R E V I E W Risk factors of allergic rhinitis: genetic or environmental? De-Yun Wang Department of Otolaryngology, Faculty of Medicine, National University of Singapore, Singapore Correspondence: De-Yun Wang Department of Otolaryngology, National University of Singapore, 5 Lower Kent Ridge Road, Singapore 119074 Tel +65 6772 5373 Fax +65 6775 3820 Email entwdy@nus.edu.sg Abstract: Allergic diseases such as allergic rhinitis represent a global health problem, affecting 10%–25% of the world population. There is clear evidence to support the concept that allergic diseases are influenced by genetic predisposition and environmental exposure. Polymorphisms of candidate genes have been associated with clinical expression of these diseases. However, characterization of these susceptibility markers in discriminating an “allergic individual” from the general population has not yet been achieved, and the value of how this genetic insight leading to recognition of specific subtypes of these disorders still needs to be confirmed. Environmental factors (eg, air pollution and bacterial/viral infection) also play an important role in the development of the diseases. A number of epidemiologic studies have supported the “hygiene hypothesis”, which is based on the observations that Th1 responses induced by microbial stimulation can counterbalance allergen-induced Th2 responses. Future studies are needed to identify the key genes or their haplotypes for atopic phenotypes and to investigate the interactions between genetic and environmental factors that influence the complex trait of allergic diseases. This will help us to further understand the etiology of the diseases and develop new avenues for genetically oriented diagnosis and more effective measures of prevention and intervention. Keywords: allergic rhinitis, genetic predisposition, environmental factors Introduction Allergic rhinitis (AR) is a common manifestation of allergic diseases affecting approximately 10%–25% of the world population (Bousquet et al 2001). The symptoms of AR include rhinorrhea, nasal obstruction, nasal itching, and sneezing. The symptoms are reversible and are caused spontaneously by exposure to allergens and triggering factors or whilst under treatment. Allergic rhinitis is a major airway disease, which causes morbidity and significantly impairs a patient’s ability to function and their quality of life. It is also associated with other conditions such as asthma, sinusitis, anosmia, otitis media, nasal polyps, lower airway infection, and even dental malocclusion (International Rhinitis Management Working Group 1994; Spector 1997; Canonica et al 1998; Apter et al 1999). Allergic rhinitis is caused frequently by exposure to perennial or seasonal allergens that exist in our indoor and outdoor environment. Among the most common allergens, pollens (grass, trees, and weeds) are the predominant causes of seasonal allergic rhinitis. House dust mites, pets, and molds are the major causes of perennial allergic rhinitis. However, in tropical and subtropical areas pollen may become a perennial allergen. Recently, the document Allergic rhinitis and its impact on asthma (ARIA), which was developed in collaboration with the WHO, has recommended to replace the old terms “seasonal” and “perennial” rhinitis by “intermittent allergic rhinitis” and “persistent allergic rhinitis”, respectively (Bousquet et al 2001). Therapeutics and Clinical Risk Management 2005:1(2) 115 – 123 © 2005 Dove Medical Press Limited. All rights reserved 115