen to reach the small bronchioles, causing symptoms of asthma. It is buoyant and “sticky,” which means it easily remains airborne and may last in a home for up to 6 –9 months after the source is removed. Dog allergen, particularly Can f 1, is present in dander, saliva, urine, and serum. There are allergens specific to dog breeds, but all breeds produce allergenic proteins (even poodles and “hairless” dogs).10 Rodent dander sensitivity occurs in occupational exposure of laboratory workers, but allergenic protein in rodent urine may also contribute to allergic disease in infested homes.

COCKROACH 

Blatella germanica (German cockroach) and Periplaneta americana (American cockroach) are the two most common species of cockroach infesting domestic homes and public buildings. The German cockroach is most prevalent in the United States and has an affinity for warm, humid environments. Increased cockroach infestations have also been noted in the inner cities. Sensitization to cockroach extract, including the best-studied allergens Bla g 1 and Bla g 2, are more common in urban settings.11 Cockroach protein, like dust-mite allergen, becomes airborne when disturbed and falls quickly

HYMENOPTERA 

 In brief, the venoms are introduced parenterally by an insect sting from either vespids (yellow jackets, hornets, and wasps) or apids (honeybees). Vespid allergens are largely cross-reactive, but people sensitive to bee venom usually are not sensitive to vespid venom. Fire ants, located in the southeastern United States, also belong to the Hymenoptera order.